bacterial chondronecrosis with osteomyelitis

8/12/2019 Bacterial Chondronecrosis With Osteomyelitis

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Bacterial chondronecrosis with osteomyelitis('femoral head necrosis') of broiler chickens:A reviewPerpetua T. McNamee a , Joan A. Smyth a & Joan A. Smyth ba Veterinary Sciences Division , Department of Agriculture and RuralDevelopment for Northern Ireland , 43 Beltany Road, Omagh, Co Tyrone,BT78 5NF, UKb Veterinary Sciences Division , Department of Agriculture and RuralDevelopment for Northern Ireland , 2 Stoney Road, Stormont, Belfast, BT43SD, UKPublished online: 17 Jun 2010.

To cite this article: Perpetua T. McNamee , Joan A. Smyth & Joan A. Smyth (2000) Bacterial chondronecrosiswith osteomyelitis ('femoral head necrosis') of broiler chickens: A review, Avian Pathology, 29:4, 253-270,DOI: 10.1080/03079450050118386

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2/19ISSN 0307-9457 (print ) /ISSN 1465-3338 (online ) /00/040253-18 2000 Houghton Trust Ltd

Avian Pathology (2000 ) 29 , 253270

*To whom correspondence should be addressed. Tel: +44 2882 243337; Fax: + 44 28 2442288; E-mail : [email protected] v.uk Received 31 March 2000. Accepted 31 March 2000.

REVIEW ARTICLE

Bacterial chondronecrosis with osteomyelitis(femoral head necrosis ) of broiler chickens:a review

Perpetua T. McNamee 1* & Joan A. Smyth 2

Veterinary Sciences Division, Department of Agriculture and Rural Development for Northern Ireland ,143 Beltany Road, Omagh, Co Tyrone BT78 5NF, UK and 2Stoney Road, Stormont, Belfast BT4 3SD, UK

Bacterial chondronecrosis with osteomyelitis (BCO ) in chickens was first reported in 1972 and is nowrecognized as an important cause of lameness in broiler chickens. Recent systematic studies of causesof lameness in birds reared in Northern Ireland have shown that it was the most common cause of lameness, being present in 17.3% of lame birds. Furthermore, it was also detected in birds presentedas found dead. Overall losses in male birds due to BCO were estimated to be 0.75% of all birdsplaced, which, in addition to welfare concerns, represents considerable economic loss. The disease hasbeen seen in birds ranging from 14 to 70 days of age, but most cases occurred around 35 days old. Itis most commonly caused by Staphylococcus aureus , but Escherichia coli , coagulase-negativestaphylococci and Enterococcus spp. are sometimes involved, as are, rarely, other bacteria. The lesionsare most commonly found associated with the growth plates of long bones, particularly the proximalgrowth plate of the femur and tibiotarsus, but other bones may also be affected. Since lesions werevisible to the naked eye in only 40 to 67% of cases, histological examination is recommended where nolesions are visible macroscopically. As the lesion may be present in only one growth plate, and becausehistological examination is often not carried out, BCO is almost certainly underdiagnosed. The exactpathogenesis of the condition is unknown, but it is thought that adherence of blood-borne bacteria toexposed cartilage at the tips of metaphyseal blood vessels is fundamental. Under controlledexperimental conditions, infection of birds with the immunosuppressive viruses chicken anaemia virusand infectious bursal disease virus increased the incidence of the disease, while restricting feed intakereduced the incidence of disease. S. aureus strains identical to, or closely related to, isolates recoveredfrom naturally occurring cases of the disease (as determined by pulsed-field gel electrophoresis ) havebeen recovered from fluff-debris in hatcheries, and also from the environment of breeding flocks,indicating that infection in the breeding farm and in the hatchery could be an important source of infection. It has also been shown that humans can carry poultry strains of S. aureus on their hands.There is a higher incidence of BCO in birds hatched from floor eggs. Thus, hygiene and managementpractice on breeder farms and in the hatchery may influence the occurrence of the disease.Bacteraemia is a prerequisite for BCO. Indeed, in some flocks suffering losses due to BCO, there arealso losses due to staphylococcal septicaemia. Thus, appropriate treatment of affected flocks shouldreduce losses due to septicaemia. It should also reduce the occurrence of bacteraemia and thedevelopment of further cases of BCO. However, birds in which BCO has already developed, areunlikely to respond to treatment. Control of BCO by vaccination seems unlikely in the short term.Simple bacterins have not been effective and much basic research is needed to identify the importantvirulence factors. Furthermore, more than one type of bacterium is capable of causing the disease.

Bacterial interference has been used successfully in humans and turkeys to prevent staphylococcaldiseases, and warrants investigation for the prevention of BCO in chickens. This may have anadvantage in that the interfering bacterium may also exclude some of the other bacteria that can causeBCO. The recent development of a disease model in which S. aureus is given by a natural route allowsthe potential for further investigation of the role of predisposing factors, and intervention strategies,including vaccination and bacterial interference, for the prevention of BCO.

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Introduction

Lameness compromises the welfare of chickens andcauses considerable economic loss (Riddell, 1992 ).Lame birds have difficulty accessing food andwater, and will therefore become dehydrated anddie. Based on data recorded daily and from weeklynecropsy examination of birds from 51 broilerflocks in Western Canada, Riddell & Springer(1985 ) found the incidence of birds culled due tolameness to range from 0.46 to 4.08%. In this study,the average incidence of chickens with skeletaldeformities was 1.72%, which included 1.1%chickens culled in the field and 0.62% chickenscondemned at processing. A national survey in theUS estimated that leg problems cost the broilerindustry between 80 and 120 million dollarsannually (Morris, 1993 ). Although there have beenseveral comprehensive reviews describing skeletalabnormalities of poultry (Wise, 1975; Riddell,1981, 1992; Thorp, 1994 ), few detailed surveys(Randall & Mills, 1981; Riddell, 1983; Riddell &Springer, 1985 ) have been conducted on commer-cial chicken flocks to define the incidence andsignificance of different disorders (Riddell, 1992 ).Historically, the majority of conditions cited asimportant causes of lameness were of non-infec-tious aetiology. Osteomyelitis was first reported asa cause of lameness in commercial broiler chickens

in Australia (Nairn & Watson, 1972 ), and Staphylo-coccus aureus was shown to be the cause. Thecondition, more correctly called bacterial chon-dronecrosis with ostemyelitis (BCO ) (see the nextsection ), has subsequently been reported in broilersfrom other parts of Australia, the US, Canada andEurope (Griffiths et al. , 1984; Riddell & Springer,1985; Thorp et al. , 1993; Randall & Reece, 1996;Thorp & Waddington, 1997; McNamee et al. ,1998 ).

Studies of commercial broilers carried out

between 1965 and 1978 showed that most skeletalabnormalities causing lameness were associatedwith long bone growth disturbances, frequentlyinvolving the growth plate (Prasad et al. , 1972 ) orwere manifest as bone deformities (Poulos et al. ,1978 ). Subsequently, a working party in the UK(MAFF/ADAS, 1981 ) identified tibial dyschon-droplasia (TD ) (Leach & Nesheim, 1965; Siller,1970 ), and angular limb deformities (Julian, 1984;Duff & Thorp, 1985 ) as the predominant causes of lameness. Studies of commercial broilers in Canada

showed that long bone deformities were the maincause of lameness, while arthritis and osteomyelitisaccounted for only 10% of losses due to lameness inthe last week before processing (Riddell &Springer, 1985 ). The MAFF working party listedleg disorders in order of priority, and osteomyelitiswas placed low on this list (MAFF/ADAS, 1981 ).Over a decade later, a report on the welfare of broiler chickens (Farm Animal Welfare Council,1992 ) indicated that osteomyelitis was being

increasingly recognized as a cause of lameness incommercial broiler chickens. A study by Thorp et al. (1993 ) in the UK, which targeted birds suspectedto have a lesion in the hip, showed a high incidenceof BCO. It was also a common diagnosis in lamebirds examined by Thorp & Waddington (1997 ).BCO was later shown to be the most common causeof lameness in commercial birds through systematiclarge-scale study of lame broilers from sevencommercial flocks (McNamee et al. , 1998,1999a ).

