Survey of porcine circovirus 2 and postweaning multisystemic wasting syndrome in New South Wales piggeries

Australian Veterinary Journal

Volume 85, No 8, August 2007 © 2007 The AuthorsJournal compilation © 2007 Australian Veterinary Association

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Blackwell Publishing Asia

Survey of porcine circovirus 2 and postweaning multisystemic wasting syndrome in New South Wales piggeries

D FINLAISON, P KIRKLAND, R LUONG and A ROSSElizabeth Macarthur Agricultural Institute, NSW Department of Primary Industries, PMB 8, Camden NSW 2570; [email protected]

Objective

To determine if postweaning multisystemic wastingsyndrome (PMWS) is occurring in the New South Wales pigpopulation and to determine the current and past seropreva-lence of porcine circovirus 2 (PCV2).

Design

Pig veterinarians were contacted seeking submis-sion of tissues from animals with clinical signs suggestive ofPMWS. Samples were also accepted from suspected cases ofporcine dermatitis and nephropathy syndrome (PDNS). Sero-logical studies were also undertaken on archival sera and serasubmitted during the study.

Procedure

Histopathological examination was undertakenon all tissues submitted. The presence of PCV2 was deter-mined by immunohistochemistry. Sera were tested for PCV2using a commercial enzyme linked immunosorbent assay kitmodified for testing of serum samples.

Results

No cases of PMWS were identified during the study.Four cases of PDNS were identified. PCV2 antibody wasdetected in 80% of archival sera from 1995 and 75.8% from2001. Seroprevalence in samples tested during 2002–2003was 87.8%. PCV2 was isolated from tissues of a case ofPDNS.

Conclusion

PCV2 is widespread in the New South Wales pigpopulation and has been since at least 1995. This studydescribes the first isolation of an Australian PCV2. No cases ofPMWS were identified in New South Wales.

Key words:

PMWS, pigs, circovirus, PDNS

Aust Vet J

2007;85:304–310 doi: 10.1111/j.1751-0813.2007.00181.x

IPMA Indirect peroxidase monolayer assayPCV1 Porcine circovirus 1PCV2 Porcine circovirus 2PDNS Porcine dermatitis and nephropathy syndromePMWS Postweaning multisystemic wasting syndrome

TBS Tris buffered saline

I

n 1991, a new disease syndrome was described in young pigsin western Canada.

1,2

This syndrome is now known aspostweaning multisystemic wasting syndrome (PMWS). A

porcine circovirus, now known as porcine circovirus 2 (PCV2),was found in association with cases of PMWS.

3

Despite this,only some animals infected with PCV2 go on to develop PMWS

and the pathogenesis of PMWS has not been fully defined.The roles of the host immune response and other infectiousagents are still being investigated.

4,5

In addition, PCV2 has beenassociated with a number of other porcine diseases includingporcine dermatitis and nephropathy syndrome (PDNS), porcinerespiratory disease complex, reproductive failure, granulomatousenteritis, exudative epidermitis, necrotising lymphadenitis andcongenital tremor.

6

PMWS is a disease of weaner and grower pigs typically seen inanimals at 5 to 12 weeks of age, however older pigs may beaffected. Clinically affected animals exhibit wasting and ill-thrift,with or without dyspnoea or jaundice, and less commonly palloror diarrhoea. Morbidity rates of 4 to 15% have been described withmortality rates of 70 to 80% in affected animals,

7,8

although thedisease in UK has been associated with mortality rates in therange of 8 to 20% in growing animals.

9

Pathologic lesions areseen most consistently in the lymph nodes and may also befound in other lymphoid tissues, lung, kidney, and liver.

9–11

Gross lymphadenopathy is associated microscopically with lym-phoid depletion and histiocytic inflammation. Multinucleategiant cells, and botryoid basophilic intracytoplasmic inclusionbodies in histiocytic cells are seen in a percentage of cases. Similarmicroscopic lesions are seen in other lymphoid tissues. Grossly,lungs fail to collapse due to underlying lymphohistiocytic inter-stitial pneumonia. Liver atrophy and jaundice are observedoccasionally and are associated microscopically with a periportallymphohistiocytic cell infiltrate, hepatocellular vacuolation andswelling, sinusoidal collapse and lobular atrophy. Renal enlarge-ment and pale foci in the cortex are associated with a multifocallymphohistiocytic interstitial nephritis. Diagnosis of PMWSdepends on several criteria being met.

