chapter 12 - parvoviridae, pages 225-235

225Fenner’s Veterinary Virology. DOI:© Elsevier Inc. All rights reserved.

10.1016/B978-0-12-375158-4.00012-2

Parvoviridae

Chapter ContentsProperties of Parvoviruses 225

Classification 225Virion Properties 226Virus Replication 227

Members of the Genus Parvovirus 228Feline Panleukopenia Virus 228

Clinical Features and Epidemiology 229Pathogenesis and Pathology 229Diagnosis 230Immunity, Prevention, and Control 230

Mink Enteritis Virus 230Canine Parvovirus 2 230


Porcine Parvovirus 232


Rodent Parvoviruses 233Rabbit Parvoviruses 234Members of the Genus Erythrovirus 234Non-human Primate Parvoviruses 234Members of the Genus Amdovirus 234Aleutian Mink Disease Virus 234Members of the Genus Dependovirus 235Goose Parvovirus 235Duck Parvovirus 235Members of the Genus Bocavirus 235Bovine Parvovirus 235Canine Minute Virus (Canine Parvovirus 1) 235Other Parvoviruses 235

Parvoviruses infect many animal species and are the causa-tive agents of several important animal diseases (Table 12.1). There probably are many more parvoviruses that cause only mild or subclinical infections, and infections with such viruses are increasingly diagnosed using molecu-lar assays. Parvovirus-induced diseases such as that caused by feline panleukopenia virus have been recognized for more than 100 years, whereas others such as canine parvo-virus disease have emerged more recently.

Despite their complex taxonomic organization, the par-voviruses are all related and probably derive from a common ancestor. They share common biological properties, including their resistance to desiccation in the environment and their requirement for cells that are passing through mitotic S phase in order to replicate their DNA. The relative availability of mitotically active cells in specific tissues during differentiation in early life confers an age-dependent susceptibility to several parvovirus-induced diseases. Thus certain parvovirus infec-tions are most severe in fetuses (after transplacental infection) and neonates. This requirement for mitotically active cells also is reflected in the tropism of some parvoviruses for rap-idly dividing hemopoietic precursors and lymphocytes, and progenitor cells of the intestinal mucosal lining.

PrOPerties Of ParvOviruses

Classification

The family Parvoviridae comprises two subfamilies: the subfamily Parvovirinae, which contains viruses of verte-brates, and the subfamily Densovirinae, which contains viruses of insects that will not be discussed further. There are five genera in the subfamily Parvovirinae. The taxonomic organization of parvoviruses can be confusing, as a single animal species may be host to more than one species of par-vovirus, but the parvoviruses are taxonomically grouped into genera according to their molecular properties and not their species of origin. The genus Parvovirus includes: feline pan-leukopenia virus and the closely related canine parvovirus, mink enteritis virus, and raccoon parvovirus; parvoviruses of rodents and lagomorphs; porcine and chicken parvoviruses. The genus Erythrovirus includes human parvovirus B19 and related viruses of non-human primates and, tenta-tively, bovine parvovirus type 3 and chipmunk parvovirus. The genus Dependovirus includes the so-called adeno-asso-ciated viruses that are themselves replication defective and do not cause disease as they are unable to replicate except in the presence of a helper virus, usually an adenovirus. This

Chapter 12

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Part | ii Veterinary and Zoonotic Viruses226

genus also includes goose and duck parvoviruses and, pro-visionally, bovine parvovirus 2. Aleutian mink disease virus is the sole member of the genus Amdovirus. Bovine par-vovirus and canine minute virus are included in the genus Bocavirus. Thus individual parvoviruses from dogs, birds, rodents, cattle, and mink are classified in several genera.

virion Properties

Parvovirus virions are non-enveloped, 25 nm in diameter, and have icosahedral symmetry (Figure 12.1). The cap-sid displays a number of surface features that are associ-ated with its functioning, include a hollow cylinder at each fivefold axis of symmetry that is surrounded by a circular

fiGure 12.1 (Top) Space-filling models of the capsid structures of canine parvovirus (CPV) (left); adeno-associated virus - 2 (AAV-2) (center) and Galleria mellonella densovirus (GmDNV) (right). Each model is drawn to the same scale and is colored according to distance from the viral center. In each case, the view is down a twofold axis at the center of the virus, with threefold axes left and right of center, and fivefold axes above and below (Courtesy of M. Chapman). (Bottom left) Diagram representing a T 1 capsid structure. (Bottom right) Negative contrast electron micrograph of CPV particles. The bar represents 100 nm. [From Virus Taxonomy: Eighth Report of the International Committee on Taxonomy of Viruses (C. M. Fauquet, M. A. Mayo, J. Maniloff, U. Desselberger, L. A. Ball, eds.), p. 353. Copyright © Elsevier (2005), with permission.]

Table 12.1 Manifestations of Parvovirus Diseases in Animalsa

Virus Disease

Feline panleukopenia virus Generalized disease in kittens, with panleukopenia, enteritis; cerebellar hypoplasia

Canine parvovirus 1 (minute virus of canines) Minimal

Canine parvovirus 2 (subtypes 2a, 2b, 2c) Generalized disease in puppies; enteritis, myocarditis (rarely), lymphopenia

Porcine parvovirus Stillbirth, abortion, fetal death, mummification, infertility.

Mink enteritis virus Leukopenia, enteritis

Aleutian mink disease virus Chronic immune complex disease, encephalopathy. Interstitial pneumonia in neonates

Mouse parvoviruses, minute virus of mice, rat parvoviruses, H-1 virus of rats

Subclinical or persistent infection; congenital fetal malformations; hemorrhagic syndrome in rats

Goose parvovirus Hepatitis, myocarditis, myositis

Duck parvovirus Hepatitis, myocarditis, myositis

a Parvoviruses have also been detected in a variety of animal species, frequently in the absence of obvious clinical disease.