There have been reports of conditions similar toBCO in other avian species. The descriptions byHole & Purchase (1931 ) of abscesses involving thearticular cartilage and long bones of pheasants arehighly suggestive of osteomyelitis, although thelesions were not examined histologically. S. aureuswas recovered from the abscesses. Staphylococcalosteomyelitis was reported by Jungherr (1933 ) ingeese. Naturally occurring bacterial osteomyelitiswith synovitis was recorded in turkeys in Australia(Nairn & Watson, 1972 ) and the US (Nairn, 1973 );S. aureus was the most common isolate. Subse-quently, Wyers et al. (1991 ) described osteomyelitisassociated with S. aureus in male breeding turkeysin France. Turkey osteomyelitis complex (TOC ) isan important cause of lameness and of carcasecondemnations in turkeys in the US (Bayyari et al. ,1994 ). The pathogenesis of the condition in turkeys

appears to be very similar to that in chickens,although TOC is associated with green livers inturkeys, but not in chickens (Mutalib et al. , 1983a ).Osteomyelitis caused by S. aureus is also asignificant disease of children ( Ish-Horowitz et al. ,1992 ).

This paper reviews the current state of knowl-edge about the disease in chickens, and describessalient features of the most commonly involvedbacterium, S. aureus .

Terminology

Over the past few decades, this condition has beenreported under a variety of names; osteomyelitis,femoral head necrosis, long bone necrosis, proximalfemoral degeneration, bacterial chondritis withosteomyelitis, and BCO. The latter terms areconsidered to be the most appropriate by the presentauthors because they describe the pathology mostaccurately and also indicate the bacterial aetiology.

The terms femoral head necrosis and proximalfemoral degeneration, while both descriptive, donot take account of the fact that the lesions mayoccur in any bone, and are common in thetibiotarsus. Furthermore, they may include degen-erative changes unrelated to bacterial infection(Julian, 1985; Duff & Randall, 1987 ). The termfemoral head necrosis has also been used todescribe lesions associated with malabsorptionsyndrome in broilers (Page et al. , 1981 ) and with

254 P. T. McNamee & J. A. Smyth

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Bacterial chondronecrosis in broilers 255

Figure 1. Commercial broiler, 28 days old. Note wing extended for support. This is typical in a broiler with bacterialchondronecrosis and osteomyelitis in the proximal end of the

femur.

Figure 2. Proximal end of the femur, midline frontal section,28-day-old lame commercial broiler. Note the large zone of

yellow tissue extending from the growth plate region (gp ) to themedullary cavity (mc).

Figure 3. Physeal cartilage in the proximal end of the femur,midline frontal section, 35-day-old lame commercial broiler. Note clumps of basophilic bacteria (bb) in metaphyseal blood vessel surrounded by poorly stained physeal chondrocytes and matrix ( ps ) , contrasting with the dark blue, normally stained cartilage (ns ) to left and right of photograph, ( Haematoxylin &eosin ). Bar = 525 mm.

Figure 4. Hypertrophic chondrocytes (hc) lining metaphysealblood vessel (mv) in the proximal end of the femur, midline frontal section, 35-day-old lame commercial broiler. Clumps of basophilic bacteria (bb ) are seen in a blood vessel and inchondrocyte lacunae ( Haematoxylin & eosin ). Bar = 59 mm.

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reovirus infection (Van der Heide et al ., 1981 ).However, the lesions seen in malabsorption syn-drome were in fact osteoporosis, not necrosis(Julian, 1985 ). Duff & Randall (1987 ) founddetachment of the proximal femoral cartilaginousepiphysis at slaughter, which they proposed wasassociated with a traumatic aetiology followinginjury at catching. They suggested that the termsfracture separation or traumatic epiphysiolysis of the cartilaginous epiphysis are more appropriatethan femoral head necrosis or degeneration todescribe such lesions. In some reports, histologicaldescriptors were applied to lesions based on theirmacroscopic appearance, which is inappropriateand potentially inaccurate. Attempts were made byJulian (1985 ) to clarify diagnosis of lesions in thefemoral head, by categorizing them on the basis of their histological appearance. He distinguishedthree types of lesion, i.e. osteochondrosis, dyschon-droplasia and osteomyelitis in 16- to 30-week-oldmale turkeys. The term osteomyelitis is inadequatebecause it does not describe the lesion in the growthplate cartilage in young growing birds.

Clinical Signs

In naturally occurring BCO of the leg bones, birdsmay show signs of lameness or may simply be

found dead. The onset of lameness may beassociated with a rising mortality rate in the flock (Nairn & Watson, 1972; Griffiths et al. , 1984 ).Nairn & Watson (1972 ) reported that the incidenceof lameness due to BCO in broiler chickens mayreach 50%, with up to 5% mortality of theflock.

Live affected birds usually have a characteristiclimping gait. When the lesion occurs in theproximal end of the femur, birds typically use oneor both wing tips for support during locomotion

(Figure 1 ) or hip flexion, and vocalize loudly whenpressure is applied to the affected region (Thorp et al. , 1993 ). In experimental studies using aerosolinfection with S. aureus , lame birds in isolatorsgenerally became moribund within 8 h, and diedovernight if not removed on the first day of clinicalsigns (McNamee et al. , 1999b ). Interestingly, afterintravenous inoculation of S. aureus to 29-day-oldbirds, lameness was generally observed after 2 daysbut birds could survive for up to a further 6 days(Emslie et al. , 1983 ). After intravenous inoculation

of 26 groups of 45-day-old birds with S. aureus ,Mutalib et al. (1983a ) found that clinical signsfollowed a similar pattern in all groups, with ruffledfeathers, reluctance to walk, heads down, closedeyes, birds sitting on their hocks, and weak response to external stimuli. A sharp decrease infeed and water intake ensued, and death usuallyfollowed 2 to 5 days after the first appearance of signs (depending on the dose of inoculum ). Thediffering survival times after the onset of lameness

in birds exposed to S. aureus by aerosol and by theintravenous route may be a reflection of differingage or bodyweight at infection, or perhaps thediffering management conditions used in eachexperimental procedure.

In the advanced stages of the disease, affectedbroilers are reluctant to move (Nairn & Watson,1972 ) and would therefore be unlikely to obtainfood or water. Weight loss has been recorded as afeature of both the naturally occurring disease(Griffiths et al ., 1984 ) and experimentally induceddisease (Emslie & Nade, 1985 ). The negative effecton growth rate has been used as a predictor of lesiondevelopment with the intravenous model (Emslie et al. , 1983 ).

Age

Nairn & Watson (1972 ) found cases in broilers aged4 to 8 weeks. It was also recorded, together witharthritis/tenosynovitis, in birds aged from 35 to 45days by Griffiths et al. (1984 ). Riddell & Springer(1985 ) found arthritis and osteomyelitis from 34days onwards in birds processed from 41 to 49days. BCO was diagnosed in lame birds aged from34 to 70 days (Thorp et al. , 1993; Thorp &Waddington, 1997 ). While studying five commer-cial flocks, McNamee et al. (1999a ) found BCO inbirds aged from 14 to 49 days, with the peak

incidence occurring most frequently at 5 weeks of age. In experimental studies, where S. aureus wasadministered by aerosol, the majority of BCOlesions developed between days 31 and 40 whenbirds were exposed to S. aureus at 1 day old andinfected with immunosuppressive viruses on day 21(McNamee et al ., 1999b ). Thus, the peak incidenceof BCO coincides with the maximum stockingdensity of the birds, and is at a time when thegrowth plate is rapidly proliferating.