11

Clinically, in the post-weaning age group there should be an increase in wastingand mortality that is not responsive to treatment. In addition,these animals have histologic lesions typical of PMWS and anabundance of PCV2 antigen or nucleic acid in association withthese lesions.

PDNS typically affects animals 12 to 14 weeks of age (range 7 to20 weeks). Clinical cases of PDNS are characterised by cutaneouspetechial/ecchymotic haemorrhages that most commonly affectthe hindlimbs and perineum, although the flanks, ventral

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abdomen, forelimbs, head and ears may also be involved. Affectedanimals are often lethargic and anorexic. Necropsy examinationmay reveal superficial lymphadenopathy with lymph nodehaemorrhage, and renomegaly with white foci and renal petechiae.The diagnosis of PDNS is based on histologic findings, whichinclude glomerulonephritis, interstitial nephritis and systemicnecrotising vasculitis.

9

PCV2 infection has been identified inassociation with cases of PDNS.

12

The pathogenesis of PDNSmay be related to excessive levels of PCV2 antibodies

13

but it hasnot yet been possible to reproduce the syndrome followingexposure of animals to PCV2. PDNS has been described as twoforms in the UK. Originally the condition was seen sporadically(1993 to 1998) and at this time PMWS had not been recordedin the UK pig population. In 1999 a much more severe form wasrecognised with up to 14% morbidity and a case mortality rateof 25 to 30% in 12 to 16-week-old animals. At the same time thefirst cases of PMWS were recognised in the UK often on thosefarms affected with PDNS. In contrast PDNS occurs sporadi-cally in the USA, and a correlation of the occurrence of PMWSwith PDNS has not been observed.

9,14

PCV2 infection is known to occur in the Australian pigpopulation,

15–17

but despite this, no cases of PMWS have beenidentified in Australia to date. PDNS has been reported sporadi-cally in pigs in Australia.

18

The study described in this paper was undertaken during 2002and 2003. The aim was prospectively to seek material from pigswith clinical signs that could be consistent with PMWS, andsubsequently to undertake laboratory investigations to confirmthe occurrence of PMWS in these animals. The scope of thestudy also included laboratory examination of tissues fromcases of PDNS and serological testing for PCV2 of a number ofsera from collections principally obtained from animals inNew South Wales.

Materials and methods

Selection and collection of clinical case material

Specialist pig veterinarians and district veterinarians in NewSouth Wales were contacted seeking cooperation andparticipation in this study. They were provided with a projectoutline and a description of clinical signs and gross pathologythat could be consistent with PMWS and PDNS, and encouragedto have such cases fully investigated. A request for tissuefrom cases of PDNS was included based on its associationwith PCV2.

12,14

To encourage sample collection and submission,participating veterinarians were reimbursed for each submission.The target population was farms in New South Wales with 10 ormore breeding sows. The aim was to sample up to 4 pigs fromeach of 40 farms so that a statistically representative numberof farms were examined and there was a 95% probability ofdetecting at least one farm with a case of PMWS. A number ofsamples were also accepted from Victoria and Queenslandduring the study. For each case, duplicate sets of tissues wererequested, including samples of lung, lymph nodes (bronchial,inguinal and mesenteric), palatine tonsil, liver, spleen, ileum and

kidney. One set of tissues was submitted fresh, while the otherwas submitted fixed in 10% formalin. Serum and faeces werealso requested from each pig. Submission of heart, skin andother organs was requested if any gross changes/abnormalitieswere observed at necropsy. In addition, a description of theclinical and gross findings were requested for each case. Freshtissues, serum and faeces were stored at or below –50

°

C untilexamined.

Histopathology

For histopathological examination formalin-fixed paraffin-embedded tissues were sectioned, and stained with haematoxylinand eosin using standard techniques.

