Rev. sci. tech. Off. int. Epiz., 1998 ,17 (1), 200-219

Genetic resistance to bacterial diseases of animals

L.G. Adams & J .W. Templeton

Department of Veterinary Pathobiology, College of Veterinary Medicine, Texas A & M University and Texas Agricultural Experiment Station, College Station, 77843-4467 Texas, United States of America

Summary Despi te t radi t ional d isease control m e a s u r e s , losses a t t r ibu tab le to infectious d iseases cont inue to i m p e d e the l ivestock industr ies . A n a l te rna t ive a p p r o a c h to this prob lem is genet ic d isease r e s i s t a n c e involving both i m m u n e and n o n - i m m u n e m e c h a n i s m s , w h i c h is t h e i n h e r e n t c a p a c i t y of a previously u n e x p o s e d an imal to resist d isease w h e n c h a l l e n g e d by p a t h o g e n s . A l though the nurtur ing e n v i r o n m e n t in f luences var iabi l i ty in d i s e a s e e x p r e s s i o n , natural res is tance has b e e n found to be inher i tab le and is t r a n s m i t t e d f r o m parent to offspring. T h u s , an a l te rnat ive a p p r o a c h to e n h a n c i n g a n i m a l hea l th m a n a g e m e n t sys tems is to i n c r e a s e the overal l level of gene t ic r e s i s t a n c e at herd and populat ion levels by using se lec t ive b reed ing p r o g r a m m e s . T h e purpose of this r e v i e w is to bring v e t e r i n a r i a n s , regu la to ry of f ic ia ls , industry r e p r e s e n t a t i v e s and an imal t e c h n i c i a n s up to da te w i t h t h e pr inc ip les and appl ica t ions of gene t ic res is tance as an a d j u n c t to t rad i t iona l in tervent ions to control bac ter ia l d iseases of l ivestock. A l t h o u g h g e n e t i c r e s i s t a n c e to bacterial d i s e a s e s is often regu la ted by mult iple g e n e s contro l l ing d i f fe rent p r o c e s s e s of t h e h o s t - p a t h o g e n in te rac t ion , t h e g e n e t i c s of na tura l r e s i s t a n c e is being unrave l led increas ing ly by ident i f icat ion and c h a r a c t e r i s a t i o n of c a n d i d a t e genes, microsate l l i te m a r k e r s and c o m p a r a t i v e g e n e m a p p i n g , to d e v e l o p m o r e practical m e t h o d s of app l ica t ion .

Keywords An ima ls - Bacter ia - Disease resistance - Genes - Genet ics - Immun i ty - Natural ly-acqui red immuni ty .

Introduction Natural resistance to bacterial diseases was first observed over one hundred years ago as familial tendencies in resistance or susceptibility to diphtheria in humans (83) , but the genetic implicadons of this observation were not appreciated at the time, and another twenty years passed before the rediscovery of the studies performed by Gregor Mendel. With the renewed appreciation of the genetic principles proposed by Mendel, experimental animals were bred in the 1920s specifically to study natural resistance to a variety of infectious diseases ( 1 3 8 , 1 7 7 ) . The fact that disease manifestations rarely occur in all members of the population exposed to bacterial pathogens had long been observed in domestic livestock, and studies of resistance to Salmonella Pullorum in poultry and Brucella suis in swine confirmed a major role for genetic control (34, 138). Subsequent research on tuberculosis in twins confirmed the importance of genetics in determining

the variability of susceptibility (45, 5 6 , 8 4 ) , while more recent studies have clearly differentiated the influence of genetics as compared to environment on premature death caused by infectious and non-infectious diseases in human adoptees (157) .

These and similar early observations were largely ignored for several reasons. First of all, antibiotics were discovered in the late 1920s, and the usefulness of these new drugs was clearly demonstrated during World War II. Secondly, vaccines were rapidly being developed for several animal diseases. Lastly, the genetics of natural disease resistance seemed unduly complicated, and there was concern that planned breeding programmes to increase natural resistance would be too slow to have an impact and would compromise productivity.

If there is no shortage of evidence for genetic control of disease resistance, then why is genetic resistance not being used more

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in modern livestock industries? Perhaps because regulatory officials, owners, producers and other industry managers do not recognise the potential for genetic resistance, not necessarily as the tool to replace traditional methods of disease control, but as another approach to reduce the impact of bacterial pathogens on animal health and to play a role in system-based approaches, such as the pre-harvest pathogen reduction programme. Significant losses caused by bacterial diseases continue to restrict livestock industries despite traditional control measures. Over the last decade, the excessive use of antibiotics has been criticised because of the possible development of antibiotic-resistant zoonotic organisms and the potential dangers of residual antibiotics in food animal products for human consumption. These problems and concerns suggest that other measures for the control of infectious disease should be sought to enhance animal health management programmes. Newer strategies to increase the overall level of resistance at herd and population levels which use selective breeding programmes to enhance natural resistance are expected to contribute significantly in this regard (3, 14, 59, 60 , 8 1 , 98 , 158, 159, 161 , 169, 176, 181).

This review will focus on progress in understanding genetic resistance to bacterial diseases in livestock species over the past two decades: readers are referred to previous in-depth reviews and texts for general and detailed knowledge of the genetics of resistance to bacterial diseases ( 3 , 3 1 , 8 1 , 1 0 3 , 1 2 2 , 159, 163, 175, 176) . Finally, the purpose of this review is to bring veterinarians, regulatory officials, industry representatives and animal technicians up to date on the principles and applications of genetic resistance as a new, alternative adjunct to traditional interventions in the control of bacterial diseases of livestock. In recent years, much progress on the identification and characterisation of candidate genes, microsatellite markers and comparative gene mapping has been made. Identification of individual candidate, genes which control natural resistance and the actions of these genes will greatly expand the knowledge of genetic resistance to bacterial diseases and the possibilities for practical application.

Background The animal genome always influences (and sometimes determines) susceptibility to bacterial diseases, yet because of the huge variety of pathogens and the multitude of complex host defence mechanisms involved, a simple understanding of resistance rarely emerges. Given this series of complex host-pathogen interactions, the fact that control of natural bacterial infection and resulting disease would rarely be exercised by a single gene is obvious, although expression of an allele at one locus can significantly affect disease pathogenesis in individuals. At the herd and population levels, where most livestock scientists are involved, many

genes are operational in controlling the spectrum of disease expression.

The only way to understand genetically controlled resistance, and to reduce this understanding to practical application, is to investigate the sequential processes affecting the outcomes of host-pathogen interactions, and to identify in a step-by-step, reductionist manner the pattern of host responses to a pathogen in an attempt to reconstruct and define the genetic regulation of these processes. The fact that the successful co-existence of a bacterial pathogen with its host is the outcome of ancient co-evolutionary relationships and selection pressures must also be borne in mind. These developments have resulted in a stalemate in which the pathogen has evolved to survive within the biological systems of the host, and the host has evolved an immune system which allows survival of infection by the pathogen, thus ultimately supporting the survival of the host-pathogen system (60) . The evolutionary development of innate, natural resistance preceded that of the adaptive, specific immunity, but as adaptive immunity evolved, its effector mechanisms combined forces with natural immune functions (122) . Indeed, recent findings suggest that innate immunity is essential to the optimal function of acquired, adaptive immunity, because most of the antigen receptors are not encoded in the germ line (53, 110) . An awareness that approximately 10,000 years of domestication of livestock species has imposed artificial selection through management practices and procedures, which - for better or for worse -influence the balance of the host-pathogen systems, is also needed. To understand these relationships, many strategies are used to analyse genetic control of natural resistance with the following minimal requirements:

- variation in host response to the pathogen

- constant environmental conditions

- standardised discriminating challenge dose of the pathogen

- constant virulence of the pathogen

- constant route of challenge, preferably approximating the natural route of exposure

- consistent phenotyping procedures.