Pathology

Location of BCO lesions

The most commonly affected sites in naturallyoccurring cases of BCO in poultry are the proximalend of the femur and tibiotarsus (Nairn, 1973;McNamee et al ., 1998 ). Lesions occur less fre-quently in the proximal tarsometatarsus, distalfemur, distal tibiotarsus, proximal end of thehumerus or vertebrae (Nairn, 1973; McNamee et

al. , 1998). In one study of naturally occurring caseswhere all growth plates of the long bones of the legs

were examined histologically, lesions were presentonly in the proximal end of the femur and in theproximal end of the tibiotarsus (McNamee et al. ,1998 ). In a subsequent field study where theproximal end of the femur and tibiotarsus wasexamined by histology, the most frequently affectedsite was the right femur (63.1% ) followed by righttibiotarsus (40.3% ), left femur (36.8% ) and left


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tibiotarsus (19.3% ). Of 57 birds with BCO in thelatter study, lesions were present in all four sites in21% of birds, in two sites in 15.8% of birds, and inone site of 63.2% of birds (McNamee, 1998 ). Usingthe intravenous model of S. aureus infection inchickens, Emslie et al. (1983 ) found that macro-scopically visible lesions were most commonlyseen in the proximal end of the tibiotarsus and in thedistal end of the femur of broiler chickens. In thatstudy, histological examination was not carried outon other growth plates, so it is possible that lesionswere present at other sites. Mutalib et al. (1983a ),who also studied broilers injected intravenously,found that the proximal end of the femur and of thetibiotarsus were most commonly affected. Lesionsoccurred less frequently in the proximal tarsometa-tarsus, distal femur, distal tibiotarsus and vertebrae(Mutalib et al ., 1983a ). BCO was diagnosed in theproximal end of the femur, following intravenousinjection of S. aureus to 35-day-broilers by Griffithset al . (1984 ), but no other site was examined in thisstudy. Thorp & Waddington (1997 ) reported ahigher incidence of BCO in the proximal end of thetibiotarsus than in the femur of broilers withhypophosphataemic rickets.

The real incidence of BCO is likely to beunderestimated during investigation of lameness inbroilers unless the growth plate of the proximal endof the femur and tibiotarsus of both legs are

examined for lesions. Naturally occurring lesions of vertebral osteomyelitis, and experimental reproduc-tion of the condition by intravenous injection of S.aureus , was reported by Carnaghan (1966 ). How-ever, the incidence in naturally affected flocks didnot exceed 1%. A lesion of vertebral osteomyelitiswas observed in one out of nine birds with BCO(McNamee et al. , 1998 ). One vertebral lesion wasalso recorded by Griffiths et al. (1984 ) 3 days afterintravenous injection of eight birds with S. aureus at35 days.

In turkeys, Nairn (1973 ) found that macroscopiclesions were evenly distributed in the proximal endof the femur, the proximal and distal end of thetibiotarsus, and the proximal tarsometatarsus fol-lowing intravenous injection with S. aureus .Lesions consistently occurred in at least two out of the three sites when the proximal end of the femur,tibiotarsus and tarsometatarsus was examined. Ver-tebral osteomyelitis also occurs in naturally infectedand in intravenously injected turkeys, and wasgenerally found in less than 10% of birds with

lesions in long bones(Nairn, 1973

).

Macroscopic appearance

At necropsy, birds with BCO were dehydrated andwere generally smaller than non-lame cohorts(Emslie & Nade, 1983; McNamee et al. , 1999b ).Macroscopically, BCO may appear as focal yellowareas of caseous exudate or lytic areas, which causeaffected bones to be fragile (Skeeles, 1997 ).

Lesions observed during experimental investiga-tions varied from a small pale area adjacent to thegrowth plate to a large zone of yellow tissueextending from the growth plate region to themedullary cavity (Figure 2 ) (McNamee et al. ,1999b ). However, it is important to note that manylesions are only visible using microscopy. Throughseveral investigations where detailed skeletal exam-inations were performed, only 40 to 67% of lesionswere visible macroscopically (McNamee, 1998 ).Lesions observed in naturally occurring cases wereusually larger than lesions observed during experi-mental investigations (unpublished observation ).This may reflect the fact that the experimental birdswere very closely monitored and were thereforeprobably culled sooner after onset of lameness thanbirds grown under commercial conditions.

In one outbreak of BCO in broilers caused by S.aureus , fractures of the neck and head of the femurwere observed. In some birds, only one femur had athin brittle cortex of the neck region of the proximalfemur and necrotic material in the medullary cavity,but there was bilateral involvement in others(Griffiths et al ., 1984 ). Riddell et al. (1983 )cautioned that separation of the proximal femoralepiphysis is a common post mortem artefact inrapidly growing broiler chickens and should not becalled femoral head necrosis unless histologicalexamination confirms the presence of lesions.

Histological appearance

In BCO lesions developing after exposure to S.aureus by aerosol, there are usually clumps of basophilic bacteria in epiphyseal or physeal bloodvessels, and these are surrounded by poorly stainedcartilaginous matrix (Figure 3 ) containing necroticchondrocytes. Emslie & Nade (1983 ) suggested thatthe pale staining of the cartilaginous matrix is thedirect result of the degradative effect of bacterial

toxin, produced by S. aureus , on acid mucopoly-saccharides in the ground substance of the cartilage.Affected blood vessels may be either completelyoccluded by bacteria or contain bacterial clumpssurrounded by inflammatory cells and fibrinthrombi. Such vessels are usually in the hyper-trophic zone of the physis and, in this region,bacteria are also frequently observed in chon-drocyte lacunae (Figure 4 ). Together with thecartilaginous lesions, osteomyelitis often occurs inthe metaphysis of the femur and/or tibiotarsus, and

occasionally in the secondary ossification centre inthe proximal end of the tibiotarsus (McNamee et al. , 1999b ). Early lesions found in cases of fieldinfection were indistinguishable from those seen inbirds infected by aerosol. More advanced lesionsappeared as large clumps of basophilic bacteriasurrounded by degenerate inflammatory cells and,in some cases, fibrous reaction. Following intra-venous infection, Mutalib et al. (1983a ) found acuteand chronic lesions similar to those already descri-


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bed. In naturally occurring cases of the disease,while lesions in the proximal end of the femur andtibiotarsus were invariably detectable on examina-tion of the first (midline frontal ) section (McNameeet al ., 1998 ), bacterial clumps were sometimes onlyfound after examination of multiple sections (Thorpet al. , 1993; McNamee et al. , 1998 ).

Following a sequential study, Emslie & Nade(1983 ) described the sequence of lesion develop-ment in broilers following intravenous infection asfollows. Bacterial proliferation occluded the vas-cular tunnels within 24 h. Within 48 h, triangular-shaped pale lesions bordering on the growth platewith the apex of the lesion pointing verticallytowards the diaphysis of the bone were observed.By 48 h, the inflammatory exudate contained manydegenerating cells and numerous secondary foci of

bacteria, and was lined by a dense layer of fibrin.By 192 h after infection, the dead cartilaginousmatrix harbouring bacteria had frequently formed asequestrum that was separated from the surroundingmetaphyseal bone by an oedematous clear zone.

It has been reported that dyschondroplasticlesions may act as foci for bacterial necrosis andsubsequent osteomyelitis in turkeys (Wyers et al. ,1991 ), but no evidence of bacterial necrosis wasdetected in association with lesions of TD in broilerchickens by McNamee et al. (1998 ).

Associated lesions in birds or flocks with BCO

There may be concurrent losses due to staphy-loccoccal septicaemia in flocks affected by BCO(Reece, 1992; McNamee et al. , 1998 ). Otherfindings in birds with BCO or their cohorts includearthritis or tenosynovitis associated with the hock

joint (Griffiths et al. , 1984; Thorp et al. , 1993 ), andsuppurative arthritis of the hip and/or hock joint(McNamee et al. , 1998 ). Septic arthritis also

developed occasionally in chickens inoculatedintravenously (Emslie & Nade, 1985 ).