Chapter | 12 Parvoviridae 227

depression, prominent protrusions around the threefold axis of symmetry, and, in most viruses, a depression at each twofold axis of symmetry. The receptor binding site of feline and canine parvoviruses, which determines their host and tissue tropism, is located on the surface of the spike, which is also the site of binding of most antibodies directed against the capsid.

The parvovirus capsid is composed of a total of 60 protein molecules, approximately 90% being VP2 and approximately 10% being the overlapping but larger VP1 protein. VP1 and VP2 are formed by alternative splic-ing of the same messenger RNA (mRNA), and the entire sequence of VP2 is encoded within the VP1 gene. In some viruses a third structural protein, VP3, is formed (only in DNA-containing capsids) by cleavage of a peptide from the amino terminus of VP2. The parvovirus capsid proteins all contain a central eight-stranded, antiparallel -barrel motif, and the strands of the -barrel are linked by four extensive loops; these loops form most of the outer surface of the virus particle and are responsible for their receptor binding, their antigenic properties, and their environmental stability. Indeed, parvoviruses are extremely stable to envi-ronmental conditions, including extremes of heat and pH, and disinfection of contaminated premises using commer-cially available disinfectants is a major challenge.

The genome consists of a single molecule of linear single-stranded DNA, approximately 4.5–5.5 kb in size (Figure 12.2). Some parvoviruses encapsidate only the negative-sense DNA strand (e.g., canine parvovirus, minute virus of mice). Others encapsidate different proportions of both negative and positive strands, so that individual viri-ons of these viruses may contain single-stranded DNA of either polarity. The genome contains two major open read-ing frames: an open reading frame in the 3 half of the genome that encodes the non-structural proteins that are required for DNA transcription and replication, and another open reading frame towards the 5 half encodes the struc-tural proteins (variously designated as CAP, VP, or S) of the capsid. Both reading frames are present on the same DNA strand of members of the Parvovirinae. The genome has terminal palindromic sequences, enabling each end to form hairpin or other complex base-paired structures required for virus replication.

virus replication

Receptor binding at the plasma membrane initiates viral infection of susceptible cells, and virions are then taken up into the cell by endocytosis. Transferrin receptor is the receptor for canine parvovirus and feline panleukopenia virus, and it also directs the virus into the clathrin-mediated uptake pathway. Utilization of the transferrin receptor probably also facilitates replication of these viruses, as it is markedly upregulated on proliferating cells; parvovirus

replication is intimately associated with cellular replica-tion, because virus replication occurs only in cells that pass through mitotic S phase. Many parvoviruses also bind sialic acid residues, consistent with their ability to hemag-glutinate erythrocytes of various species; sialic acid is an essential component of the cell receptor binding process utilized by some rodent parvoviruses. Other determinants of parvovirus tropism are not well understood. The known receptors for most animal parvoviruses do not appear to be sufficiently tissue specific to explain the tropism of the viruses, although it is likely that the binding affinity of spe-cific virions to their receptors might influence the patho-genesis of infections with these viruses.

Once inside cells, virions traffic through the endosomal pathways within the cytoplasm, including the early and late endosomes and, in some cases, the recycling endosomes. Exactly how the particles exit from the endosomal sys-tem is unclear. However, the viral VP1 protein contains a phospholipase A2 enzyme activity in its N-terminal unique region that may be involved in modifying the endosomal membrane and facilitating capsid release. This unique region of VP1 is buried inside the newly made particle, and so exposure within the endosome requires a structural trans-formation of the capsid to release that activity. The parti-cles that enter the cytoplasm are trafficked to the nuclear

3' 5'NS1 and NS2 VP1 and VP2

NS1

An 1

An 2 Readingframes

An 3

An 3

NS2

VP1

VP2

1 2 3 4 5 kb

fiGure 12.2 Genomic DNA of canine parvovirus and its transcription strategy. The genome has terminal palindromic sequences enabling each end to form hairpin structures; these structures serve as the origin of DNA replication and also facilitate encapsidation (packaging) of viral DNA within nascent virions. The 5 ends of RNA transcripts are capped (black circles) and the 3 ends are polyadenylated (An). VP1 and VP2, which are produced in very large amounts, are encoded in the same mRNA. They are formed by alternative initiation codons (arrowheads)—the entire sequence of VP2 is encoded within the VP1 gene. The non-structural protein NS1, also produced in very large amounts, serves a number of functions: (1) it binds to DNA and is required for viral DNA replication; (2) it serves as a helicase; (3) it serves as an endonuclease; (4) it interferes with cellular DNA replication, causing the arrest of the cell division cycle in the S phase. NS2, which is encoded in two open reading frames and is formed by splicing, also regulates viral gene expression. Among the dif-ferent parvoviruses, there is a remarkable diversity in transcription details (frameshifting, splicing, etc.) and products that cannot be shown using any one virus as a model. (Courtesy of C. R. Parrish.)


pore, and the more-or-less intact particle enters the nucleus, where replication occurs.