Natural disease resistance refers to the inherent capacity of an animal to resist disease when exposed to pathogens, without prior exposure or immunisation (81) . Although some of the observed variation in natural resistance is related to environmental factors, a significant component of variation in natural disease resistance appears to be heritable and, therefore, to be passed stably from parent to offspring. Processes by which an animal might resist disease include both immurie and non-immune mechanisms, as supported by important previous findings in the 1960s. These earlier discoveries of the role of genetics in susceptibility to mycobacterial infections were suggested by epidemiological studies (112) , substantiated by twin studies (45, 84) and further documented by resistance to mycobacteria in strains

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of inbred rabbits (99). Almost simultaneously with these studies, the cellular basis for specific immune responsiveness to mycobacteria was defined (101, 102). The combination of these findings then formed the basis for two major concepts. Firstly, the host response to mycobacterial infection has two phases: the innate stage of resistance to the establishment of infection, and the later development of specific, acquired cellular and humoral immunity, which results in either clearance of infection or disease progression. Secondly, host resistance mechanisms are under genetic control during both phases, but each phase is governed by different groups of genes.

These concepts established the foundations for the creation of recombinant inbred and congenic strains of mice to explore the genetics of natural resistance and to develop strategies for pursuing the molecular genetics of natural resistance in specific host-pathogen systems, particularly those of intracellular bacterial pathogens. Several approaches were successful, for example, positional cloning, candidate gene analysis, gene replacement knockout mice and other procedures. While questions and concerns regarding the traditional health management procedures are being raised, remarkable progress is being made in identifying specific genes which contribute to natural disease resistance. The creation of gene maps for domestic animals by identifying the chromosomal location of a large number of genes is also making significant progress ( 6 , 1 8 , 58 , 182). In time, any gene involved in disease resistance should be genetically linked to a useful marker gene. Further advances in genetic technology should provide practical methods for using knowledge of disease resistance genes to improve the overall health of livestock. Together with current approaches to health management, genetic manipulation to increase natural disease resistance is predicted to contribute significantly to improvements in the health and productivity of domestic animals.

General mechanisms of genetic resistance From the innate or natural non-specific immunity perspective, resistance to bacterial pathogens encompasses the following:

- resistance due to impenetrable barriers

- absence of appropriate receptors for binding and penetration of cellular membranes

- failure to survive after entering the host

- inability to replicate in the host

- killing and elimination by host defence mechanisms, especially by phagocytes.

These factors, although termed 'non-specific', are in fact dependent upon precise, genetically determined molecular interactions. Resistance to bacterial pathogens from the point of view of the 'acquired' or adaptive specific immunity involves the following:

- lymphocyte-mediated host responses, including cytotoxic T cells, helper/inducer T cells, delayed-hypersensitivity T cells and B cells

- non-lymphoid cells, natural killer (NK) cells and particularly macrophage- and granulocyte-mediated phagocytosis and inflammation

- humoral-mediated responses, including antibodies and complement

- production and regulation cytokines (175) .

Importantly, the processes for both innate and adaptive immunity are genetically determined and controlled: innate immunity is controlled by non-major histocompatibility complex (MHC) genes whereas adaptive immunity is controlled by MHC genes, although the division between innate and adaptive immunity is strictly operational and major co-operation between the two systems occurs for optimal immunity to most bacterial pathogens (31). To illustrate this observation, Biozzi bred two strains of mice for their ability to make specific antibodies after optimised immunisation of alternate consecutive generations with either sheep erythrocytes or Salmonella antigens (16 , 17). After twenty generations, one strain producing high titres of specific antibody and one strain producing low titres were developed. Approximately ten genes were estimated to be responsible for the differences in the two strains. Several differences in the macrophages of these two strains were identified. Macrophages from the low-response strain cleared antigens from the body more rapidly and killed intracellular bacteria more rapidly. Apparently, the low antibody response in these mice was caused by the rapid clearance of antigen by their mononuclear-phagocytic system. With higher levels of macrophage activity, these low-response mice were also more resistant to infections with the intracellular pathogens B. suis, Mycobacterium bovis, Salmonella Typhimurium, Yersinia pestis, Listeria monocytogenes and others (16 , 37).

The host usually employs at least three phases of host defence to overcome bacterial infections. The first defence mechanism encountered by the pathogen is the epithelial barrier. The epithelium provides a physical barrier blocking entry of potential pathogens, but also provides cell surface receptors for attachment of some pathogens, which is the first step in the establishment of many bacterial infections (86 , 111 , 175). Inappropriate conformation or absence of the receptor will prevent colonisation of the host. The second phase of host defence is non-specific, or innate, immune mechanisms (103, 111). Cells involved in non-specific immunity include neutrophils, macrophages and NK cells which lyse infected cells (141) . Humoral mediators of non-specific host defences include the complement system (61) and the interferon

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system (80). The third phase of host defence is specific, or adaptive, immunity ( 1 1 1 , 175) . Stimulation of adaptive immune mechanisms is the goal of vaccination programmes, but these mechanisms may also be significant in speeding recovery and clearing the infection. However, in a primary infectious process, the development of this phase is delayed as it requires activation, proliferation, and differentiation of antigen-specific T and B lymphocytes (103) . Several classes of T lymphocyte activity have been defined, including helper/inducer activity, suppressor activity, cytotoxic activity and activities inducing delayed hypersensitivity. Humoral mediators of immunity include the immunoglobulins secreted by plasma cells and lymphokines secreted by certain classes of activated T lymphocytes.

Host genes involved in resistance to bacterial pathogens Interestingly, the best defined host-pathogen systems from the perspectives of effector mechanism and genetics are facultative or obligate intracellular bacterial pathogens (some of which are capable of extracellular replication). This may suggest that an evolutionary pathway was followed independently by a broad spectrum of bacteria, with selection in favour of organisms able to establish and sustain themselves inside host cells which had phagocytic and microbicidal function. All processes of host defence are subject to genetic

control. Most of the natural defence mechanisms are controlled, at the molecular level, by proteins. Lipids and carbohydrates which contribute to host defence mechanisms are synthesised by enzymes. All proteins, whether enzymatic or structural, are synthesised from messenger RNA, which in turn is synthesised from the gene coding for the protein. Variant forms of a gene are called alleles, and alleles may give rise to different structural forms of the protein. Alterations in protein structure may affect the ability of the protein to function. If the protein is part of the host defence mechanism, then a decrease in its functional capacity may lower resistance to one or more infectious agents.