Prevalence of BCO

Pattison (1992 ) suggested that the condition des-ignated femoral head necrosis was the mostcommon cause of lameness in UK broilers, but alsostated that this terminology encompassed a varietyof gross lesions in the proximal end of the femur

(infectious and non-infectious ). While cases of BCO have been cited as an important cause of lameness by other workers (Thorp et al. , 1993;Thorp & Waddington, 1997 ), until the work of McNamee (1998 ), no study reported the numbers of birds actually culled or dying due to BCO frombroiler flocks. To obtain an accurate estimate for theincidence of BCO in commercial broiler flocks, itwas first necessary to determine the total incidenceof birds culled because of lameness (lame culls ),and mortalities, and then to determine the incidenceof different disorders contributing to each category.The number of birds culled because of lamenessand the number of birds found dead was recordeddaily in 28 male and 19 female flocks, selected atrandom from two broiler production companies(McNamee, 1988 ). Overall, the mean incidence of

male birds culled due to lameness was 0.52% of 28flocks, and that of females was 0.38% of 19flocks.

A detailed longitudinal study of five randomlyselected broiler flocks was then carried out(McNamee et al. , 1999a ). All cull birds andmortalities occurring on one day each week weresubjected to a detailed necropsy examination, andthe proximal end of each femur and tibiotarsus wasexamined by histology (McNamee et al. , 1999a ). Atotal of 191 birds culled because of lameness, 138

birds that were found dead, and 89 birds that wereculled for reasons other than lameness (other culls )were examined. On average, 0.52% of all birdswere culled due to lameness from these flocks. Of the 191 birds culled because of lameness, BCO wasfound in 17.3% of cases, limb deformities in 13.6%,severe TD in 8.4% and spondylolisthesis in 7.8% of cases. BCO was present in many of the birdscategorized as other culls, and in birds found deadin the broiler house. The latter finding suggests thatmany birds with BCO are not detected by the

stockman, but subsequently die and are recorded asmortalities rather than lame culls. Overall, BCOwas diagnosed in 13.7% of the total deaths and culls(416 birds ) examined by histology (Table 1 ).

Therefore, recent work indicates that the ratio of lameness due to BCO, to lameness of non-infectious origin, appears to be changing. It is notknown whether this is due to a real increase in theincidence of BCO, a relative increase comparedwith other causes of lameness such as TD and


Table 1. Incidence of BCO (diagnosed by histological examination ) in birds culled due to lameness, dead birds, other culls, and inthe total number of birds examined from one female and four male commercial broiler flocks (reproduced with permission from

McNamee, 1998 )

Totalbirds

Totalmales

Totalfemales

Lamebirds

Lamemales

Lamefemales

Deadbirds

Otherculls

BCO (%) 13.7 14.4 10.6 17.3 17.2 17.6 12.3 8.0Number examined 416 341 75 191 157 34 138 87

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An Estimate of Financial Losses due to BCO

In the UK, it was estimated that the conditiondesignated femoral head necrosis caused a lossin sales value of 3.78 million to the broilerindustry (Pattison, 1992 ). In that study the termfemoral head necrosis was used to encompass avariety of gross lesions in the proximal end of thefemur. The work of Thorp et al. (1993 ) showedthat of 67 birds exhibiting lameness characteristicof a lesion in the proximal end of the femur, 61%had evidence of bacterial infection in this site.Therefore, by extrapolation, bacterial infection of the hip may have been responsible for a loss of approximately 2.3 million in the UK. The meanincidence of total losses in male flocks was 5.2%(McNamee, 1998 ). It was estimated that BCOoccurred in 17.2% of lame male birds, and in14.4% of all losses in male birds placed in fourhouses (Table 1 ). From these figures, it has beenestimated that for male birds produced in NorthernIreland, annual losses due to BCO are approx-imately 185 625. On the basis of the findingsfrom one female flock (McNamee, 1998 ), lossesdue to BCO represent approximately 118 000

annually. By extrapolation, this equates approx-imately to losses of 3 million due to BCO inmale and female broilers in the UK. These figuresdo not include the cost of any treatment that mighthave been used, the value of the bird or the costof rearing the bird to this age. This cost may besubstantial since the majority of losses due toBCO occur close to processing.

Bacterial Causes of BCO

S. aureus is the most common bacterium recov-ered from leg and joint infections of poultry(Skeeles, 1997 ), and it has been frequently recov-ered from bone infections of commercial broilers(Nairn & Watson, 1972; Griffiths et al. , 1984;Randall & Reece, 1996; Riddell, 1997; McNamee,1998; McNamee et al ., 1998 ). Other bacteriarecovered from infected bone of broilers includecoagulase-positive staphylococci other than S. aur-eus , e.g. Staphylococcus hyicus , coagulase-neg-ative staphylococci, e.g. Staphylococcus xylosus

and Staphylococcus simulans, Escherichia coli, Mycobacterium avium, Salmonella spp. and Enter-ococcus spp. (Reece, 1992; Thorp et al. , 1993;McNamee, 1998; McNamee et al ., 1998 ). Therelative occurrence of these pathogens in cases of BCO in broiler flocks has been examined in recentstudies in Great Britain and in Northern Ireland(Thorp et al. , 1993; McNamee, 1998; McNameeet al. , 1998 ). Thorp et al. (1993 ) recoveredbacteria from the proximal femur of birds in

which a lesion suggestive of BCO was evident. Of the lesions swabbed, coagulase-positive staphylo-cocci (22.2% ), coagulase-negative staphylococci(11.1% ) and E. coli or mixed cultures (13.3% )were recovered. The bacteria were not identifiedto species level in that study. Riddell (1997 )reported Staphylococcus spp. as the most commonbacteria isolated from arthritis/tendonitis/osteomy-elitis in broilers from Western Canada. He alsonoted an increase in the incidence of musculoske-letal infection associated with E. coli infection.

Bacteriological examination was carried out on38 bones with BCO, identified during a longitudinalstudy of five broiler flocks (McNamee, 1998 ). S.aureus was recovered from 63.1%, non-haemolytic

E. coli from 13.1%, Staphylococcus xylosus from10.5%, S. hyicus from 10.5% and S. simulans fromone lesion (2.6% ). In another study, S. aureus wasrecovered from 62.5% of eight lesions of BCO(McNamee et al. , 1998 ). Thus, all the availableinformation suggests that at this time, S. aureus isthe predominant cause of BCO in commercialbroiler chickens. However, it seems probable that if measures are introduced which are effective incontrolling/preventing S. aureus infections, otherbacteria will become relatively more common.However, it remains to be seen whether the actualincidence of bone infection by the other bacteriawould increase. Ultimately, therefore, attempts to

eliminate BCO will also have to consider theinvolvement of bacteria other than S. aureus in theaetiology of this condition.

Characteristics of S. aureus That MayInfluence the Development of BCO

The reason S. aureus causes more cases of BCOcompared with other bacteria is unknown but it maybe a reflection of specific intrinsic characteristics of the organism. S. aureus is a common, economically

significant pathogen in several species, e.g. it is asignificant cause of mastitis in cattle and sheep(Lee, 1996 ), and of osteomyelitis in children (Ish-Horowitz et al. 1992 ). S. aureus is a major cause of hospital-acquired and community-acquired infec-tion (Foster, 1991; Lee & Pier, 1997 ). The majorityof strains of S. aureus recovered from diseasedhumans or animals possess capsules (Sompolinskyet al ., 1985; Karakawa et al. , 1988 ). The capsuleimpairs phagocytosis and is generally associatedwith virulence (Jonsson & Wadstrom, 1993 ). It has

been suggested that the capsule may enhanceadherence to chicken cartilage (Speers & Nade,1985 ). However, Cunningham et al. (1996 ) statedthat the role of capsules in staphylococcal bone and

joint infection, if any, remains unknown.Gram-positive bacteria such as S. aureus express

surface proteins on their cell wall, which contributeto virulence by influencing, among other things,bacterial adherence. Examples of surface proteins of S. aureus associated with virulence include protein


angular limb deformities that appear to be decreas-ing due to breeding selection, or due to increasedawareness of the condition.