Immunohistochemical examination of tissue sections for PCV2

All tissues submitted during the early stages of the project wereexamined for PCV2 antigen using immunohistochemistry.Tissues submitted from the last two properties were notstained due to the absence of lesions suggestive of PMWS. Themethod was modifed from that of Kennedy et al.

19

The PCV2monoclonal antibody (F217G) was supplied by Dr G Allan(DARD, Belfast, Northern Ireland). All rinses were performedwith 5 mM Tris buffered saline, pH = 7.6. Lymph node sectionsfrom a pig from Northern Ireland with PMWS and abundantPCV2 antigen were used as a positive controls. The primaryantiserum was replaced with an irrelevant monoclonal antibodyto provide a negative control.

Serology

A total of 1114 pig sera from New South Wales in archivalcollections were tested for PCV2 antibody. Details of ages ofanimals, the origin and year of collection are listed in Table 1. Inaddition, 87 sera collected during the period of the current studywere also tested. These samples were collected as part of thePMWS study or were received as diagnostic samples at thelaboratory during the study period. Ages of these animals andtheir origin are described in Table 2. One hundred and five serafrom feral pigs were also tested, and all but 6 of these were fromferal pig populations in the Northern Territory (Table 3). Allsamples were tested for antibody by a commercial ELISA kit(SERELISA Circo AB Mono Blocking, Synbiotics, Lyon, France),using a specific monoclonal antibody against PCV2. This testwas developed for testing of faecal samples but was used in thisstudy to test sera at a dilution of 1/10. Reference ranges forserum samples tested in this kit were determined by testing 246samples in both the ELISA and an indirect peroxidase monolayerassay (IPMA). Results for a batch of tests were accepted if theresults for the positive and negative controls, included within thekit, met the kit guidelines.

The IPMA was undertaken in 96-well plates using PCV1 freePK15 cells (PK15A) inoculated with PCV2 (both supplied byDr G Allan, DARD, Belfast, Northern Ireland). The targetproportion of infected cells in a monolayer was 10%. The plateswere incubated at 37

°

C in an atmosphere of 5% CO

2

forapproximately 48 h until a complete monolayer had formed.


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The supernatant was then removed and the monolayer washedtwice with phosphate buffered saline, pH = 7.2 (PBS). The plateswere then air dried, sealed by wrapping in plastic film and storedat

−

20

°

C until required. For use, stored plates were broughtto room temperature and fixed by adding 3% formaldehyde

(100

µ

L/well) for 10 min at room temperature. The formalde-hyde was removed and the plates washed four times in purifiedwater containing 0.05% TWEEN® 20. Serum dilutions (1/10,1/100, 1/1000 and 1/10,000) were prepared in 0.01M PBS with10% horse serum and 0.05% TWEEN® 20. Diluted samples

Table 1. PCV2 ELISA results for archival pig sera from New South Wales.

Population Year Negative total (%)

Inconclusive total (%)

Positive total (%)

Total Comments

Culled sows 1995 12 (6) 29 (14) 167 (80) 208 5 per farm

8-week-old pigs 1998a 5 (25) 7 (35) 8 (40) 20

12-week-old pigs 1998a 1 (5) 5 (25) 14 (70) 20

16-week-old pigs 1998a 0 (0) 5 (25) 15 (75) 20

20-week-old pigs 1998a 1 (5) 0 (0) 19 (95) 20

24-week-old pigs 1998a 1 (5) 2 (10) 17 (85) 20

28-week-old pigs 1998a 0 (0) 2 (10) 17 (90) 19

Sows 1998a 0 (0) 0 (0) 10 (100) 10

Slaughter age animals 2001 78 (10) 110 (14) 589 (76) 777 Approx. 30 per farm

aCross-sectional survey of a farm

Table 2. PCV2 ELISA results for samples collected during 2002–2003.