Viral DNA replication and capsid assembly take place in the nucleus and require host-cell functions of S phase of the cell division cycle. The requirement for cycling cells for virus replication is due to a viral requirement for host DNA replication machinery for replication of the viral DNA, as the virus does not encode or package such an enzyme. Instead, cellular DNA polymerases replicate the viral DNA to form a double-stranded DNA intermediate, which is then used as a template for transcription of viral mRNAs. Alternative splicing gives rise to several mRNA species that are translated into four major proteins, and additional small and less well-characterized proteins. The most abun-dant mRNA, which is encoded in the 5 half of the genome, directs the synthesis of the structural proteins. The non-structural protein (NS1) that is encoded in the 3 portion of the genome serves a number of functions: (1) it becomes attached to the 5 end of the viral DNA during replication; (2) it serves as a helicase during replication and DNA pack-aging; (3) it serves as a site-specific nickase; (4) it mediates arrest of the cell in the G1 phase of the cell cycle.

The mechanism of replication of the genome is described as a rolling-hairpin replication; it is complex, and some details still are not completely understood. The 3-terminal hairpin on the negative-sense DNA genome serves as a self-primer for the initiation of synthesis of a double-stranded DNA replicative intermediate. The detec-tion of a dimeric form of the replicative intermediate—that is, a head-to-head concatemer of two covalently linked double-stranded forms—has led to a model in which the growing DNA strand replicates back on itself to produce a tetrameric form from which two complete positive strands and two complete negative strands are generated by a com-plicated series of reopening of closed circular forms, reini-tiation of replication at transiently formed hairpins, and single-strand endonuclease cleavages (Table 12.2).

A major determinant of the pathogenesis of parvoviruses is their requirement for cycling cells for virus replication. Parvovirus infections of the fetus (pig or cat) or newborn (dog or cat) at critical stages of organogenesis when there is considerable cell division may result in widespread infection and tissue destruction that cause developmental defects. Thus feline panleukopenia virus infection selec-tively destroys the developing cerebellum in feline fetuses or kittens infected in the perinatal period, whereas the devel-oping heart (myocardium) may be affected in parvovirus-infected pups and goslings. Typically, replication of these same viruses is restricted in older animals with differenti-ated organs; however, continuously dividing cells such as hemopoietic precursors, lymphocytes, and progenitor cells of the intestinal mucosa are susceptible in animals of all ages. Selective parvovirus infection and destruction of these rapidly dividing cell types leads to tissue injury analogous to that induced by radiation—hence the designation of some

parvovirus infections as being “radio-mimetic.” The tropism of Aleutian mink disease virus also changes with age; in neonates lacking maternal immunity there is infection and destruction of type II pneumocytes, leading to acute intersti-tial pneumonia, whereas older animals (or neonates in which antibodies are present) develop chronic infections with less infection of type II pneumocytes.

While many parvoviruses cause acute infections that last only a few days, others persist for long periods in the face of apparently robust host immune responses. The pre-cise mechanisms of parvovirus persistence are not well understood, as most of the viruses appear to be susceptible to antibody-mediated neutralization. Aleutian mink disease virus persistently replicates to high levels in many mink, perhaps because of capsid-associated phospholipids that reduce antibody binding or neutralization. Disease devel-ops in persistently infected mink as a result of the high lev-els of circulating antigen–antibody complexes that deposit in tissues and initiate a type III hypersensitivity reaction that results in tissue injury and destruction.

MeMbers of The genus ParvovirusVirions of some members of the genus Parvovirus contain exclusively negative-sense DNA, whereas those of other viruses in the genus also include variable proportions of positive-sense DNA.

feline PanleukOPenia virus

All members of the family Felidae are probably susceptible to infection with feline panleukopenia virus, which occurs worldwide. Some members of the families Viverridae, Procyonidae and Mustelidae also are susceptible, includ-ing the raccoon, mink, and coatimundi. The associated

Table 12.2 Properties of Parvoviruses

Five genera: Parvovirus, Erthrovirus, Dependovirus, Amdovirus, Bocavirus

Virions are icosahedral, 25 nm in diameter, and composed of 60 protein subunits

The genome is a single molecule of single-stranded DNA, approximately 4–6 kb in size; some viruses encapsidate exclusively negative-sense DNA, whereas others encapsidate both positive- and negative-sense DNA

Replication occurs in the nucleus of dividing cells; infection leads to large intranuclear inclusion bodies

Viruses are very stable, resisting 60°C for 60 minutes and pH 3 to pH 9

Most viruses hemagglutinate red blood cells


disease, feline panleukopenia, can be very severe and cause substantial mortality in susceptible animals.

Clinical features and epidemiology

Feline panleukopenia virus is highly contagious. The virus may be acquired by direct contact with infected cats or via fomites (bedding, food dishes); fleas and humans may act as mechanical vectors. Virus is shed in the feces, vomitus, urine, and saliva, and is very stable in the environment.

Feline panleukopenia is most common in kittens infected around the time of weaning when maternal anti-body wanes, but cats of all ages are susceptible. The incu-bation period is approximately 5 days (range 2–10 days). At the onset of clinical signs, there is a profound leukopenia and the severity of the disease and the mortality rate par-allel the severity of the leukopenia; the prognosis is grave if the white blood cell count falls below 1000 cells per ml of blood. Clinical signs include fever (greater than 40°C), which can persist for 24 hours or more. Death occurs dur-ing this phase in the peracute form of the disease. In cats that survive, temperature returns to normal and increases again on the 3rd or 4th day of illness, at which time there is lassitude, inappetence, a rough coat, and often repeated vomiting. A profuse, persistent, and frequently bloody diarrhea may develop at approximately the 3rd or 4th day of illness. Dehydration from severe malabsorption diarrhea frequently is a major contributing factor to fatal infections.