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A, fibronectin binding protein, collagen bindingprotein, fibrinogen binding protein and bone sialo-protein (Holderbaum et al. , 1985; Ryden et al. , 1989;Switalski et al. , 1993; Foster & McDevitt, 1994 ).Protein A increases the severity of S. aureusinfections by inactivating complement, blocking theFc portion of immunoglobulin involved in opsoniza-tion (Kloos & Bannermann, 1995 ) and preventingthe accumulation of neutrophils around bacteria(Colburn et al. , 1980 ). A study carried out in ananimal model of peritonitis confirmed that a S.aureus strain producing protein A was more virulentthan a protein A-negative mutant (Patel et al. , 1987 ).No direct relationship was found between theproduction of protein A and the ability to adhere totissue by strains recovered from turkeys (LeFevre &Jensen, 1987 ). However, they suggested that proteinA, when combined with other factors, may contrib-ute to the ability of S. aureus to cause staphylococcalinfections in turkeys.

Fibronectin-binding deficient mutants of S. aur-eus have reduced ability to adhere to heart valves ina rat model of endocarditis, and antibodies raisedagainst a fusion protein containing regions of fibronectin offered some protection to cows againstmastitis (Foster & McDevitt, 1994 ). Specific bind-ing by several strains of S. aureus to collagen wasdemonstrated by Holderbaum et al. (1985 ). S.aureus strains isolated from humans with septic

arthritis or osteomyelitis possessed a collagenbinding receptor (Switalski et al. , 1993 ). S. aureuscan bind specifically to several other host proteins,e.g. bone sialoprotein and fibrinogen binding pro-tein (Foster & McDevitt, 1994 ), and these may beimportant in the pathogenesis of BCO (see follow-ing Pathogenesis section ).

The role of cell-surface proteins in the pathogen-esis of BCO merits further investigation as block-age of S. aureus adhesion to tissue, perhaps byproduction of antibodies to staphylococcal adhe-

sins, might prevent initiation of disease by prevent-ing adhesion to host tissues.

Source of S. aureus Infection and Methods of Spread of the Organism

Staphylococcus spp. are normal inhabitants of skinand mucous membranes of poultry, and are ubiqui-tuous in environments where poultry are hatched,reared or processed (Steeles, 1997 ). S. aureus canbe recovered from the skin and nares (Harry,

1967a,b), and the dorsal and plantar surface of thefeet of clinically healthy wild birds and chickens

(Cooper & Needham, 1976 ). Thus, wild birds maybecome infected as a result of contact with poultrywhen visiting commercial poultry sites in search of food, and could potentially act as a reservoir of S.aureus infection for domestic poultry. S. aureus hasbeen recovered from litter (Vaid et al. , 1979 ), fromfeeders and drinkers (Vaid et al ., 1979; Thompsonet al. , 1980 ), and from the air in poultry houses

(Vaid et al. , 1979; Sauter et al. , 1981 ). S. aureus hasalso been isolated from the oral cavity, eye, cloacaand faeces of healthy chickens (Shimizu, 1979 ), andfrom hatcher debris ( shell, shell membrane, fluff,etc. ) and from work surfaces in the sexing andvaccinating areas, and from the hands of personnelin hatcheries (Thompson et al. , 1980; McCullagh et al ., 1998; Rodgers et al. , 1999 ).

During the development of an aerosol model of BCO, a significantly lower incidence of S. aureusrecovery from birds simultaneously exposed to S.aureus and inoculated with chicken anaemia virus(CAV ) and infectious bursal disease virus (IBDV ) atday 21 was observed than that in birds exposed to S.aureus at day 10 and inoculated with CAV and IBDVat day 21. Furthermore, 1-day-old chicks exposed toS. aureus developed a higher incidence of BCO(38.4% ) than 10-day-old chicks exposed to S. aureus(8.3% ). These findings suggest that broilers may beat greater risk of developing BCO if exposed at earlystages of the production cycle. In order to testwhether S. aureus recovered from hatcheries mightbe a source of organisms causing BCO, 47 isolates of S. aureus collected from broilers with musculoskele-tal disease and 62 isolates from fluff samplescollected from chick hatchers were compared usingpulsed-field gel electrophoresis (PFGE ) (McCullaghet al. , 1998 ). The PFGE patterns of the isolatesdemonstrated either full or closely related identity

between 85% of isolates from clinical sources and71% of the hatchery isolates. Many of the isolatesfrom clinical sources had been recovered fromlesions of BCO. The similarity between isolatesrecovered from BCO and isolates from the hatcheryconfirms that the hatchery is a potential source of infected birds who develop disease (McCullagh et al. , 1998 ). Therefore, the prevention of S. aureuscontamination in hatchers to avoid colonization of hatching chicks should be considered an integral partof the control of BCO.

The possibility of poultry acquiring S. aureusinfection from human contact was investigated byHarry (1967a,b ), who identified similar haemolysispatterns and bacteriophage types in human andpoultry strains. More recently, Hu et al . (1995 )found the same strains of S. aureus in the nares of adult humans for up to 2 years, confirming thathumans could act as carriers of S. aureus for a longperiod of time. Rodgers et al. (1999 ) investigatedwhether humans could carry poultry-associatedstrains of S. aureus capable of causing BCO in

chickens. In this study, personnel from one broilerhatchery, and workers on 18 separate broiler parentfarms that supply the hatchery, were tested for handand nasal carriage of S. aureus . In both types of location, nasal carriage of S. aureus was morecommon than hand carriage, but there was noevidence of nasal carriage of poultry-associated S.aureus strains by humans. Hand carriage wasdetected in parent farm personnel but not inhatchery personnel, and some of the S. aureus


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strains had identical or closely related PFGEpatterns to strains causing BCO. This raises thepossibility that handling may contribute to thedissemination of S. aureus associated with BCO inbroilers (Rodgers et al. , 1999 ).

PathogenesisMuch information on the pathogenesis of BCO hasbeen obtained through observations of the naturallyoccurring disease and by studying experimentalmodels of BCO. Many similarities exist in thedevelopment and maturation of avian and mamma-lian growth plates (Howlett, 1979 ), and thereforethe broiler chicken has been used extensively as amodel for acute haematogenous staphylococcalosteomyelitis of human infants (Emslie & Nade,1983, 1985; Emslie et al. , 1983; Speers & Nade,1985; Alderson et al. , 1986; Daum et al. , 1990 ).This model uses intravenous inoculation of bacteriathat, although unlikely to occur naturally in broilers,nonetheless yields useful information.

Attempts to determine the most likely naturalroute of infection in poultry have resulted inconflicting hypotheses. Smith (1954 ) proposed thatstaphylococcal infection was likely to be associatedwith husbandry conditions in which wounding,particularly of the feet, was likely to occur. Afterhatching, the open navel is a possible portal of entry

for S. aureus (Harry, 1957 ). Insect bites (Hinshaw& McNeil, 1952 ), cuts in the skin (Hole &Purchase, 1931 ), and abrasions, vaccination orminor surgical procedures are all possible routes of S. aureus entry into the bloodstream (Jensen et al. ,1987; Steeles, 1997 ). Interestingly, Riddell (1997 )noted an increase in the incidence of musculoskele-tal infection, including osteomyelitis, associatedwith E. coli infection, which he suggested wasassociated with an increase in the incidence of cellulitis due to abdominal scratching. Similarities

were found by Harry (1967a ) between strains of S.aureus recovered from skin, upper respiratory tractand bone lesions, and he suggested that staphylo-coccal infections may occur following mechanicaldamage, or disease which favours tissue invasion,in previously colonized birds. A low percentage(10.0% ) of 10 chickens developed osteomyelitiswhen a high challenge of S. aureus organisms (5 10 11 ) was given intratracheally in a single dose,compared with 100% in 10 birds after a lowerchallenge (5 10 6 ) was given intravenously(Mutalib et al. , 1983b

). Only the 10 birds exposedto S. aureus by the intratracheal route were

colonized by S. aureus in the respiratory tract. Thiswork showed that tracheal inoculation with S.aureus may result in respiratory tract colonizationbut the authors suggested that additional, uni-dentified factors are necessary to induce invasionand subsequent BCO. An observation by Riddell,cited in the publication of Mutalib et al. (1983a ),indicated that outbreaks of osteomyelitis were

associated with stressful situations such as severefeed restriction, poor nutrition, overcrowding inchickens and adverse weather conditions in turkeys.However, although they were unable to demonstratea relationship between the occurrence of osteomye-litis and stress under experimental conditions,Mutalib et al. (1983a ) suggested that stress mightinitiate outbreaks in chickens that have a latentstaphylococcal colonization from an early age.