Population State Negative total (%)

Inconclusive total (%)

Positive total (%)

Total Comments

Unsuckled piglets NSW 0 (0) 0 (0) 0 (0) 12 Diagnostic samplesa

Weaner pigs NSW 0 (0) 2 (14) 12 (86) 14 PMWS study

Grower pigs VIC 0 (0) 3 (100) 0 (0) 3 PMWS study: PDNS cases

Sows and gilts NSW 1 (4) 2 (8) 21 (88) 24 Diagnostic samplesa

Parity 2 sows VIC 1 (5) 0 (0) 19 (95) 20 Diagnostic samplesb

18 week gilts VIC 0 (0) 1 (7) 13 (93) 14 Diagnostic samplesb

aSamples are from same farmbSamples are from same company

Table 3. PCV2 ELISA results for feral pig populations.

Animal location State Negative total % Inconclusive total % Positive total % Total

Kakadu National Park (2002) NT 19 95 1 5 0 0 20

Ngukurr region (2002) NT 20 100 0 0 0 0 20

Ramingining region (2002) NT 14 70 1 5 5 25 20

Twin Hills Station (2002) NT 13 69 1 5 5 26 19

Maningrida region (2002) NT 4 20 2 10 14 70 20

Toongabbie (2002) NSW 0 0 0 0 1 100 1

Lurnea (2003)a NSW 0 0 0 0 4 100 4

Leumeah (2003) NSW 0 0 0 0 1 100 1

Total 70 67 5 5 30 28 105

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were added to the plates (50

µ

L/well) and incubated on thePCV2 infected cells for 1 h at 37

°

C. After washing plates fourtimes with PBS containing 0.05% TWEEN® 20, a goat anti-swine horseradish peroxidase conjugate (Jackson ImmunologicalsWest Grove, PA) was added (50

µ

L/well) and the plates wereincubated for a further 1 h at 37

°

C. Plates were washed againfour times in PBS containing 0.05% TWEEN® 20 and theenzyme substrate, AEC, (Zymed Laboratories, South SanFrancisco, CA) was added (50

µ

L/well) for 10 min at roomtemperature in the dark. The PCV2 titre of a test serum wasdetermined by examining the cell monolayer for presence of specificstaining of PCV2 infected cells. Cell staining was typicallycytoplasmic although nuclear staining was also observed. Theendpoint was the highest dilution of serum at which any typicalstaining was observed.

Virus isolation

Virus isolation was attempted on inguinal lymph node, spleenand kidney from a case of PDNS. Tissues were homogenised inminimum essential medium containing streptomycin andpenicillin to make approximately a 10% suspension. Homogenateswere frozen and thawed once and clarified by centrifugation at2000

g

for 30 min. The supernatant was filtered through a0.45

µ

m and 0.2

µ

m filter prior to inoculation onto cell culture.Isolation was carried out based on the method described by Allanet al.

20

Briefly, 0.5 mL of supernatant was added to 18 mL ofPK15A cells (4

×

10

5

cells/mL) suspended in minimum essentialmedium, 10% foetal calf serum, non-essential amino acids,sodium pyruvate, streptomycin and penicillin. Six mL aliquotsof the inoculated cell suspension were used to seed 2

×

25 cm

2

cell culture flasks and the remainder was used to seed wells ina 96-well plate (150

µ

L/well). These cultures were incubatedfor approximately 18 h at 37

°

C in 5% CO

2

. The medium wasremoved from the resulting semi-confluent monolayers, whichwere then treated by adding 2 mL per flask or 50

µ

L per well ofpreheated (37

°

C) 300 mM D(+)-glucosamine (Sigma) in Hanksbalanced salt solution (HBSS). After 30 min incubation at37

°

C this solution was removed and replaced with the previouslydescribed growth medium. The cells were incubated for a further48 to 72 h after which one flask was frozen and thawed threetimes. The cells were removed from the second flask bytreatment with a trypsin /versene (0.5%/0.5%) solution. Thesepassaged cells were mixed with 1 mL of the freeze thaw lysate per5 mL of cell suspension and new flasks and a plate seeded asalready described. The 96-well plates were processed as describedfor IPMA but stained with a PCV2 monoclonal antibody(F217G) at a dilution of 1/1000 and using a rabbit anti-mouseHRP conjugate (DAKO®, DAKO corporation, Carpinteria,California) to determine if PCV2 antigen was present.