Theoretically, a defect in any single gene which controls the structure or regulates the expression of any host defence mechanism could modify the level of natural host resistance to a specific infectious disease, or to infectious diseases in general. In reality, many genetic defects would be so overwhelmingly disadvantageous that they would be rapidly selected out of the population, although selection pressure has diminished with the introduction of antibiotics and management procedures. There remain, however, other genes which strongly influence the level of host resistance in an individual, and several of these have already been identified. Most genes controlling specific processes of natural disease resistance have been identified using inbred strains of mice (Table I) (103, 105) , due to the ease of creating mice for genetic experiments. Most genes identified in mice will probably be functional in domestic animals as well. Whether variation in any individual gene accounts for variations in disease resistance in domestic animals is known for only a few genes (Table II).

Table I Genetic resistance to bacterial pathogens in mice

Immune mechanism Pathogen Gene Functional role References

Microbicidal Mycobacterium, Nrampl Endocytic membrane protein, effector transport in phagosome? (63, 64, 70,106,171,172) Salmonella Typhimurium [Ity/Lsh/Bcg] Mycobacterium paratuberculosis Nrampl Endocytic membrane protein, effector transport in phagosome? (57) Mycobacterium N0S2 Inducible macrophage nitric oxide synthase (100) Gram-negative bacteria, Salmonella Cryp Paneth cell defensins (147) Salmonella Typhimurium Lps Macrophage effector function (118,120) Listeria monocytogenes Lsrl Macrophage activation (61,154) Legionella pneumophila Lgnl, Lps Early inflammatory macrophages? (10,11,184,185) Francisella tularensis — Macrophage function? 15) Brucella abortus — Macrophage clearance & MHC? (38,76)

Tcell Rickettsia tsutugamuchi Ric T-cell mediated elimination (68) Antibody Salmonella Typhimurium xid B-cell controlled anti-polysaccharide immunoglobulins (117) Antigen Mycobacterium, Salmonella Nrampl Enhanced macrophage antigen presentation (20,30,92) processing, [Ity/Lsh/Bcg]

(20,30,92)

presentation Listeria monocytogenes MHC class I-b Non-classical MHC antigen presentation to CD8+ T cells (91)

MHC : major histocompatibi l i ty complex CD8+ : cluster of di f ferent iat ion antigen 8 +

Nramp : natural resistance-associated macrophage protein N0S2 : nitric oxide synthase-2

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Table II Genetic resistance to bacterial pathogens of domestic animals

Animal species Disease Phenotype Pathogen Gene References

Cattle Brucellosis In vivo Brucella abortus NRAMP1 1 (54) In vitro Brucella abortus NRAMP1 ? (73,74,130,132)

Tuberculosis In vivo Mycobacterium bovis - (124) In vitro Mycobacterium bovis NRAMP1 1 (132,166)

Salmonellosis In vitro Salmonella Dublin NRAMP1 1 (132) Swine Colibacillosis In vivo Escherichia coli K88+ - (146)

Brucellosis In vivo Brucella suis - (32,33, 34, 35) Chicken Pullorum In vivo Salmonella Pullorum S a l ? (26,138)

Fowl typhoid In vivo Salmonella Gallinarum Sal? (26) Salmonellosis In vivo Salmonella Enteritidis S a l ? (26) Salmonellosis In vivo Salmonella Typhimurium S a l ? (25)

Sheep Footrot In vivo Dichelobacter nodosus - (51,121,133,156,179)

?: preliminary evidence

Porcine K88 + receptor The first defence barrier, the epithelium, provides receptor sites for attachment of many pathogens. Cell surface receptors for viral or bacterial attachment are a heterogeneous group of cell surface molecules, and some have been carefully defined and studied. Genetically determined variation in the structure of a cellular receptor would cause some individuals to be resistant to colonisation by a specific pathogen. The classic example of such genetic variability is the resistance of certain neonatal piglets to the K 8 8 + strain of Escherichia coli (which causes colibacillosis) as a result of non-expression of an appropriate receptor for attachment of this organism to intestinal epithelium (146) .

Natural resistance-associated macrophage protein 1 (Bcg/lty/Lsh) Variations in the functional capacity of phagocytes, affect disease susceptibility (17, 64, 111, 141), an example of which is the expression of the natural resistance-associated macrophage protein-1 (Nrampl) gene. Studies over the last two decades have led to the identification of a gene, Bcg/lty/Lsh, on mouse chromosome 1 which alters the early stages of resistance to diverse and antigenically unrelated facultative and obligate intracellular pathogens, including M. bovis (bacillus Bilié-Calmette-Guérin) (Bcg), M. intra­cellulare, S. Typhimurium (Ity) and Leishmania donovani (Lsh) (23, 47, 66 , 127, 155), although these murine models do not usually discriminate between infection and disease. Resistance to one disease in animals was previously thought to have little effect on resistance to other diseases, but activation and signalling pathways in host cells may have similarities with regard to survival of intracellular bacterial pathogens (19, 43 , 180) . A candidate gene for Bcg/lty/Lsh, designated Nrampl, has been identified by positional cloning (171) and shown to encode an integral phosphoglycoprotein membrane protein, with 12 transmembrane domains, which is expressed only in the macrophages of reticuloendothelial organs (19, 82,

105). Other sequence motifs of Nrampl include predicted extracellular glycosylated loops, sites for phosphorylation by protein kinases, a proline-rich src homology 3 (SH3)-binding domain and a 'binding-protein-dependent transport system inner-membrane component signature' motif in one of the intracellular loops. Gene disruption (knockout) experiments have confirmed that Nrampl and Ity/lsh/Bcg are the same gene (172, 173) , and that this gene acts early to regulate the pathway priming/activation for anti-microbial activity, particularly generation of NO (nitric oxide) following tumour necrosis factor a (TNF-a)-dependent, sustained expression of Nos2 encoding inducible nitric oxide synthase (iNOS) (7, 9, 92, 135, 137). Nrampl has many pleiotropic effects influencing phenotypic expression, which include the following:

- inhibition of bacterial growth

- granuloma formation

- production of reactive oxygen and nitrogen products

- antigen processing and MHC class II molecule expression

- enhanced phago-lysosomal fusion

- regulation of production and release of KC chemokine and TNF-a and interleukin-1(3 (IL-1(3) cytokines (19, 4 3 , 4 8 , 9 2 ) .

Susceptibility to M. bovis BCG infection in inbred strains of mice is caused by a recessive deficiency allele associated with a single glycine to aspartic acid substitution at position 169 of the putative transmembrane region of the protein (106). Recent findings provide evidence that the membrane phosphoglycoprotein of Nramp 1r is associated with late endocytic (late endosome/lysosome) compartments (70) and a- and -tubulins (89) , is constitutively expressed in professional phagocytes of the myeloid line (44, 186) and is also inducible by interferon-y (IFN-y) followed by lipopolysaccharide (LPS) (42, 62) , while the Nramp1s form of the protein is degraded and is not expressed (174) .

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Subsequently, Nrampl has been shown to belong to a family of related proteins, including Nramp2 (69) , which is expressed in many tissues in mice and humans and has also been found recently in divalent cation transporter (DCT1) (71), which is expressed primarily in duodenum and kidney of mice. Several polymorphisms of the human homologue (41, 87), Nrampl, have been described (21 , 97) , but no association has been found with tuberculosis (115) , leprosy (93,148) or leishmaniosis (178) , although an association with rheumatoid arthritis has been demonstrated (149) . Moreover, controversy exists regarding the role played by Nrampl in natural resistance to tuberculosis, because in other studies which compared antimicrobial resistance to Mycobacterium tuberculosis or BCG, M. tuberculosis-infected Nrampl r mice actually had reduced survival times when compared to congenic Nramps mice (107, 109, 116); however, Nrampr mice were able to stabilise BCG pulmonary infections (108) . Extensive controversy continues over the mechanism of action of the Nrampl gene as a possible nitrate (or other substance) transporter, SH3-binding domain-associated cell signal transducer, cytoskeletal attachment mechanism or tyrosine kinase phosphorylation associated with macrophage activation (19, 2 1 , 43 , 88 , 171).