Jensen et al. (1987 ) suggested that staphylo-coccal infections did not result primarily frominjection of the organism due to injury, insect bitesor cuts to the skin, rather that the respiratory tractwas the most likely portal of entry of S. aureus inpoultry on the basis of preferential S. aureusadherence to cells of the respiratory tract comparedwith cells from other tissues. Such adhesion of S.aureus to the surface of the host cell has beenshown to be important in bovine mastitis andendocarditis in humans (Foster, 1991 ), in humanosteomyelitis (Ryden et al. , 1989 ) and in osteomye-litis in chickens (Alderson & Nade, 1987 ).

In studies carried out during the development of a model of BCO (McNamee et al. , 1999b ), S.aureus was recovered from the nasal cavity of birds6 weeks after exposure to the bacterium by aerosolat 1 day old. Interestingly, the number of birds withS. hyicus infection of the nasal cavity after 6 weekswas much lower under the same experimental

conditions. Perhaps S. aureus has an enhancedability to adhere to the mucosa of the nasal cavity,which may increase the opportunity for bacterialinvasion and subsequent bacteraemia. Such anability, if present, could contribute to the dominanceof S. aureus as a cause of BCO. Therefore,investigation of the role of the earlier describedadhesion factors in poultry is warranted.

Injection of S. aureus intravenously

Since bacteraemia appears to be necessary for thedevelopment of BCO, much work has been carriedout studying disease development, by injecting S.aureus intravenously. In 1973, Nairn found thatintravenous injection of turkeys with S. aureus orwith E. coli produced osteomyelitis indistinguish -able from that seen in the spontaneous disease. Heconcluded that osteomyelitis commenced in theterminal vessels of the growth plate, and that theturkey or chicken would make a satisfactory modelfor the study of the disease in humans. Subse-

quently, S. aureus was injected intravenously inbroilers (Emslie & Nade, 1983; Emslie et al. , 1983;Alderson et al. , 1986; Daum et al. , 1990 ) as a modelof acute haematogenous staphylococcal osteomyeli-tis in infants. Studies using this model suggest thatosteomyelitis is caused by the spread of bacteriafrom small foci of infection in the growing ends of metaphyseal blood vessels in the hypertrophicregion of the cartilaginous growth plate (Emslie &Nade, 1983; Alderson et al ., 1986 ). Howlett (1980 )


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carried out a detailed ultrastructural examination onthe proximal growth plate of the tibia of birdsperfused intravenously with Tyrodes solution. Inthe hypertrophic region, it was found that chon-drocyte lacunae are opened by chondroclasts andthe thin walled metaphyseal blood vessels expandinto them forming small vascular saccules (Howl-ett, 1980 ). At the advancing capillary tips of metaphyseal vessels, the attenuated endothelium isfenestrated and frequently discontinuous, and it wassuggested that these gaps facilitate contact betweenbacteria and the physeal cartilage matrix (Howlett,1980; Emslie & Nade, 1985 ). Howlett (1980 )proposed that blood-borne elements such as bacte-ria and red cells often extravasate through such gapsinto the extravascular tissues. Sluggish circulationin the tips of the long narrow metaphyseal vesselsof the growth plate (Trueta, 1959 ) may facilitate theaccumulation of bacteria in this region and theestablishment of bacterial infection (Thorp et al. ,1993 ). In addition, broilers fed ad libitum spend alarge proportion of their time in the sitting position,which may lead to blood flow restriction and poorcirculation to this area (Thorp, 1988 ). In addition topoor circulation, Thorp et al. (1993 ) also cited adeficiency of macrophages in cartilage and therapid formation of occlusive thrombi as reasons forlocalization of bacteria in metaphyseal vessels.

Nade & Speers (1987 ) and Alderson & Nade

(1987 ) suggested a specific tropism of S. aureus forthe growth plate cartilage and the articular cartilageof long bones. Histological studies revealed thattransphyseal blood vessels, connecting the met-aphyseal and physeal vessels to the epiphysealvessels, were present in growing chickens and were alikely explanation for concurrent acute osteomyelitisand adjacent joint infection due to extension of infection from the metaphyseal blood supply (Alder-son et al. , 1986 ). Using the intravenous model of BCO, osteomyelitis and septic arthritis due to S.

aureus was produced only in broiler chickens thatwere continuously bacteraemic (Daum et al. , 1990 ).It was also found that continuously bacteraemicchicks developed osteomyelitis.

Ultrastructural studies showed that S. aureus wasanchored to collagen fibres of exposed cartilagematrix by the extensive network of S. aureusglycocalyx in the metaphyseal region of long bonegrowth plates, but were not adherent to adjacentvascular linings or red blood cells (Speers & Nade,1985; Alderson et al ., 1986 ).

Inoculation of S. aureus via aerosol

Attempts by Devriese et al. (1972a ) to reproduceBCO using a suspension of S. aureus by aerosolresulted in colonization of the upper respiratorytract from 3 and 14 days, respectively, until 7 and 8weeks of age without pathological changes in theskeleton. Several subsequent attempts to reproduceBCO using S. aureus by aerosol in broilers also

proved unsuccessful (Mutalib et al. , 1983b ), despitethe additional stresses of wetting and chilling.Jensen et al. (1987 ) found that the lung and liver of 19-day-old turkeys became colonized by S. aureusfollowing aerosol exposure to this pathogen. Theyconcluded that the presence of liver infectionindicated passage of the bacteria from the respira-tory tract into the blood or lymphatic system,although the exact mechanism of spread was notdetermined. In a subsequent study, Nicoll & Jensen(1987a ) exposed 14-day-old leghorn chicks to S.aureus by aerosol, and again found evidence of colonization of lung and liver tissue followingbacterial culture of portions of each tissue. Noevidence of clinical disease was detected. Theyhypothesized that factors exist under field condi-tions which upset the hostpathogen balance, thusresulting in clinical disease.

A series of experiments was recently designed inan attempt to reproduce BCO in broiler chickensusing the aerosol route of infection (McNamee,1998; McNamee et al. 1999b ). Birds in isolatorswere exposed to a suspension of S. aureus byaerosol, or exposed to S. aureus and subsequentlyinoculated with CAV alone, or with CAV and IBDV.In all groups, some birds became lame, BCO wasconfirmed and S. aureus was recovered (McNameeet al. , 1999b ). The incidence of BCO was higher inbirds receiving the viruses than in birds exposed to

S. aureus alone. These findings support the hypoth-esis of Thorp et al. (1993 ), who suggested thatviruses with potentially immunosuppressive effectsmight have a role in the development of BCO inbroilers. It is difficult to say exactly why exposureto S. aureus administered by aerosol (in the absenceof viruses ) produced clinical disease in theseexperiments, when others were unsuccessful. Per-haps the possession of virulence factors by thestrain of S. aureus used, particle size of theaerosolized inoculum, broiler genotype, or hatching

and rearing environment may have contributed tothe success of the model described (McNamee et al. , 1999b ).