Results

Specimen submission

Submissions were received from 10 properties betweenSeptember 2002 and June 2003. Due to the low submissionrate from New South Wales, samples were also accepted from

interstate. Six of the properties were located in New South Wales,three in Victoria and one in Queensland. Specimens were availablefor examination from 22 pigs. In 18 of the 22 cases, specimenswere collected from pigs that had clinical signs that may havebeen consistent with PMWS (New South Wales and Queenslandproperties only). These cases were obtained from seven differentfarms. The remaining cases from three different Victorian farmswere animals exhibiting clinical signs suggestive of PDNS.

Weight loss was the most common (12/18) clinical or gross findingdescribed for pigs that were submitted to this study as possiblecases of PMWS. Other findings included pneumonia (6/18),lymph node enlargement (5/18), jaundice or liver pathology (3/18),diarrhoea (3/18), inflamed intestines (3/18), pallor (1/18), palemottled kidneys (1/18), polyserositis (1/18) and pleurisy (1/18).Dyspnoea was not described by any of the submitting veterinarians.The ages of these animals were 4 to 10 weeks of age.

Tissues from four pigs with clinical signs suggestive of PDNSwere submitted. These animals were 12 and 15 weeks of age.Clinically, red erythematous skin lesions around the head, flanksand legs were seen. Two cases had renal enlargement and one casehad pale kidneys. Inguinal lymph node enlargement was notedin two cases of which one case was described to have markedinguinal lymph node enlargement. Only one case was noted tohave wasting. Animals with these clinical signs were reported asbeing seen on an occasional basis in the affected herds.

Histopathological changes

Tissues submitted from five of the seven farms with pigs thatmay have been clinically consistent with PMWS had nohistological lesions consistent with those seen in PMWS. For theremaining two properties some or all of the submitted cases hadsome histopathological observations consistent with (but notspecific for) PMWS. These lesions consisted of mild lymphoiddepletion and non-suppurative interstitial pneumonia.

Histopathology resulted in an aetiological diagnosis consistentwith PDNS in all four cases with suggestive clinical signs. Mor-phological changes consistent with a diagnosis of PDNS wereobserved in the skin, lymph nodes and kidneys of these animals.

Immunohistochemistry for PCV2

Immunohistochemistry failed to detect PCV2 antigen in allsections examined from cases with clinical signs that may havebeen suggestive of PMWS. In the lymph node submitted fromone of the four animals with lesions consistent with PDNS, asmall number of positive staining cells were identified. Thesecells were observed in necrotic lymphoid follicles and hadmorphology consistent with histiocytes. PCV2 antigen was notobserved in the other tissues submitted from this animal (skin,spleen, intestine, kidney and liver). Strong staining was observedin the reference material from Northern Ireland.

Serology

The results of evaluation of the antibody ELISA against theIPMA are summarised in Table 4. The ELISA kit was designed


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for detection of PCV2 antibody in faecal samples andcomparison of the ELISA results with the IPMA results showedthat the manufacturers test interpretation values had to bereassessed for serum samples. The following reference rangeswere used for interpretation of results for PCV2 antibodythroughout this study:

Positive for PCV2 antibody: percentage inhibition greater thanor equal to 80%

Inconclusive for PCV2 antibody: percentage inhibition greaterthan or equal to 60% and less than 80%

Negative for PCV2 antibody: percentage inhibition lessthan 60%.

These reference ranges are comparable with those that have beenindependently determined and subsequently recommended bythe manufacturer for this kit.

Using these reference ranges the sensitivity of the test was 90% andthe specificity was 96% when compared with the IPMA results.