Perinatal or in-utero infection of kittens can cause abnormal development of the cerebellum (cerebellar hypo-plasia/atrophy syndrome). Affected kittens are noticeably ataxic when they become ambulatory around 3 weeks of age (so-called spastic or wobbly cat syndrome); they have a wide-based stance and move with exaggerated steps, tending to overshoot the mark and to pause and oscillate about an intended goal.

Pathogenesis and Pathology

After virus entry in the oropharynx, initial virus replica-tion occurs in pharyngeal lymphoid tissue. From here the virus is distributed in a free and cell-associated viremia to other organs and tissues via the blood stream. Cells that have appropriate receptors and are in the S phase of the cell cycle are infected and killed or prevented from enter-ing mitosis; there also may be “indirect” effects on unin-fected cells through receptor binding, or resulting from the regulatory and cytotoxic effects of virus-induced cytokines such as tumor necrosis factor. The characteristic profound leukopenia involves all white blood cell elements, includ-ing lymphocytes, neutrophils, monocytes, and plate-lets. These cells are destroyed—both those present in the circulation and those in lymphoid organs, including the thymus, bone marrow, lymph nodes, spleen, and Peyer’s patches. Resting peripheral leukocytes may be stimulated

to proliferate, thereby becoming permissive for virus repli-cation. The presence of virus bound to the surface of cells may also render them targets for cytotoxic lysis.

Rapidly dividing epithelial cells lining the intestinal glands (crypts) are also highly susceptible to infection. These cells are progenitors of the entire intestinal mucosa, so their destruction results in mucosal collapse with contrac-tion and fusion of the villi of the small intestine, and attenu-ation of the lining epithelium. The functional consequence is maldigestion and malabsorption, with resultant diarrhea. At necropsy, there may be segmental congestion of the mucosa and/or petechial hemorrhages on the bowel serosa, although gross lesions are often very subtle, even in severely affected cats. Histologically, in addition to marked contraction of the intestinal villi and attenuation of the lining epithelium, the crypts are dilated and distended with mucus and cell debris. Attenuation of the enterocyte lining of the intestinal mucosa in acute infections occurs as individual cells spread out, pre-venting exposure of the basement membrane to intestinal contents, but ulceration and breach of this important barrier is frequent. Rarely, intranuclear inclusions may be present in crypt enterocytes. Proliferation and expansion of crypt entero-cytes are prominent in the recovery phase of infection as those cells attempt to repopulate the damaged mucosa. Maldigestion and malabsorption may occur during the repair phase because of immaturity of the intestinal mucosal lining. Lymph nodes may be enlarged and edematous; histologically, there is evi-dence of widespread destruction of lymphocytes.

In fetuses infected during the last 2 weeks of pregnancy and the first 2 weeks of life, dramatic lesions are present in the external granular layer of the cerebellum—this is the basis for the characteristic cerebellar hypoplasia/atrophy that occurs in cats infected at this stage of development (Figure 12.3). During this period, cells of the external granular layer of the cerebellum normally undergo rapid division and migrate to form the internal granular and Purkinje cell layers; this proliferation and migration is arrested, and affected kittens remain permanently ataxic.

fiGure 12.3 Cerebellar hypoplasia/atrophy (arrow) induced by feline panleukopenia virus in a young kitten. (Courtesy of J. Peauroi and University of California, Davis.)


Diagnosis

Clinical signs, hematological data, and postmortem find-ings are characteristic and sufficient for presumptive diag-nosis of feline panleukopenia. The usual confirmatory tests include either antigen-capture enzyme immunoassay or immunofluorescence for the detection of antigen in tissues, or polymerase chain reaction (PCR) assay for the detection of viral DNA in feces or tissues. Virus isolation or hemag-glutination assays also can be used. Serologic diagnosis is by hemagglutination-inhibition assays, enzyme immu-noassay, or indirect immunofluorescence.

immunity, Prevention, and Control

Following natural infection in previously healthy cats there is a rapid immune response. Neutralizing antibody can be detected within 3–5 days of infection and may increase to very high levels. The presence of high-titer antibody is cor-related with protection against reinfection, and immunity after natural infection or vaccination with modified live vac-cines is probably life-long. The titer of passively acquired antibody in kittens is related to the maternal antibody titer, and falls at a constant rate. Thus kittens are protected for varying periods related to their initial titer, and varying from a few weeks to as long as 16 weeks. Vaccination is widely practiced, with both inactivated and live-attenuated virus vaccines available. Although each vaccine type has its own inherent perceived advantages and disadvantages, attenuated live vaccines are safe and potentially better than inactivated vaccines for the control of disease.

The stability of the virus and the very high rates of virus excretion result in high levels of environmental con-tamination, hence it may be difficult to disinfect contami-nated premises. The virus may be acquired from premises after the introduction of susceptible cats weeks or even months after previously affected cats have been removed. The virus may also be carried a considerable distance on fomites. In large catteries, strict hygiene and quarantine of incoming cats are essential if the virus is to be excluded; cats should be held in isolation for about 2 weeks before entry, sick cats should be removed and isolated, and vac-cines should be used rigorously. For disinfection, 1% sodium hypochlorite applied to clean surfaces will destroy residual contaminating virus, but it is less effective in the presence of organic matter. Organic phenolic or iodine- or glutaraldehyde-based disinfectants, together with thorough cleaning with detergent-based cleansers, can also be used in these circumstances.

Mink enteritis virus

Mink enteritis is caused by a parvovirus that is related very closely to feline panleukopenia virus. In mink, the virus produces a syndrome similar to that caused by feline

panleukopenia virus in cats, except that cerebellar hypopla-sia/atrophy has not been recognized. The disease in mink appears to have resulted from the introduction of feline panleukopenia virus into commercial mink farms in Ontario, Canada, during the 1940s.