During these experiments, CAV and IBDV weregiven at day 21 and it was found that reducing the ageat which the birds were exposed to S. aureusincreased the incidence of BCO and bacterialrecovery (McNamee et al. , 1999b ). In one of theseexperiments, birds fed 100% of the recommendedfeed intake for the breed developed a 68.7%incidence of S. aureus infection and/or BCO, while a

significantly lower incidence(10%

) developed inbirds fed 60% of the recommended intake. However,

feed restriction to such an extent is not a practicaloption for the control of BCO in broilers.

McNamee et al. (1999b ) found the highestincidence of BCO (36.8% ) occurred when 1-day-old broilers were exposed to S. aureus by aerosol,and subsequently inoculated with CAV and IBDVat day 21, while fed ad libitum . Using this model,birds of a more slow-growing genotype were


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investigated (McNamee, 1998 ). Although S. aureuscolonized the nasal cavity of the birds, no BCOdeveloped. The reason for this is unknown, but itmay be that in the slower growing bird, theendothelial gaps previously described in the met-aphyseal blood vessels did not develop signifi-cantly, thus limiting the possibility of bacterialinvasion and adherence to cartilage in this region.This might also explain why clinical signs failed todevelop in the studies of Nicoll & Jensen (1987a ),who also used leghorns.

Floor eggs

Sparks (1994 ) proved a positive relationshipbetween faecal contamination of the egg at ovi-position and bacterial contamination of the egg-shell. Eggshells are most susceptible to penetrationby bacteria at oviposition (Sparks & Board, 1985 ).Therefore, the microbiological status of the envi-ronment into which eggs are laid influences theincidence of bacterial contamination of the egg(Bruce & Drysdale, 1994 ). Thus eggs may becolonized by S. aureus , through contact with faecescarrying S. aureus . Such eggs may play an impor-tant role in the dissemination of strains of S. aureuswith the potential ability to cause skeletal infectionto newly hatched broiler chicks. Eggs laid bybroiler parents outside the nest-boxes on the litter-

covered floor are known as floor eggs. Theirincidence is generally highest at the onset of production and declines to a minimum shortly afterpeak production (Wilson, 1996 ).

McNamee (1998 ) examined birds hatched fromfloor eggs for BCO and by bacteriological culture.Following culture of bones, S. aureus (12.5% ) andS. hyicus (12.5% ) were recovered from birdshatched from floor eggs, and Enterococcus spp.(12.5% ) only from birds hatched from nest eggs.There was a significantly higher incidence of birds

with bacterial infection of the bone and/or BCO inbirds hatched from floor eggs compared with birdshatched from nest eggs. The results of this studysuggest that use of floor eggs should be dis-continued. If they must be used, they should beincubated separately from clean eggs in anattempt to reduce the likelihood of contamination of chicks derived from clean eggs.

Non-bacterial factors that may influence thedevelopment of BCO due to S. aureus

S. aureus is an opportunistic pathogen (Jensen et al. , 1987 ) that frequently colonizes healthy chick-ens, and it appears that under field conditions,factors exist that upset the hostpathogen balancein some birds, resulting in the development of clinical disease (Nicoll & Jensen, 1987b ).

Several authors have suggested that the occur-rence of trauma or pre-existing pathology of thegrowth plate cartilage may contribute to the onset of

osteomyelitis and BCO. It was noted that osteomye-litis most frequently occurs in the growth plate of the distal end of the femur, humerus and radius of foals where shear forces and focal clefts predom-inantly occur (Firth & Goedegebuure, 1988 ). Thorpet al. (1993 ) suggested that pre-existing pathologyin the presence of a bacteraemia might predisposethe cartilage to bacterial infection. In order toexamine this hypothesis, the incidence of physealosteochondrosis and the incidence of bacterialinfection were examined in lame and sound broilers(McNamee et al. , 1998 ). No significant differenceswere found between the overall incidence of mild,moderate or severe physeal osteochondrosisbetween lame and sound birds examined in thisstudy. It is therefore unlikely that this conditionalone contributed to the lameness observed. How-ever, significantly more bacterial isolates wererecovered from the bone of the lame birds than fromthe bone of sound birds. This difference, togetherwith the fact that the condition was just as commonin the sound birds, suggests that bacterial infectionwas associated with lameness, but not with physealosteochondrosis.

The report of the Farm Animal Welfare Council(1992 ) cited arthritis and tenosynovitis due to viralinfection (Jones et al. , 1975 ) as important causes of lameness. The possible involvement of viruses inthe aetiology of BCO was investigated by

McNamee et al. (1998 ), who found that althoughvirus isolation procedures yielded adenovirus andreovirus isolates from bone samples, there was noassociation between the occurrence of virus andskeletal lesions. Thorp et al . (1993 ) also recoveredadenovirus and, more surprisingly, infectious lar-yngotracheitis virus from the proximal end of thefemur. They considered it unlikely that either wasinvolved in the changes in the joint or bone.

Heterophils collected from commercial broilerswith osteomyelitis due to S. aureus infection had

reduced chemotactic ability (Andreasen et al. ,1993 ). The latter is an important factor in the fightagainst bacterial infection. Santivatr et al. (1981 )have shown that IBDV infection affects the abilityof avian heterophils to phagocytose S. aureus ,although the exact mechanism involved was notdetermined. Yuasa et al. (1980 ) suggested that amore profound suppression of the immune responsemight occur when chicks are infected with bothCAV and IBDV compared with CAV or IBDValone. During the development of the aerosol model

of BCO, it was found that dual infection with CAVand IBDV after S. aureus exposure, compared withS. aureus only, increased the number of lesionsproduced and reduced the age at which BCOdeveloped (McNamee et al. , 1999b ). Therefore,CAV and IBDV together play an important role inthe pathogenesis of BCO, although the precisecontribution of each has not been investigated. Anyattempt to reduce the incidence of BCO in broilerflocks should consider the role of these agents.


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Treatment

Dolman (1955 ) stated that among the more chasten-ing chapters in the annals of microbiological researchis the story of our apparently dismal failure to controlthe depradations of the staphylococcus. S. aureus isinherently a rather resistant organism; many strains

produce penicillinases and infections have alwaysbeen difficult to treat. In Belgium, Devriese et al.(1972b ) found that 80% of strains of S. aureus strainsrecovered from arthritis/synovitis in broiler breederswere resistant to penicillin G, 50% were resistant totetracycline, and 10% to streptomycin. As in otherspecies, antibiotic resistance to isolates of S. aureusfrom commercial poultry is common (Devriese,1980 ). Erythromycin-resistant strains of S. aureuswere recovered from outbreaks of dermatitis andsynovitis in chickens (Witte & Kuhn, 1978 ).

Takahashi et al. (1986 ) reported that 80% of S. aureusstrains recovered from poultry infections wereresistant to ampicillin, oxytetracycline, erythromy-cin, kanamycin, streptomycin, chloramphenicol andsulphadimethoxin. Systemic S. aureus infections inpoultry have been treated successfully with a range of drugs including penicillin, streptomycin, tetracy-clines, erythromycin, novobiocin, sulphonamide,lincomycin and spectinomycin. Reece (1992 ) cau-tioned that antibiotic treatment might limit the spreadof S. aureus in the flock or suppress systemic diseasebefore osteomyelitis develops, but will not curepoultry with advanced lesions of BCO.

The general trend towards increasing resistance toantibiotics is a worrying development in human andveterinary medicine, and S. aureus is among thebacteria where problems have been developing. Inhuman medicine, there has been a dramatic increasein disease associated with methicillin-resistan tstrains of S. aureus worldwide (Kluytmans et al. ,1997 ). The first detection of methicillin-resistance incoagulase-negative staphylococci recovered fromchicken in Japan is of concern to the poultry industry(Kawano et al. , 1996 ) and the situation should bemonitored. The evidence of widespread resistance of S. aureus to antibiotics indicates that sensitivitytesting is essential when treatment regimens arebeing considered for clinical infections.