In domestic animals, homologues for Nrampl have been identified, sequenced and/or mapped in cattle (54) , chickens (78, 79), swine (168) and sheep (126 , 125) , although at present, associations with disease resistance have been documented only in cattle (54) . Other homologues for Nrampl have been identified in Drosophila melanogaster (fly), Caenorhabditis elegans (nematode), Oryza sativa (rice), Saccharomyces cerevisiae (yeast) and M. leprae (bacteria), thus indicating conservation of an ancient origin for the NRAMP family associated with a common membrane structure and possible transport activity (43) .

Major histocompatibility complex The third phase of host defence on the primary encounter with a pathogen is the specific immunological response. The MHC is an important genetic complex controlling immune responses (12, 77, 98 ) , and consists of a series of closely linked genes coding for at least three classes of proteins in all mammalian species studied. There are multiple copies of the class I genes, which encode glycoproteins that associate non-covalently with a second protein, ^-microglobulin, on the surface of all nucleated somatic cells. Cytotoxic T lymphocytes use these class I MHC molecules in recognising infected cells (144) . Thus, the antigen and the class I MHC molecule together constitute an antigenic complex recognised by the cytotoxic T lymphocyte. There are also multiple copies of the class II MHC genes. These consist of genes for the a - cha in and the (3-chain of class II glycoproteins. The two chains associate non-covalently on the surface of B lymphocytes and cells of the mononuclear-phagocytic system. T lymphocytes of the helper/inducer and delayed-hypersensitivity classes use class II molecules

for recognition of foreign antigens (144) . For these T lymphocytes, both the foreign antigen and the class II MHC molecule make up the antigenic complex required for T lymphocyte recognition. The recent characterisation of human MHC peptide-binding motifs and the identification of peptides from pathogens binding these MHC molecules and eliciting protective immune responses provide novel approaches to optimise vaccines (4) . Additionally, peptide-binding motif analysis offers new rationales for understanding hyporesponsiveness in certain individuals (152) .

An important characteristic of the class I and class II MHC genes is their high degree of polymorphism. Polymorphism indicates that a gene exists in more than one allele in a population. For most genes, only one to a few alleles exist in a population. By contrast, over fifty alleles have been serologically identified for each of two class I genes in both man and mice (77) . Additional alleles are being defined by improved serological techniques and by other, more precise, techniques. A similar degree of polymorphism exists for class II genes (77) . The result of the multiplicity of MHC genes is that each individual has a number of class I and class II molecules with which its T cells recognise foreign antigens. The consequence of the high degree of polymorphism for each gene is that the set of MHC class I and class II genes is unique to each individual; over 100 million combinations are possible. The ability of an individual to respond to certain antigens is under genetic control of the MHC. This was shown most clearly in mice using simple synthetic antigens (12) . Most of these effects are determined by the class II genes, and presumably relate to the ability of the class II molecules available to the individual to form an immunogenic complex with the foreign antigen, and thereby to activate helper/ inducer T lymphocytes to initiate an immune response (144) .

In humans, genetic associations between the MHC and susceptibility to certain infectious diseases have been identified (129) . A striking example is that of leprosy. Two polar forms of leprosy exist: lepromatous and tuberculoid. Lepromatous leprosy is associated with a minimal T-cell response, disseminated lesions and a poor prognosis. Tuberculoid leprosy is associated with a strong T-cell response, multiple confined granulomata and a good prognosis. The tendency to develop lepromatous or tuberculoid leprosy is inherited and is associated with certain MHC class I and class II alleles (170) .

In mice, some immune response defects are associated with the gene complex for the immunoglobulin heavy chains (IgH) (13) . The IgH complex consists of gene segments for the variable region and for the consistent region of the antibody molecule. The variable region of the antibody binds to specific antigens, and the constant region carries out other biological functions, such as complement fixation. Presumably, immune response defects are caused by an absence of variable region genes capable of forming antibodies which effectively interact with the antigen. In some human diseases, association with

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certain allelic variants, or allotypes, of immunoglobulin heavy chains has been reported (65). The genes encoding the T-cell antigen receptor have recently been identified and mapped (90).

Hereditary immunodeficiency syndromes are clear examples of genetic alteration of susceptibility to infectious diseases. In humans, horses and mice, inherited forms of severe combined immune deficiency, in which the individual lacks functional T and B lymphocytes, have been defined (123) . Specific defects in T- or B-cell function have also, been identified in humans and mice. In mice, the xid gene, on the X chromosome, controls certain aspects of B-cell maturation, and in humans, X-linked defects in B-cell function have also been characterised (55, 150). In mice and rats, the nude gene controls thymic development and, consequendy, T-cell immune functions (151) .

Genetic resistance in domestic animals As stated earlier, resistance may be associated with a single gene but usually is associated with several. Studies which demonstrate genetic control of natural disease resistance in domestic animals are highly dependent on methodology. Important methodological parameters include the nature of the exposure to the pathogen, the method used to assess resistance and the type of genetic analysis used. Exposure to a pathogen may be by natural (field) exposure or by an experimentally controlled and discriminating inoculum. The advantages of the latter include the certainty that all animals in the study are exposed and that the exposure is uniform for all animals. For these studies, the dose of the pathogen used for challenge is chosen to maximise differences between resistant and susceptible animals, while approximating a natural exposure. Resistance and susceptibility may be graded according to the severity of disease induced in the animal or according to isolation of the pathogen from lesions,- or both. Examples of studies of genetic resistance to specific bacterial diseases in which some of the above criteria have been used are illustrated in Table II. If isolation is a criterion, relative differences in susceptibility may be assigned qualitatively (i.e., any isolation of the pathogen implies susceptibility) or quantitatively (i.e., isolation of a minimum number of organisms implies susceptibility; this method will include a class of animals with intermediate susceptibility). If severity of disease manifestations is used as the criterion for assessing relative susceptibility, additional genetic mechanisms are likely to be measured. This arises from the fact that three classes of animals are involved in these experiments, as follows:

- infected and diseased

- infected and disease-free

- uninfected and disease-free.

Thus, one would be measuring genes which allow, or prevent, infection, in addition to those genes which allow, or prevent, disease manifestations in infected animals.

Different types of genetic analyses are also possible. All types of analysis assume that animals can be identified as resistant or susceptible. (Indeterminate types are also included, but cannot be analysed.) The examples given below were selected to represent the various methodologies and approaches which have been used to identify genetic control of disease resistance and susceptibility. Two examples have been noted above: brucellosis and K 8 8 + colibacillosis in pigs. The study of susceptibility to B. suis in pigs represents the ultimate goal of most studies of natural disease resistance: the ability to breed selectively to increase herd resistance to disease (34) . Here, the mechanisms by which some pigs were more resistant were not determined, and genes controlling resistance were not identified. By contrast, the mechanism for resistance to K88 +

colibacillosis is well characterised and relates to genetic variability in the expression, or non-expression, of a receptor for K 8 8 + E. coli on the intestinal epithelium of neonatal pigs (146) .