Canine ParvOvirus 2

Canine parvovirus disease, caused by canine parvovirus 2, was first described as a new disease in 1978. After its ini-tial recognition, the virus spread rapidly around the world, causing a “virgin-soil” panzootic that was marked by high incidence rates and high mortality rates. Sequence analyses and retrospective serologic studies indicate that the imme-diate ancestor of the virus began infecting dogs in Europe during the early or mid-1970s; this conclusion is based on the finding of virus-specific antibodies in sera from dogs in Greece, the Netherlands, and Belgium in 1974, 1976, and 1977, respectively. During 1978, antibodies were first found in dogs in Japan, Australia, New Zealand, and the United States, confirming that the virus spread around the world in less than 6 months. The stability of the virus, its efficient fecal–oral transmission, and the near-universal susceptibility of the dog population of the world probably explain the occurrence of this remarkable panzootic.

All members of the family Canidae (dogs, wolves, foxes, coyotes) are susceptible to natural infection with canine parvovirus 2. Infection by some virus strains has been described in members of the families Mustelidae and Felidae—specifically cats, mink, and ferrets. The virus con-tinues to be a very important cause of infectious diarrhea in both wild and domestic canids.

Canine parvovirus 2 is distinct genetically from a previ-ously described parvovirus of dogs, minute virus of canines, which is now called canine parvovirus 1. Since its emergence in the 1970s, continuing genetic variation has resulted in the appearance of novel strains of canine parvovirus 2, with three major variants (2a, 2b, and 2c) having now been recognized. Interestingly, some of the more recently emergent variant 2a and 2b viruses are more infectious to cats than the original strains of canine parvovirus 2 that first emerged in the 1970s.


The epidemiological features of canine parvovirus 2 infec-tions are similar to those of feline panleukopenia. The virus is highly contagious and very stable in the environ-ment, so most infections result from the exposure of sus-ceptible dogs to virus-contaminated feces. Severe disease is most common in rapidly growing pups between 6 weeks and 6 months of age; however, many dogs that are naturally infected with canine parvovirus 2 exhibit only mild or sub-clinical disease.


Canine parvovirus 2 is the cause of an enteritis syndrome analogous to feline panleukopenia, although leukopenia is often less severe in dogs. Further, intestinal hemorrhage with severe bloody diarrhea is more characteristic of canine parvo-virus disease than of feline panleukopenia. The incidence of the enteritis syndrome has fallen since the virus first emerged, thanks to widespread vaccination, but canine parvovirus 2 is still an important cause of infectious diarrhea in young dogs. Vomiting is often the initial sign and can be severe and protracted; there is accompanying anorexia, lethargy, and diarrhea that quickly can lead to severe dehydration. The feces are often streaked with blood or are frankly hemorrhagic and remain fluid until recovery or death. Death is uncommon except in young pups. Some genetic strains of canine parvovi-rus may be more virulent than others, and it appears that some dog breeds are more susceptible to severe disease than others.

A myocarditis syndrome that results from infection in the first week of life is usually manifest as acute heart failure and sudden death in pups, often without preceding clinical signs. Pups that survive acute myocardial injury may subsequently develop cardiomyopathy at 4–8 weeks of age. This syndrome was relatively common when the virus first emerged, but is now rare as a result of the widespread immunity in breeding bitches that protects most puppies during the susceptible period.


The pathogenesis of canine parvovirus infection in the dog is similar to that of feline panleukopenia virus infection in the cat, but the absence of cerebellar hypoplasia/atrophy and the occurrence of myocarditis in pups distinguish the

diseases. Parvovirus infection of the myocardium can occur because of the rapid proliferation of myocytes that occurs in the first week after birth. Infection leads to myocardial necrosis and inflammation in affected puppies, which in turn results in pulmonary edema and/or hepatic congestion from acute heart failure. Eccentric hypertrophy (dilated cardiomyopathy) occurs in pups that survive for some time, with associated lymphocytic myocarditis and myocardial fibrosis.

Parvovirus infection of dogs results in systemic infec-tion following oropharyngeal entry of the virus (analogous to feline panleukopenia virus infection). Intestinal lesions in affected dogs result from infection and destruction of enterocytes populating the intestinal crypts, with subse-quent mucosal collapse, maldigestion and malabsorption diarrhea (Figure 12.4). Mucosal and serosal hemorrhage can be severe, perhaps reflecting terminal disseminated intravascular coagulation in affected dogs. Hemorrhages may occur in other organs, and hemorrhage in the central nervous system can cause neurological signs, for exam-ple. Lymphoid tissues also are affected, with widespread destruction of lymphocytes, and the resultant immunosup-pression can predispose to secondary infections.

Diagnosis

The sudden onset of foul-smelling, bloody diarrhea in young dogs is suggestive, but certainly not diagnostic, of canine parvovirus infection. Fecal enzyme immunoassays now facilitate rapid detection of the virus, although virus shedding is transient (between days 3 and 7 after infec-tion). Laboratory diagnosis of canine parvovirus infection

(B)(A)

(D)(C)

fiGure 12.4 Canine-parvovirus-induced intestinal lesions. (A) Serosal hemorrhage. (B) Mucosal hemorrhage. (C) Crypt necro-sis. (D) Immunohistochemical staining of parvovirus antigens in crypt epithelium. (Courtesy of P. Pesavento, University of California, Davis.)


also can be made using hemagglutination of pig, cat, or rhe-sus monkey red blood cells (pH 6.5, 4°C) by virus present in fecal extracts, and the specificity of this hemagglutina-tion is determined by titrating the sample in parallel in the presence of normal and immune dog serum. Fecal samples from dogs with acute enteritis may contain many thousands of hemagglutinating units of virus, reflecting very high titers of virus. Electron microscopy, virus isolation, and amplifi-cation of viral DNA using PCR assay on fecal samples are also used for laboratory confirmation of clinical diagnosis. Retrospective diagnosis can be done with serology, typically using the immunoglobulin IgM and/or IgG-capture enzyme-linked immunosorbent assay on paired sera.