Response to treatment of staphylococcal diseasesin other species, e.g. bovine mastitis, is poor (Lee,1996 ), and the prevalence of antibiotic resistanceamong clinical isolates of S. aureus from humans ishigh. Human staphylococcal infections and bovinemastitis due to S. aureus are significant concerns, andso vaccine strategies to control staphylococcalinfections are being considered and research is wellunderway (see later and Lee, 1996; Lee & Pier,1997 ).

Prevention and Control of BCO

As discussed in the previous section, the use of antibiotics is unlikely to provide a long-termsolution to the problem of staphylococcal infec-

tions, due to the inherent resistance of the organismand the concerns over increasing development of antibiotic resistance. There have been difficulties intreating and controlling other staphylococcal-asso-ciated diseases, e.g. osteomyelitis and joint infec-tions in humans, and mastitis in cattle. By analogy,the control/prevention of BCO due to S. aureus inchickens will not be straightforward. There arethree possible approaches to control.

Vaccination

To date, the use of vaccines in the fight againststaphylococcal infection in avians has had poorsuccess. Jungherr & Plastridge (1939 ) had nosuccess with an autogenous bacterin of S. aureus toprevent staphylococcal septicaemia and arthritis.Smith (1954 ), using a live S. aureus vaccine, hadonly limited success, and birds inoculated intra-venously with S. aureus developed lesions in boneand in other sites.

Currently, there are intensive efforts to developS. aureus vaccines for the prevention of bovinemastitis and human S. aureus infections. Knowl-edge gained from these studies may be relevant tothe poultry problem. Whole-cell vaccines against S.aureus infection were unsuccessful in rabbit mod-els of catheter-induced endocarditis (Greenberg et al. , 1987 ) and in humans undergoing peritoneal

dialysis (Poole-Warren et al. , 1991 ). Whole-cellvaccines expressing pseudocapsule were foundto provide a significant level of protection tolactating ewes and cows against staphylococcalmastitis (Watson, 1992 ). Encapsulated bacteria areresistant to phagocytosis by leukocytes, antibodiesspecific for the capsule promote phagocytosis of S.aureus by human leukocytes (Karakawa et al. ,1988 ). It has been suggested that capsular poly-saccharides conjugated to protein carrier moleculesmay lead to an improved immune response (Lee,

1996 ). Systemically administered cellular vaccinesare associated with adverse effects and because of this, Lee & Pier (1997 ) suggested that it wasimportant to identify surface-associated antigenswhich can serve as targets for protective antibodies.Adhesion of S. aureus to the surface of the host cellhas been shown to be important in bovine mastitis,and Lee (1996 ) reported on the successful reduc-tion of the incidence of mastitis, through theproduction of antibodies by a subcellular vaccinecontaining fibronectin-binding protein, in a mouse

model of S. aureus mastitis. To date, a cell-surfaceprotein that binds fibronectin has been purified, andthe gene has been cloned and sequenced. The roleof this and other binding proteins in bacterialadherence in vivo could be examined by comparingthe virulence of deletion mutants lacking thespecific binding protein in suitable infection mod-els (Foster, 1991 ). Many problems are likely to beencountered during vaccine development sincehumans with chronic staphylococcal infection such


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as osteomyelitis demonstrate a poor immunologicresponse; furthermore, there is no evidence thatpatients who recover from S. aureus infections areimmune to re-infection (Lee & Pier, 1997 ). Todevelop a successful novel vaccine, a good knowl-edge of virulence factors would be needed, and thisis lacking for poultry.

Using the aerosol model of BCO, the incidenceof BCO was higher in birds exposed to S. hyicusand infected with CAV and IBDV (23.1% ) than inbirds exposed to S. hyicus (9.1% ) without viruses(McNamee, 1998 ).

It is also important to remember that vaccinationagainst S. aureus alone may be of limited use in theprevention of BCO, since other bacteria can causethe condition, and may result in a higher incidenceof BCO due to other opportunistic pathogensparticularly when birds are immunosuppressed.

Management

At the present time, the main bacterium found incases of BCO is S. aureus , accounting for 62 to63% of cases in recent surveys (McNamee 1998;McNamee et al. 1998 ). Using the aerosol model of the disease, McNamee et al. (1999b ) showed thatthe younger the birds were on exposure to S.aureus , the higher the incidence of BCO. McCul-lagh et al. (1998 ) showed that S. aureus could be

recovered in high numbers in hatcheries, so clearlyexposure of chicks in the hatchery represents asignificant risk. Thompson et al. (1980 ) andMcCullagh et al. (1998 ) suggested that appropriatedisinfection and improved hatchery hygiene prac-tices might reduce the levels of initial S. aureuscolonization of chicks. Rodgers et al. (1999 )highlighted the potential importance of hand car-riage in the dissemination of S. aureus in broilerparent farms. Personnel should regularly use abacteriocidal hand-wash, as part of their general

disease biosecurity measures, to limit carriage andspread of S. aureus . Continuous bacteriologicalmonitoring in hatcheries using selective staphylo-coccal media to give early warning of potentialproblems should be carried out (J. McCullagh,personal communication ).

Use of the aerosol model has also shown thatexposure to the immunosuppressive viruses CAVand IBDV increases the incidence of BCO(McNamee et al. , 1999b ). Effective control andprevention strategies for these viruses is therefore

important. It has also been shown that birds hatchedfrom floor eggs have a higher incidence of BCO andbacterial infection, so these should be avoided.

Birds that were growing less quickly had a lowerincidence of BCO, so, theoretically, the incidenceof BCO could be reduced if broilers were notallowed to grow to their maximum potential. Anexamination of the effects of feed restriction atdiffering times in the production cycle may be anarea worthy of further investigation.

Bacterial interference

One strategy that has been successfully used to pre-vent virulent staphylococcal infection both in humannurseries (Kluytmans et al. , 1997 ) and in turkeyflocks (Jensen et al. , 1987 ) is bacterial interference.Bacterial interference, using S. aureus strain 502A,

was used successfully in human nurseries to curtailepidemics of S. aureus infection and to interruptcycles of recurrent furunculosis in adult humans(Aly et al. , 1974 ). Staphylococcus epidermidis hasbeen used successfully when administered by aero-sol, both experimentally and in larger-scale controlprogrammes, to reduce the incidence of S. aureusinfections in turkeys (Meyers & Jensen, 1987; Nicoll& Jensen, 1987b ). Preliminary studies of bacterialinterference to control staphylococcal infections inchickens have also proved successful (Nicoll & Jen-

sen, 1987a ). The mechanism of interference isthought to be a function of both competition by theinterfering bacterium for the same tissue receptorsites and the secretion of a bacteriocin that is bacter-iocidal for S. aureus (Wilkinson & Jensen, 1987 ).This approach is worthy of further investigation as apotential method to control BCO in chickens, andmay have the advantage of interfering with otherbacteria that can cause BCO.

Conclusion

BCO is a significant problem of broilers, and isalmost certainly underdiagnosed. A detailed studyhas shown that it accounts for losses in the region of 0.75% of all broiler placements (McNamee, 1998 ).This was estimated to cost the UK broiler industry 3million per annum. The most common cause of BCO, presently, is S. aureus , but other bacteria mayalso cause the disease. S. aureus has been a difficultpathogen to control in humans and cattle, where italso causes significant problems. There have beenadvances in the knowledge and understanding of fac-tors contributing to infection and disease develop-ment in chickens, which show that it should be possi-ble, by good management practices and biosecurity,to at least reduce the incidence of BCO. Develop-ment of an effective vaccine would appear to besome way off. It should be realized that since otherbacteria can cause the disease, a vaccine against S.aureus will not totally prevent BCO. If other bacteriacan occupy the niche vacated by S. aureus , then theincidence of BCO may not even be reduced; ratherdifferent bacteria will be involved. The use of bacte-rial interference in the prevention of BCO warrantsfurther investigation, and the availability of a diseasemodel using a natural route of infection will facilitatesuch investigations .


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