Prevalence of PCV2 antibodies in archival sera

Three collections of archival sera were tested and the results aresummarised in Table 1. The culled sow sera from 1995 and thesera collected from slaughter age animals in 2001 provide anoverview of the PCV2 antibody status in New South Wales pigpopulations at these times. Individual farm results were notavailable for the 1995 collection. The seroprevalence of PCV2antibodies in this population was 80%. The 2001 sampleswere collected from 27 piggeries. In 2001, the seroprevalencewas 75.8%. Only two of the herds in this collection showedsignificant numbers of negative animals. One farm was completelyseronegative (30 animals sampled) and a second had one positiveand four inconclusive animals (30 animals sampled). Theaverage proportion of seropositive animals when these two farmsare excluded was 83% (range 43% to 100%; median 86.7%).The third collection of archival sera was a cross-sectional surveyfrom a single farm. Forty percent of 8-week-old pigs were positive

for PCV2 antibody. The percentage of seropositive animalsincreased at each sampling time point. One hundred percent ofthe sow population sampled were seropositive.

Prevalence of PCV2 antibodies in sera collected during 2002–2003

The results for these samples are summarised in Table 2. Allsamples submitted as part of the PMWS study gave positive orinconclusive results for PCV2 antibody. A group of parity 2 sowsand 18-week-old gilts from a Victorian herd had a seroprevalenceof PCV2 antibody greater than 90%. A second group of sowsand gilts from New South Wales also sampled in 2003 had aseroprevalence of PCV2 antibody greater than 85%. A smallnumber of unsuckled piglets from this farm were all found tobe seronegative.

Prevalence of PCV2 antibodies in feral pig populations

The results from the feral pig populations are summarised inTable 3. In 2 populations from the Northern Territory noseropositive animals were identified, a further 2 had a lowseroprevalence, and the final population in the Maningridaregion had a seroprevalence similar to that seen in commercialpig populations. All feral animals sampled from New SouthWales were seropositive.

Virus isolation

PCV2 was isolated from the inguinal lymph node of the animalin which PCV2 antigen was observed by immunohistochemistryafter four passages, and from the kidney and spleen after sixpassages in cell culture.

Discussion

No cases of PMWS were identified by histopathology during thisstudy. The failure to identify PCV2 by immunohistochemistryin these animals supports the conclusion that PMWS was notoccurring in these herds. Unfortunately we were unable to reachthe target number of animals and tissues were only availablefrom a small number of animals. However, communicationswith pig veterinarians practicing in New South Wales at the timesuggested clinical signs of suggestive of PMWS were not beingseen. Therefore PMWS was either absent or may have been beoccurring at a frequency in the New South Wales pig populationthat was too low to be detected with our sample numbers,despite the fact that case material was specifically targetedfrom animals with clinical signs that may have been consistentwith PMWS.

Four cases of PDNS were identified during the study, all inVictorian animals. Animals with this clinical syndrome werereported as being observed occasionally by the submitting veter-inarian. Although no direct link between PDNS and PMWS hasbeen recognised, the apparent association between PDNS andPMWS in the UK is of concern for Australia. In the UK, epidemicPDNS appears to be a common sequela in herds affected byPMWS. However, PDNS cases in Australia appear only to besporadic in nature, which is typical of the situation in the USA.

Table 4. Comparison of the results of 246 sera tested by both IPMA andELISA.

ELISA PIa IPMA antibody titre

Negative 10 100 1000 10,000 Total

> 80 1 0 9 43 137 190

70–80 7 0 3 4 10 24

60–70 3 0 0 1 3 7

50–60 3 0 0 0 1 4

< 50 21 0 0 0 0 21

Total 35 0 12 48 151 246

aPercentage inhibition.


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PDNS has yet to be reproduced in an experimental setting, andtherefore its pathogenesis and the role of PCV2 as an aetiologicalagent is yet to be confirmed. Our study did identify PCV2 byimmunohistochemistry and PCR (results not included) in casesof PDNS, which is consistent with previous studies in the litera-ture.

12

As some researchers are now classifying PDNS as part ofthe group of syndromes associated with PCV2 infection, thissuggests that monitoring this syndrome should continue in casethere is a change in the nature of its occurrence. That is, anyincrease from a sporadic occurrence may be a cause for concern.