Following natural infection there is a rapid immune response. Neutralizing antibodies can be detected within 3–5 days of infection and increase rapidly to very high titers. Immunity after natural infection appears to be life-long. Most maternal antibody is transferred with colostrum; and the titer of the antibody in pups parallels the maternal antibody titer and is therefore quite variable, providing protection for only a few weeks or for as long as 16 weeks. Cytotoxic T cells are also generated after both infection and vaccination.

Live-attenuated virus vaccines are available and widely used; however, vaccine failure in weanling pups may occur as a result of maternal antibody interference during immu-nization, and is the most common cause of failure. Pups receive about 10% of their maternal antibody via transpla-cental transfer and 90% through colostrum (the half-life of canine IgG is 7–8 days). It has been determined that an antibody titer of 80 or greater is protective (as measured by the hemagglutination-inhibition assay); thus pups born to bitches with low antibody titers may become susceptible to wild-type virus as early as 4–6 weeks after birth, whereas those born to bitches with high titers may be immune to infection for 12–18 weeks. Of course, pups born to seroneg-ative bitches are susceptible at birth. The level of maternal antibody that is able to protect pups against infection by the wild-type virus is different than that which interferes with an attenuated vaccine virus. In addition to the difference in their intrinsic properties, the wild-type virus is introduced via the oronasal rather than the parenteral route. In effect, as maternally acquired immunity wanes, there is an approx-imately 1-week period when antibody titers have declined to levels where pups are susceptible to wild virus but are still refractory to immunization. The time of this gap may be estimated for each pup by serologic testing, but this is expensive and, in most instances, impractical. The usual approach is to administer pups a series of vaccinations at 2- to 3-week intervals, starting at 6–8 weeks of age and continuing through 16–20 weeks of age. Another approach has been to use very high-titer vaccine, thereby partially

overcoming immune interference. Yet another approach has been to use vaccine containing a lower passage, slightly more virulent virus, favoring more virus replication in the recipient and a better chance to overcome interference.

Problems in parvovirus disease prevention and control are encountered commonly in breeding colonies or facili-ties that house large numbers of puppies, such as shelters, breeding facilities, or kennels, and in veterinary clinics, where high viral loads can occur. Along with any vaccina-tion strategy, in contaminated environments it may help to isolate pups to minimize their chances of becoming infected during their most vulnerable period. It is especially impor-tant in kennels to isolate pups from other dogs, beginning around 6 weeks of age and continuing until their vaccina-tion series is complete. In household settings, if true isola-tion is not possible, pups should at least be kept from areas where puppies or infected dogs congregate.

POrCine ParvOvirus

Porcine parvovirus disease is an infectious cause of repro-ductive failure in swine throughout the world. When the virus is introduced into a fully susceptible breeding herd, it can have devastating effects. Some manifestations of the disease are described by the acronym, SMEDI (stillbirth, mummification, embryonic death, infertility). Infection of older swine causes only a mild or subclinical disease, but the virus has also been associated more rarely with respira-tory disease and vesicular disease, and systemic disease of neonates. Although there are genetic differences between some porcine parvovirus strains, only a single serotype is recognized.


Porcine parvovirus occurs worldwide and is enzootic in many herds, although the occurrence of disease has been dramatically diminished by vaccination. Because the virus is so stable, premises may remain infected for many months, even where hygiene appears satisfactory. Losses are most extreme if the virus is introduced into a seronega-tive herd at a time when many sows are pregnant. There is a possibility that some pigs infected in utero may survive as long-term immunotolerant carriers, but this is unproven. In most herds, a large proportion of gilts are infected naturally before they conceive, and hence are immune. Passively acquired maternal antibody can persist for up to 6 months or more, which interferes with active immunization follow-ing either natural infection or vaccination. Consequently, some gilts may conceive and then, when their resid-ual maternal antibody levels decline to non-protective levels, their pregnancy is at very high risk. Boars play a significant role in the dissemination of virus, in that they may shed virus in semen for protracted periods.


The major impact of porcine parvovirus results from infection of pregnant gilts or sows, and the stage of ges-tation at which infection occurs determines the particular clinical signs seen, and runs the full gamut of the SMEDI syndrome. The first sign of infection in a herd is frequently an increase in the number of gilts or sows returning to estrus 3–8 weeks after breeding. Some sows may remain “endocrinologically pregnant,” not returning to estrus until after the expected time of farrowing. These clinical fea-tures are caused by fetal infection and resorption. Infection occurring later in gestation is evident at farrowing by smaller than normal litters and by mummified fetuses, due to only some of the fetuses becoming infected and the vari-able course of the disease in those fetuses that do become infected (Figure 12.5). In addition, some piglets at birth may be smaller than normal, or so weak that they do not survive. In young pigs, infection has been associated with a vesicular disease of the feet and mouth.