Bovine brucellosis Studies to determine whether resistance to B. abortus is heritable and to identify genes controlling resistance to brucellosis in cattle began in the late 1970s (165). Unvaccinated and previously unexposed sexually mature bulls or heifers at mid-term gestation (150 + 3 0 days) were challenged with a standardised, discriminating challenge inoculum of B. abortus S2308 , then scored for the outcome of parturition, and quantitative cultures were collected from tissues and secretions three to five months later. Resistant cows did not abort, and no Brucella organisms were cultured from the cow or calf. Resistant bulls similarly gave negative culture results for Brucella in semen and at slaughter. Immune correlates with respect to macrophage function, bovine leucocyte antigen (BoLA) alleles and immunoglobulin allotypes associated with natural resistance to Brucella were determined retrospectively (Table III) and breeding experiments were begun to identify the genes which control differential immune responses.

The expression of anti-LPS immunoglobulin G 2 a (IgG 2 a) allotypes in the two groups of heterozygous cattle was significantly different from the I g G 2 a A l allotype predominating (P < 0 .05) in B. abortus-susceptible cattle (2, 52) . However, previous studies have disputed the role that antibodies play, if any, in natural resistance to B. abortus in cattle ( 7 2 , 1 6 7 ) . No association of resistance with BoLA class I alleles was demonstrated; however, mammary macrophages from resistant cows, and monocyte-derived macrophages from resistant cows and bulls, were significantly more active than macrophages from susceptible cows and bulls in terms of:

a) respiratory burst in response to opsonised B. abortus (74)

b) ability to control replication of B. abortus ( 3 6 , 1 3 0 , 1 3 2 ) .

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Table III Characteristics of macrophages, T cells and antibody responses in cattle naturally resistant or susceptible to Brucella abortus

Biological activity Activity in resistant cattle Activity in susceptible cattle

Intracellular growth of Brucella abortus, Mycobacterium bovis, Salmonella Dublin Phago-lysosomal fusion Production of reactive oxygen intermediates LPS + IFN-y induced nitric oxide production B. abortus binding to macrophages

Antibody response to B. abortus LPS IgG2a Al and A2 allotypic response to B. abortus LPS at 6 wks Oligoclonal T-cell response

Restrictive

Increased High High RGDS tetrapeptide, B. abortus LPS and Mab anti-LFA-1 inhibit 90% binding Minimal, short duration

-50:50 Only B. abortus stimulates response

Permissive

Reduced Low Low Minimal inhibition

Massive, long duration 95% A1 B. abortus, B. suis, B. melitensis and B. canis stimulate response

LPS IFN-y IgG2 a

RGDS LFA

lipopolysaccharide interferon-y immunoglobulin G 2 a

tetrapeptide of asparagine-glycine-glutamine-serine CDIIa/CD18, lymphocyte function-associated antigen

The differential response between macrophages from resistant and susceptible cattle was similar to the differences observed in mice strains resistant and susceptible to M. bovis BCG, L donovani, M. paratuberculosis and 5. Typhimurium (48, 134,135). Furthermore, macrophages from cattle which were selected for in vivo resistance to B. abortus restricted the intracellular replication of B. abortus, M. bovis BCG and Salmonella Dublin significantly better than macrophages from cattle which were susceptible to B. abortus challenge (132) . By assaying the brucellacidal capacity of macrophages before cattle were challenged with B. abortus, resistance or susceptibility to in vivo challenge was predicted with > 8 0 % accuracy in preliminary studies (L.G. Adams and J.W. Templeton, unpublished data). The striking similarities between bovine macrophage function and resistance to brucellosis and murine macrophage function and resistance to Salmonella, Mycobacterium and Leishmania supported the hypothesis that a bovine homologue of the murine Bcg/Lsh/Ity gene was one of the major candidate genes determining resistance to brucellosis.

Consequently, the bovine homologue to the murine Nrampl gene (designated bovine NRAMP1) was cloned and the complementary DNA (cDNA) sequenced (54) . The DNA sequences of the murine Nrampl, human NRAMP1 and bovine NRAMP1 have remarkable conservation of DNA sequence and predicted amino acid sequences. The predicted amino acid homology for murine Nrampl and bovine NRAMP1 is 87%, and that between human NRAMP1 and bovine NRAMP1 is 8 9 % (41, 54) . All three of the predicted Nrampl proteins are polytopic membrane proteins which are expressed predominantly in the macrophages or reticuloendothelial cells. Common features in all three species are a conserved protein of approximately 60 kiloDaltons (kDA) with an N terminus SH3 motif, four phosphokinase C (PKC) phosphorylation sites and a 'binding protein-

dependent transport system inner-membrane component signature', all predicted to be expressed on the inner membrane (19, 54, 171) . The hypothesis was further supported by the conservation of the Bcg/Lsh/ity-bovine NRAMP1 amino acid sequences and the conservation of the linkage group on mouse chromosome 1 (19, 104, 128, 178) and bovine chromosome 2 ( 6 , 1 8 , 5 8 , 1 8 3 ) . Northern blotting confirmed that bovine NRAMP1 was primarily expressed in macrophages of the reticuloendothelial system (RES). As discussed previously, there is a point mutation at position 169 of the murine Nrampl which distinguishes the resistant (Gly-169) and susceptible (Asp-169) alleles of the murine Bcg gene. Neither cattle nor any other species (including humans, rats, chickens, bovine, bison, water buffalo, sheep, red deer and moose) sequenced for the Nrampl homologue by the authors or other researchers have the coding region mutation at position 169. Single-stranded conformational analysis (ssCA) (142) disclosed a highly significant association of a single-stranded conformation polymorphism located in the 3 ' untranslated (UT) portion of bovine NRAMP1 with cattle naturally resistant to brucellosis (Table IV). The only difference found in NRAMP1 sequences from cattle, bison, water buffalo, goats, sheep, red deer, white-tailed deer, fallow deer and moose is an imperfect dinucleotide ( G T ) n DNA microsatellite in the 3 ' UT portion of bovine NRAMP1 homologues. These data also indicate that more than a single gene influences susceptibility to B. abortus infection and abortion in cattle.

In classical breeding studies, natural resistance to Brucella was demonstrated to be increased dramatically by simple mass selection in one generation of selective breeding (164) . The frequency of natural resistance to brucellosis in challenging unvaccinated cattle was 2 0 % (30/150) . Mating a naturally resistant bull with naturally resistant cows increased the frequency of natural resistance in their progeny to 5 8 . 6 %

208 Rev. sci. tech. Off. int. Epiz., 17 (1)

Table IV Association of bovine natural resistance-associated macrophage protein-1 single-stranded conformational analysis polymorphism with bovine brucellosis-resistant and -susceptible phenotypes

/NRAMP1ssCA In vivo resistant In vivo susceptible type phenotype phenotype

Resistant ssCA g (a) 2 Susceptible ssCA 2 9 Total 11 11

a) Significant association: P= 0.0089 (Fisher's exact test) NRAMP1 : bovine natural resistance-associated macrophage protein ssCA : single-stranded conformational analysis

(17/29). When resistant bulls were bred to resistant cows and susceptible bulls to susceptible cows, both matings segregated resistant and susceptible phenotypes in the progeny. The genetic analysis of these mating results is consistent with two or more genes controlling the resistant phenotypes. Subsequently, the authors have produced 5 sibships of 48 backcross and 8 F l half sibs for a more thorough genetic analysis of natural resistance. Bovine NRAMP1 single-stranded conformational polymorphism (ssCP) typing is now being evaluated in these five sibships challenged with B. abortus and in large populations of naturally exposed cattle which have been phenotyped for disease expression and confirmed by bacteriological culture to be free or infected with M. bovis, S. Dublin or B. abortus as a method for genetic selection against these diseases. This study indicates that natural resistance involves interacting genes, which leads to complex genetic types and consequently to the need to identify other genes contributing to natural resistance. The study also illustrates that individual genes, or gene effects, which have an effect on natural resistance can be identified in spite of complex patterns of inheritance, and that the frequency of these genes realistically could be increased through selection or gene manipulation procedures.