The results from the IMPA for PCV2 antibody indicate that pigsdevelop high titres of antibodies against PCV2. In the collectionof samples examined, 94% of seropositive animals had titresgreater than or equal to 1000. Antibody titres raised againstPCV1 are reported to be low (< 1000) (F McNeilly personalcommunication). The high titres detected against PCV2 suggestthat, as the IPMA was undertaken using PCV2 infected cells, falsepositive results due to cross-reactivity with PCV1 are unlikely.The ELISA results correlated well with the IPMA results. Thisindicates that the ELISA is a sufficiently sensitive test but alsosupports the suggestion that the high antibody titres againstPCV2 that we observed in the IPMA are not the result of cross-reactive PCV1 antibody as the blocking ELISA is PCV2 specific.

All three sera collected from PDNS cases gave inconclusiveresults in the PCV2 antibody ELISA, and these three sera were nottested in the IPMA. There is evidence that PDNS is associatedwith extremely high PCV2 antibody titres.

13

The inconclusiveresults by ELISA are not suggestive of very high antibody titres,although the sera examined by Wellenberg et al

13

were testedby IPMA.

The serological results from the archival sera collected in 2001suggest that exposure to PCV2 is widespread in the New SouthWales pig population, with the exception of the occasional unex-posed herd. In addition, the seroprevalence of PCV2 antibodiesin New South Wales animals in 1995 was 80% and this suggeststhat PCV2 has been widespread in the New South Wales pigpopulation for some time. Our results suggest a much higherexposure to PCV2 in New South Wales compared with the studyundertaken by Raye et al,

17

where PCV was detected in approxi-mately 30% of Western Australian pigs tested. However, the testused for the Western Australian study was an indirect immun-ofluorescence assay using PCV1 infected cells and is likely tohave significantly underestimated the seroprevalence of PCV2.Although only a limited number of animals from Victorian herdswere sampled, the results also indicate a high prevalence. Theseserological results are comparable with studies undertaken inCanada,

21

Northern Ireland

22

and the USA.

23

Despite reports of inutero infections with PCV2, we did not identify any PCV2 anti-body in the limited number of unsuckled piglets sampled.

24,25

The serology results for the feral pig populations in the NorthernTerritory suggest that PCV2 is not well established in all remoteferal pig populations in Australia. These findings contrast withthe small number of feral pig sera from New South Wales that

were tested, where all samples were seropositive. These animalswere all in urban fringe areas of greater Sydney. The feral pigssampled from the Northern Territory have either had limitedexposure to PCV2 or transmission is significantly reduced in thisextensive environment compared with intensive housing. Theextent of feral pig culling in each of these regions is not known.This is comparable with the situation in European wild boar inSpain, where the seroprevalence and titre of PCV2 antibodies werecorrelated to the management of the wild boar populations.

26

The prevalence of PCV2 antibodies was higher in intensivelymanaged, farm-like populations compared with wild boars free-living in more natural, open and low density populations.

The serological profile across several age groups within one herdindicated an increase in the number of seropositive animals withage, although in some of the older age groups seronegativeanimals still remain. These results suggest maternal antibody isbeing lost around 8 weeks of age and that seroconversion isoccurring at approximately 12 to 16 weeks of age.

In conclusion, the serologic data suggest that PCV2 is wide-spread in the New South Wales pig population and has beensince at least 1995. This study describes the first propagation ofan Australian PCV2 isolate in cell culture. Most importantly,this study was not able to identify any cases of PMWS in NewSouth Wales.

Acknowledgments

This project was funded by New South Wales Agriculture andAustralian Pork Limited, Project number 1840. We gratefullyacknowledge provision of reagents, methods and advice from GAllan, F McNeilly and S Kennedy (Department of Agricultureand Rural Development, Northern Ireland). We thank thoseveterinarians who submitted samples for the study: E Bunker, HDunlop, S Hobson, G Ireland, R Smith, B Squire, A Thompson,M White, and R Wilson, and L Melville for providing the feralpig sera from the Northern Territory. We thank M Frost fortechnical assistance.

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Documents

Survey of porcine circovirus 2 and postweaning multisystemic wasting syndrome in New South Wales piggeries