It has been shown experimentally that it takes about 15 days after maternal infection for the virus to reach the fetus. When infection occurs less than 30 days after conception, the fetus dies and is resorbed; when infection occurs between 30 and 70 days after conception, the fetus often fails to develop an immune response and is usually affected severely and dies. Fetuses infected 70 or more days after conception, although frequently developing lesions, are affected less severely and mount an immune response (immunocompetence of swine fetuses starts at 55–70 days). The virus replicates in lymph nodes, tonsils, thymus, spleen, lungs, salivary glands, and other organs. It replicates well in blood lymphocytes, and both infection and the immune response stimulate cell pro-liferation, thereby increasing the viral load. Monocytes and macrophages also can become lytically infected. More so

than with the other parvoviruses, swine parvovirus causes persistent infection, with chronic shedding.

Diagnosis

Infected fetuses may contain very large amounts of virus. Frozen-section immunofluorescence of fetal tissues using standardized reagents is rapid and reliable and the preferred diagnostic test. Hemagglutination of guinea pig red blood cells by virus contained in extracts of fetal tissues may also be used. PCR assay is very sensitive, but the interpretation of results is important, as the assay may detect viral DNA even when the virus is not the primary cause of the dis-ease. Serologic tests are of limited value, because the virus is so widespread in swine, and vaccination may interfere. Diagnosis is difficult if infection occurs in the first few weeks of gestation; commonly, fetuses are resorbed completely and there may be no suspicion of the presence of the virus, and hence no specimens collected for laboratory diagnosis.


Vaccination is practiced widely as the only means of assur-ing that all gilts are protected. Inactivated and attenuated virus vaccines are used. There is often only a brief win-dow of opportunity to immunize gilts that are bred before 7 months of age. The duration of immunity is uncertain, but there seems to be good immunological memory, and infec-tion in vaccinated pigs rarely leads to fetal disease.

rODent ParvOviruses

More than 30 distinct parvoviruses in at least 13 sero-groups have been isolated from laboratory rodents, thus they represent a broad genetic spectrum. Several of these viruses commonly cause enzootic infections in rodent colo-nies: parvoviruses of mice, including minute virus of mice, mouse parvovirus types 1, 2, and 3; parvoviruses of rats, including Kilham’s rat virus, Toolan’s H-1 virus, rat minute virus type 1 and rat parvovirus type 1; a hamster parvo-virus, which is genetically identical to mouse parvovirus 3 and therefore represents cross-species transmission. There is also a high prevalence of mouse and rat parvoviruses in wild mice and rats, respectively. The major importance of these viruses is their confounding effect on research, espe-cially immunology and cancer research. They may also contaminate cell lines and tumor virus stocks, sometimes causing little cytopathology, which can allow them to be introduced into clean colonies.

Rodent parvoviruses most commonly cause subclini-cal infection, but they may rarely cause fetal and neonatal abnormalities, with granuloprival cerebellar hypoplasia, as in feline panleukopenia. Rodent parvoviruses destroy dividing cells, but with a more limited spectrum compared with parvoviruses of other species. Most importantly, none

fiGure 12.5 Porcine parvovirus infection. Infected fetuses in vari-ous stages of mummification, consistent with stillbirth, mummification, embryonic death and infertility (SMEDI) syndrome.


of the rodent parvoviruses infects intestinal epithelium, but rather they tend to have primary tropism for hemopoietic and lymphoid tissues. The overwhelming majority of parvovirus infections in rodents are clinically silent, but often with significant effects upon immune response. In rats, clinical disease is most often associated with Kilham’s rat virus, resulting in cerebellar injury, hemorrhagic encepha-lopathy and hepatitis in young rats, and outbreaks of peri-testicular and intra-abdominal hemorrhage in older rats. The hemorrhagic lesions are probably a result of tropism of the virus for vascular endothelium, as well as tropism for megakaryocytes, resulting in thrombocytopenia. Periodontal and craniofacial deformities have been observed in ham-sters naturally infected with hamster parvovirus (mouse par-vovirus 3), and can be experimentally induced with several other rodent parvoviruses.

One consequence of rodent parvovirus infections, par-ticularly mouse parvovirus, can be persistent virus carriage, even in the presence of high titers of neutralizing antibody. This is important, because some experimental manipula-tions, especially those that are immunosuppressive, may cause virus reactivation and recrudescent shedding. In turn, infection can be immunosuppressive (e.g., abrogating cyto-toxic T lymphocyte responses and helper T cell dependent B cell responses), again affecting experiments in which infected animals are used unknowingly.

Diagnosis is primarily based on serology (hemagglutina-tion-inhibition, indirect immunofluorescence, neutralization, or enzyme immunoassay) and virus isolation in rodent cell cultures. Reference reagents are used to identify particular virus strains, or viral DNA can be identified by PCR and the specific virus type determined by DNA sequencing. Both serologic and nucleic acid detection methods may be chal-lenging with some mouse strains, such as C57BL/6 mice, which may be infected with undetectable levels of antibody or viral DNA. Thus direct-contact sentinel animals of a more susceptible genotype are needed to detect infection within such colonies.

In laboratory colonies, these viruses are transmitted horizontally by contact and fomites. Young animals born from infected dams are protected by maternal antibody for the first few weeks of life, but then are infected via the oronasal route. As with other parvoviruses, rodent viruses are extremely stable and resistant to desiccation, and may be carried between rodent colonies by fomites; the strict-ness of facility quarantine must be rigorous. When virus is detected, elimination is effected by depopulation, meticu-lous disinfection of the premises, and introduction of new founding stock that is screened and free of virus and/or antibody. Unlike the situation in rebuilding a colony after eliminating some other rodent viruses, colonies that have had parvovirus infections cannot always be repopulated by cesarean section and use of foster mothers. Under such cir-cumstances, embryo transfer may be effective.

rabbit ParvOviruses

Serologic evidence indicates that lapine parvovirus is very common among domestic rabbits, but is clinically silent. Experimental infection of young kits has been shown to result in disseminated infection, mild enteritis, clinical signs of depression, and anorexia.