Porcine brucellosis In regard to swine brucellosis, a series of studies of genetic control of resistance to B. suis was conducted by Cameron e£ al. (32, 33 , 34) . In the first experiments, two sows and one boar which did not develop antibody titres when exposed to B. suis were saved for breeding. These swine were classified as naturally resistant (R) to brucellosis. The progeny of these three apparently resistant swine were challenge-infected and their antibody titres to B. suis were compared to control groups of unselected pigs, which were also challenge-infected. A total of 7 3 % (24 of 33) of the progeny from the resistant swine were resistant to the challenge infection, compared to 9% resistant (3 of 24) progeny in the control group. In subsequent breeding challenge-infection studies, R X R matings produced 128 progeny, of which 76 .6% were R, 22 .6% were of uncertain status and 0.8% were susceptible. Thus, in one generation of mass selection, resistance to B. suis could be increased remarkably by approximately 5 4 % over unselected controls. Another important observation of natural

disease resistance was the demonstration that pigs which did not develop brucellosis in a natural outbreak of B. suis were naturally resistant to the disease, and that 76 .6% of their progeny were also resistant to an experimental challenge with B. suis (34). Obviously, very few genes are involved in controlling resistance to B. suis in swine.

Bovine tuberculosis Natural resistance to tuberculosis in cattle has been observed and statistically evaluated in families and breeds exposed to M. bovis under field conditions (39, 4 0 , 124, 136). No association between the expression of tuberculosis and the Latvian Brown or Mottled Black breeds was found (124). Zebu cattle were observed to be much more resistant to M. bovis than European breeds (Jersey, Holstein and Brown Swiss) and Ankole cattle in another study (40, 136) , which is somewhat analogous to the findings of racial differences in tuberculosis susceptibility in humans (160) . Additionally, Zebu cattle derived more protection from BCG vaccination against experimental virulent M. bovis challenge than Sussex cattle (50) . From extensive family studies, the coefficient of heredity of resistance to M. bovis in cattle was estimated to be 0.18 (124) . Families having low levels of tubercular disease and high productivity were documented in these extensive studies over time and in different climatic zones. Recently, macrophages from cattle phenotyped for in vivo and in vitro resistance to B. abortus (36, 130, 132) were found to be significantly superior (P = 0 .009) to macrophages from susceptible cattle in their ability to restrict the growth of M. bovis (BCG) (132) . The previously mentioned mono­zygotic and dizygotic twin study of human tuberculosis gave the first clear evidence that disease expression was genetically influenced (45, 56 , 84) . The Beg gene was subsequently identified and characterised in mice (22, 67) and documented to control mycobacterial infections (28, 39 , 143) , including M. bovis (BCG) and M. paratuberculosis (57) . Bcg was confirmed to be identical to the Ity and Lsh genes controlling salmonellosis and leishmaniosis and to be allelic with Nrampl by gene disruption (173) . Considering these observations and discoveries, the data from cattle, mice and humans are highly suggestive that resistance to M. bovis is genetically influenced by the candidate gene, Nrampl (166) .

Avian salmonellosis Some of the earliest observations of natural resistance to bacterial diseases of domestic animals were made in elegant, long-term experiments in the 1920s and later years by forward-thinking poultry scientists who convincingly demonstrated that resistance to S. Pullorum (138) and S. Typhimurium (64) is under genetic control. More recently, studies of not only S. Typhimurium but also S. Gallinarum and S. Enteritidis in inbred lines of chickens have extended the earlier findings and provided clear evidence of natural resistance to clinical disease and mortality due to salmonellosis (25, 26) . Other studies have demonstrated that survival of S. Typhimurium in the liver and spleen was significantly lower in susceptible than in resistant inbred lines

Rev. sci. tech. Off. int. Epiz., 17 (l) 209

of chickens (8); these findings are parallel to the murine salmonellosis resistance (95, 96 , 119) . Linkage analysis in segregating backcross chicken progeny suggested a major dominant autosomal gene not linked to the MHC (25) , which certainly implies that a homologue of the murine Nrampl should be considered as a candidate, as recently proposed (78). With the availability of the chicken gene maps (25, 94) and inbred lines of resistant and susceptible chickens (8, 25 ) , this hypothesis will certainly be tested.

Ovine footrot Genetic factors for natural resistance to Dichelobacter nodosus-induced footrot have been observed in sheep under various climatic and management conditions (24 , 5 1 , 1 7 9 ) in addition to breed-associated resistance (51 , 133, 156) , although some studies had limited numbers of observations. One study suggested that development of footrot lesions was inversely related to the humoral immune response to membrane antigens of D. nodosus (179) , while others imply that resistance may be related to non-immune mechanisms, e.g., receptors (24) . The mechanism of resistance remains largely undefined, as does the mode of inheritance, although resistance to footrot did respond to selection pressure (24) , but is difficult to phenotype accurately because of the extreme sensitivity to variable challenge inocula. Other studies conclude that insufficient data exist to permit inclusion of selection for resistance to footrot in production-oriented breeding programmes (49) .

Future strategies for developing genetic resistance in domestic animals Since breeding for resistance to all bacterial pathogens will not be possible, should the selection for resistance be specific or general (59)? Should selection be for resistance to infection or to disease (122)? Can selection for optimal disease resistance be achieved without compromising productivity (59, 6 0 , 1 2 2 , 169)? These basic questions provoke major controversy, some aspects of which will be discussed.

Caveats Clearly, any form of resistance to an infectious disease is relative rather than absolute. This is true for vaccine-induced resistance as well as for natural resistance. Thus, breeding animals to increase their level of natural resistance will not completely prevent infectious diseases. However, the increased level of natural resistance conferred by selective breeding would reduce morbidity and economic losses caused by infectious diseases. In combination with a vaccination programme, inherited natural resistance could provide even better protection than the use of either measure alone. Indeed, a strategy could include selection for appropriate immune responses to vaccination. Another

concern is that, in selecting for natural resistance, production traits may be compromised, even though information on correlations between disease resistance, immune parameters and important production is practically non-existent. A compromise might occur if there were a negative correlation between enhanced disease resistance and one or more production traits. On the other hand, selection for high egg production and other related traits in chickens resulted in slightly improved resistance to Marek's disease (60) . Additionally, the predicted relative economic benefits will need to be determined to justify the inclusion of selection for resistance in a breeding programme for maximum productivity.

If a negative correlation between disease resistance and production is observed, at least three approaches might be used to avoid negative selection for the production traits. One simple approach would be to test unrelated breeder stock for correlation between resistance and production traits; a negative correlation in one set of breeder stock does not imply a correlation in all stock. An alternate approach would be to survey the breeding stock for animals which have undergone genetic recombination which reassorted the genes for disease resistance and production traits. This approach is being greatly facilitated by the rapid development of linkage and physical gene maps of domestic animals, cattle, chicken and swine (27, 58 , 139, 182, 183) , particularly microsatellite saturated maps, in which selection for maximal polymorphism on each chromosome and genetic diversity would be possible. These maps are a major resource for comparative genetic analysis for homologues of resistance genes in other species (46) . A third approach could be taken if the specific gene conferring resistance were identified and isolated. This would involve transferring the resistance gene into an embryo, so that the resistance gene would effectively be added to the desirable production traits determined by the genome of the embryo. This last approach will be feasible only when resistance genes are isolated and the technology for transferring genes efficiently has been developed.

The generality of disease resistance is also a major concern. Several studies have demonstrated the feasibility of breeding animals resistant to a single pathogen. Some have been able to demonstrate that animals bred for resistance to one pathogen are also more resistant to another pathogen, for example, Ostertagia circumcincta and Haemonchus contortus in sheep (145) . As noted above, both the Lsh and Lps genes in mice confer resistance to several unrelated intracellular pathogens (47, 118, 140, 155). In addition, Biozzi et al. bred mice for high and low antibody titres in response to immunisation with sheep erythrocytes, and were able to demonstrate segregation of resistance and susceptibility to a variety of pathogens in those strains (17) , thereby demonstrating that different immune functions play crucial roles in resistance to different pathogens. Chickens selected in the same way also differed in their levels of natural resistance to various infectious diseases (153) .

210 Rev. sci. tech. Off. int. Epiz., 17(1)

These findings indicate that the ability to select for resistance to a group of pathogens is possible, but may not be probable, particularly given the complexity of bacteria and the multitude of host defence mechanisms. Perhaps selection for resistance to a reduced spectrum of bacterial diseases could be tailored for pathogens of cold, temperate or tropical climatic conditions or specific management systems, because selection for all pathogens will not be feasible. Efforts to enhance the general immune response through selection would be less likely to contribute to the generalised enhancement of the disease resistance but rather would be expected to enhance the quantity and quality, as opposed to the specificity, of the immune response (15, 16, 17). This possibility is further supported by the fact that selection on components of a trait is rarely as effective as selection on the whole trait. In addition, the carrier state at the population level is frequently associated with low levels of persistent infection concurrent with protective immune responses, which leads to the idea that the adaptive immune response is in reality a modulator of the host-pathogen relationship; as a result, good immunity allows life to continue with infection, i.e., a successful co-existence. Thus, selection should be made preferably for resistance to disease rather than for resistance to infection.

Strategies for application of genetic resistance Remembering that under natural conditions, host, pathogen and environment are normally interacting in expression of disease, careful approaches are necessary to understand the basis for genetic resistance. Several strategies for improving genetic disease resistance have been proposed previously from various perspectives (1 , 59, 169) , as follows:

- variation in host response to the pathogen

- environmental conditions

- standardised discriminating challenge dose of the pathogen

- constant virulence of the pathogen

- constant route of challenge, preferably the natural route of exposure

- constant phenotyping procedures of the resistant trait

- methods to measure components of innate and adaptive immune responses

- defined strains or families of the host.

Current techniques of animal breeding, using direct marker-assisted genetic selection for measurable traits as illustrated above for bovine brucellosis, could be applied to

. the problem of increasing natural disease resistance. This would require that the gene, a linked marker, the gene product, or an in vitro correlate of resistance be identified in individual animals. Animals which possess the resistance trait would then be selected for breeding, and this procedure would be followed until a high proportion of animals carry the gene for the resistance trait. This could be accomplished by using a marker gene closely linked to the resistance gene, by identifying the gene product using monoclonal antibodies

or by directly identifying the gene using specific DNA probes. Gene products involved in controlling resistance to infectious diseases, e.g., specifically class I MHC gene products, can be identified on embryos (162) . This should enable the selection of only those embryos which have inherited the appropriate gene for transfer to surrogate mothers.

Recent developments in the area of molecular biology are certainly enhancing the utilisation of known genes for natural disease resistance. With the current molecular genetic technology, more bacterial resistance genes will be identified and cloned in mice and humans, and given the extensive domestic animal (especially cattle)-human and domestic animal-mouse homology, identification of domestic animal resistance homologues will be greatly facilitated. Furthermore, the recent progress in the isolation and characterisation of candidate genes and new microsatellite-based markers for family linkage studies of the entire genome are expediting the identification of new resistance genes. With the progress in development of genetic and physical maps of domestic animal genomes (27, 58 , 182, 183) which have informative markers throughout the genome, identity-by-descent and identity-by-state analyses of affected sib pairs become very powerful tools to identify major disease resistance loci (75) , which can then be confirmed by gene disruption (knockout) procedures (85) . These methodologies are making the candidate gene approach preferable to the positional cloning approach for identifying new resistance genes. In this way, the linked markers or the gene can be used to determine the relative risk of clinical disease in exposed individuals. The accuracy of predicting relative risk is affected by the distance between the linked marker and the resistance gene, in addition to the relative contribution of the resistance gene to the overall disease phenotype on the variable genetic and environmental backgrounds of outbred domestic animals.

The transfer of innate and adaptive disease resistance genes into embryos taken from animals which have superior production traits by microinjection of pronuclei, retroviral insertion and stem cell insertion is becoming increasingly possible ( 1 1 3 , 1 1 4 , 1 3 1 ) . The transgenic animal derived from that embryo would then have a probability of approximately 70% of carrying the desired gene in its germ cells, and therefore would have an approximately 5 0 % likelihood of passing the gene to its progeny (131) . Understanding of gene regulation will be necessary to make the system more efficient. The technology for transferring genes to embryos is being improved for all species of domestic animals, and controls for assuring the safety of such experiments are being carefully clarified and applied. The application of direct gene transfer technology for genetic resistance to bacterial diseases depends upon public support for the introduction of appropriate laws permitting use of transgenic animals in animal production (113) .

Rev. sci. tech. Off. int. Epiz., 17 (1) 211

Conclusions Without doubt, vaccination programmes, antibiotics and

other modalities have contributed significantly to the

reduction of losses from bacterial pathogens. However, with

the exception of a few diseases in specific geographical areas,

bacterial diseases have not been eradicated and remain a

major cause of animal arid economic loss. The aim of applying

genetic selection to the problem of controlling infectious

diseases in domestic animals is not so far into the future. Of

necessity, a significant part of this review has been devoted to

comparative studies in mouse or other models from which

much information has been gleaned for application to

bacterial diseases of domestic animals at genetic and

functional levels. Poultry breeders have successfully applied

selective breeding programmes to produce stock with

increased genetic capacity to resist specific diseases (59, 60 ,

169). Investigators in several branches of veterinary science

are bringing the realisation of this aim even closer by

providing basic information about the genetic constitution of

domestic animals and about specific genes controlling

mechanisms involved in natural and adaptive resistance to

bacterial diseases. Utilisation of such basic information to

enhance herd health would be a significant adjunct to current

and future bacterial disease control methods for diseases of

domestic animals such as brucellosis, tuberculosis and

salmonellosis within the next decade.