MeMbers of The genus ErythrovirusPopulations of mature virions of viruses within the genus Erythrovirus contain equivalent proportions of positive- and negative-sense DNA. The genus includes parvoviruses of humans, non-human primates, and bovine parvovirus 3.

nOn-huMan PriMate ParvOviruses

Several parvoviruses have been identified in macaques, including simian parvovirus in cynomolgus monkeys (Macaca fascicularis), rhesus parvovirus in rhesus macaques (M. mulatta) and cynomolgus parvovirus in cynomolgus monkeys. Considering the large number of species and sub-species of macaques, it is likely that there are many other parvoviruses among non-human primates. Of the three characterized to date, they are genetically related to but dis-tinct from each other, and are also related to B19 virus of humans. These viruses may be associated with clinical ane-mia and fetal abnormalities.

MeMbers of The genus amdovirusVirions of viruses in this genus include genomes exclu-sively of negative-sense DNA.

aleutian Mink Disease virus

The Aleutian Mink Disease Virus naturally infects mink, skunks, and ferrets, generally causing a mild or subclini-cal disease. When clinical disease occurs in mink, it is characterized by chronic antigenic stimulation leading to expansion of plasma cells in multiple tissues (so-called plasmacytosis), hypergammaglobulinemia, splenomegaly, lymphadenopathy, arteritis, glomerulonephritis, hepatitis, anemia, and death. Lesions result from chronic infection in which there is a sustained production of virus and a failure to eliminate virus–antibody (immune) complexes. Despite extremely high levels of virus-specific antibody, the virus is not neutralized, and infectious virus can be recovered from circulating immune complexes. Immune stimulation and immune-complex-mediated disease follow. The disease occurs primarily in mink that are homozygous for the reces-sive gene for a commercially desirable pale (“Aleutian”)


coat color. This coat color gene is linked to a gene associ-ated with a lysosomal abnormality of the Chediak–Higashi type that inhibits destruction of internalized immune com-plexes. The level of the hypergammaglobulinemia is cycli-cal, with death typically occurring during a peak response between 2 and 5 months after infection. Immunization of mink carrying the Aleutian gene with inactivated virus vaccine increases the severity of the disease. Conversely, immunosuppression diminishes the severity of the disease. As the virus appears to be only poorly transmissible, and mink are seasonal breeders, Aleutian disease can be con-trolled in a farmed mink population by serological testing and elimination of seropositive animals.

MeMbers of The genus dEPEndovirusPopulations of mature virions contain equimolar amounts of positive- and negative-sense DNA. The genus includes adeno-associated viruses from several animal species, as well as certain avian parvoviruses and bovine parvovirus 2.

GOOse ParvOvirus

Goose parvovirus causes a lethal disease in goslings 8–30 days of age that is characterized by focal or diffuse hepatitis and widespread acute necrosis and degeneration of striated, smooth, and cardiac muscle. Inclusion bodies occur in the liver, spleen, myocardium, thymus, thyroid, and intestines. Control is achieved by the vaccination of laying geese with attenuated virus vaccine; maternal antibody per-sists in goslings for at least 4 weeks, the period of maximum vulnerability.

DuCk ParvOvirus

An apparently new disease of Muscovy ducklings was described in France in 1989. Although clearly a distinct virus type, it is most closely related to adeno-associated viruses and so is classified in the genus Dependovirus. Mortality has been high, and clinical and postmortem find-ings have resembled those found in geese infected with goose parvovirus. Ducks that survive are stunted and feath-ering is delayed. Effective vaccines are available, including one that consists of recombinant VP2 and VP3 viral pro-teins expressed in a baculovirus system.

MeMbers of The genus BocavirusIn contrast to other parvoviruses, the bocaviruses contain an additional open reading frame that encodes a non-struc-tural protein (NP1) of unknown function. Bocaviruses recently have been identified in humans, specifically in children with lower respiratory disease.

bOvine ParvOvirus

A parvovirus has been isolated from cows, which is wide-spread but only rarely associated with clinical disease. In neonatal calves, the bovine parvovirus may cause mild watery to mucoid diarrhea. Infection of enterocytes occurs throughout the intestine, especially the small intestine. Disease lasts for 4–6 days, and virus may be shed for up to 11 days after infection.

Canine Minute virus (Canine ParvOvirus 1)

A parvovirus isolated from a clinically normal dog in 1967 was originally named the minute virus of canines (also known as canine minute virus or canine parvovi-rus type 1). By serological testing, it appears that this virus is widespread in dogs, but that the vast majority of infections are very mild or subclinical. The most com-mon clinical disease associated with canine minute virus is diarrhea or sudden death in neonatal puppies. Some cases were apparently associated with primary infection with the canine minute virus, but in other instances the affected dogs were also infected with another pathogen. Fetal infections have been reported, although these appear to be rare.

oTher ParvovirusesIntestinal parvovirus infections recently were identified in chickens and turkeys. Novel parvoviruses related to human parvovirus 4 also were recently isolated from cattle and pigs. Human parvovirus 4 has been detected in human plasma and liver tissue, and these viruses all share a dis-tinctive genome organization, so they probably constitute a new genus within the family Parvoviridae, subfamily Parvovirinae.

chapter 12 - parvoviridae, pages 225-235

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