Acknowledgements This work was supported by the United States Department of

Agriculture (USDA)-Co-operative State Research Service

(CSRS) grants USDA-CSRS 9 0 - 3 7 2 4 1 - 5 5 8 3 and USDA-CSRS

9 3 - 3 7 2 0 4 - 9 4 9 1 , USDA-Office of International Co-operative

Development (OICD)/Egyptian National Agriculture Research

Project - 58-319R-3-005, Formula Animal Health Grant

USDA-Formula Animal Health - Texas (FATEX) 0-8050 from

the United States Department of Agriculture, and the Texas

Agriculture Experiment Station Project H-6194 and Project

H-8409.

Résistance génétique des animaux aux maladies bactériennes

L G . Adams & J .W. Templeton

Résumé

M a l g r é les m e s u r e s d e prophy lax ie t rad i t ionne l l es , le s e c t e u r d e l ' é levage

con t inue de subir de lourdes per tes du fa i t des m a l a d i e s i n f e c t i e u s e s . Or la

r é s i s t a n c e g é n é t i q u e aux m a l a d i e s pourra i t ê t re un au t re m o y e n de fa i re f a c e à ce

p r o b l è m e . Fa isant in terveni r à la fois des m é c a n i s m e s i m m u n o l o g i q u e s e t non

i m m u n o l o g i q u e s , el le c o r r e s p o n d à la c a p a c i t é innée d'un a n i m a l non

p r é c é d e m m e n t e x p o s é à un a g e n t p a t h o g è n e , de rés is ter à la m a l a d i e

n o r m a l e m e n t c a u s é e par c e t agent . M ê m e si l 'expression de la m a l a d i e es t

i n f l u e n c é e par les f a c t e u r s e n v i r o n n e m e n t a u x , la r é s i s t a n c e na ture l l e à la

m a l a d i e es t hér i tab le et se t r a n s m e t à la d e s c e n d a n c e . L'autre solut ion pour

a m é l i o r e r la s a n t é a n i m a l e cons is te ra i t donc à a m é l i o r e r la r é s i s t a n c e g é n é t i q u e

.g loba le aux n iveaux du t r o u p e a u e t de l ' ensemble du c h e p t e l g r â c e à des

p r o g r a m m e s de s é l e c t i o n .

L'objet de ce t te é t u d e es t d 'appor te r aux v é t é r i n a i r e s , aux s e r v i c e s of f ic ie ls , aux

r e s p o n s a b l e s des f i l iè res de product ion e t aux t e c h n i c i e n s d ' é l e v a g e les

c o n n a i s s a n c e s les plus r é c e n t e s sur les pr inc ipes e t les app l ica t ions de la

r é s i s t a n c e g é n é t i q u e c o m m e vo le t c o m p l é m e n t a i r e des m é t h o d e s t rad i t ionne l les

de prophy lax ie des m a l a d i e s a n i m a l e s d 'or ig ine b a c t é r i e n n e . Cer tes , la r é s i s t a n c e

g é n é t i q u e a u x m a l a d i e s b a c t é r i e n n e s d é p e n d s o u v e n t d e n o m b r e u x g è n e s

c o n t r ô l a n t d i f férents p r o c e s s u s de l ' in teract ion h ô t e - a g e n t p a t h o g è n e .

C e p e n d a n t , les m é c a n i s m e s g é n é t i q u e s de la r é s i s t a n c e na ture l le sont de m i e u x

212 Rev. sci. tech. Off. int. Epiz., 17 (1|

en mieux compr is g r â c e à l ' identi f icat ion et au t y p a g e de g è n e s en c a u s e , aux

m a r q u e u r s microsate l l i tes e t à la c a r t o g r a p h i e c o m p a r a t i v e d e s g è n e s , qui

p e r m e t t e n t plus d 'appl icat ions pra t iques .

Mots-clés An imaux - Bactér ie - Gènes - Génét ique - Immuni té - Immun i té na ture l le - Résistance

aux malad ies.

Resistencia genética a las enfermedades bacterianas de los animales

L.G. Adams & J . W . Templeton

Resumen Pese a las m e d i d a s t r a d i c i o n a l e s de control san i tar io , las p é r d i d a s a c h a c a b l e s a

e n f e r m e d a d e s in fecc iosas s iguen const i tuyendo un p e s a d o lastre para la

industr ia a g r o p e c u a r i a . Una so luc ión a l te rnat iva a e s t e p r o b l e m a podr ía hallarse

en la res is tenc ia g e n é t i c a a la e n f e r m e d a d l igada a m e c a n i s m o s tanto

inmunológ icos como no inmuno lóg icos , esto es , la c a p a c i d a d i n h e r e n t e a un

an imal de resistir a la p r e s e n c i a de p a t ó g e n o s en su o r g a n i s m o sin h a b e r tenido

con tac to previo con el los. A u n q u e los modos de a l i m e n t a c i ó n y cr ía influyen

sobre la var iab i l idad en la expres ión de la e n f e r m e d a d , ha podido o b s e r v a r s e que

la res is tenc ia natura l es hered i ta r ia y se t r a n s m i t e de un progeni tor a su

d e s c e n d e n c i a . Por el lo, una posible a l te rnat iva para m e j o r a r los s is temas de

gest ión zoosani ta r ia res ide en la ap l i cac ión de p r o g r a m a s de s e l e c c i ó n que

permi tan a u m e n t a r el nivel g e n e r a l de res is tenc ia a la e n f e r m e d a d , e n términos

tan to de r e b a ñ o s c o m o de p o b l a c i o n e s .

Esta r e s e ñ a intenta p roporc ionar a ve te r ina r ios , f u n c i o n a r i o s de órganos

r e g l a m e n t a r i o s , r e p r e s e n t a n t e s de la industr ia y t é c n i c o s de p r o d u c c i ó n animal

in fo rmac ión ac tua l i zada sobre los pr incipios y a p l i c a c i o n e s de la resistencia

g e n é t i c a , c o n c e b i d a como a c c e s o r i o a las m e d i d a s t r a d i c i o n a l e s de control de

las e n f e r m e d a d e s b a c t e r i a n a s del g a n a d o . A u n q u e en la r e g u l a c i ó n de la

res is tenc ia gené t ica a las in fecc iones b a c t e r i a n a s s u e l e n in terven i r numerosos

g e n e s , c o n contro l sobre dist intos p r o c e s o s de la i n t e r a c c i ó n e n t r e huésped y

p a t ó g e n o , la ident i f icac ión y c a r a c t e r i z a c i ó n de g e n e s c a n d i d a t o s , el uso de

m a r c a d o r e s microsaté l i te y la car togra f ía g e n é t i c a c o m p a r a d a e s t á n permitiendo

d e s e n t r a ñ a r p r o g r e s i v a m e n t e los m e c a n i s m o s g e n é t i c o s de la resistencia

na tu ra l , lo que a la postre ha de conduc i r al desarro l lo de m é t o d o s ap l icados más

e f i c a c e s .

Palabras clave An ima les - Bacter ias - Genes - Genética - Inmunidad - Inmunidad natura l - Resistencia a la en fe rmedad.

Rev. sci. tech. Off. int. Epiz., 17 (1) 213

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