PROGRAMME - British Society of Animal Science

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Aetiology, pathogenesis and immunology of post-weaning multi-systemic wasting syndrome in pigs:
genetic environmental interaction. Results so far
D.Werling 1 , P. Alarcon 1 , C. Browne 1 , T. Demmers 1 , S. Gauss 1 , D.Armstrong 2 , D. Pfeiffer 1, A.Nevel 1 , B.Schmidt 1 , C.Wathes 1 ,
M. Velasova 1 , B.Wieland 1
1 Royal Veterinary College, London, UK, 2 BPEX, Stoneleigh Park, Kenilworth, UK Email: dwerling@rvc.ac.uk
Background Post-weaning multi-systemic wasting syndrome (PMWS) causes major economic losses for the world-wide pig
industry and severity of clinical signs and economic impact vary considerably between affected farms. PMWS emerged as a
significant disease affecting pig production in the 1990s although the causal agent, porcine circovirus type 2 (PCV2), and the
disease itself, had existed in swine for many years prior to this. In the United Kingdom the first outbreaks of the disease were
reported in Northern Ireland in 1997 and in the south east of England in late 1999. Subsequently, the disease moved northward
and reached Scotland in 2002. A telephone survey with veterinary practitioners in England and Wales indicated that between
18 and 20% of the holdings were affected by PMWS and PDNS by 2000 and a retrospective cohort study in England and
Wales resulted in 83% of reporting British farmers having observed clinical signs of PMWS in their farm between 2000 and
the end of 2003. Reports based on diagnostic sample submissions indicate a peak of the epidemic in 2002–2003 with
significant decrease in the following years. During these early years of the epidemic stage, the cost of the disease has been
estimated at £35 million per year for the United Kingdom and between €562 and 900 million across the European Union.
PCV2 is a small and rather simply organized virus and expresses only few proteins. Therefore, it was expected that the
factor(s) triggering PMWS should be easily identified, but more than one decade of PCV research has not yet singled out a
molecule inducing the disease onset. Typically, the disease affects pigs aged between 8 and 15 weeks and the most frequent
clinical signs are wasting, skin pallor, respiratory distress, diarrhoea, jaundice and enlarged lymph nodes. Economic losses are
caused through increase in post-weaning mortality, reduced growth rates, increased feed conversion ratios, and secondary
infections resulting in increased management and treatment costs. However, since its first occurrence, the disease appears to
have evolved from epidemic to endemic status and farmers and veterinary practitioners have reported changes in the clinical
pictures and severity of PMWS observed on farms. The highly beneficial effects of the PCV2 vaccines, which have been in use
since 2008, highlight that the role of subclinical disease might have been underestimated over the last five years. Even farms
that had minor PMWS problems seem to have been able to significantly increase their overall productivity since PCV2
vaccination.
The present paper gives a progress report on aspects of a collaborative project aimed to understand the current PMWS/PCV2
situation in England, to identify risk-factors associated with the outbreak of PMWS on farms, to potentially identify key-genes
being affected by the virus in the host, and to assess the genetic as well as the environmental factors potentially contributing to
the occurrence of PMWS. Unravelling the molecular features of PCV and the channels through which the virus interacts with
its host are key to manage, prevent and treat PMWS and other PCV-associated diseases.
Methods An initial investigation into samples obtained and stored during an outbreak of PMWS on a large commercial farm
showed that the virus was present on the farm a long time before the actual outbreak of the disease (1). The investigations also
identified a potential genetic component as well as indicated a switch from a PCV2a to a PCV2b genotype of the virus.
Complementary strategies of epidemiology, immunology and animal husbandry were applied to identify on-farm risk factors
for PMWS using a cross-sectional study on 147 farms. Nearly 6000 blood-samples were analysed for a variety of porcine
pathogens, as well as post-mortems performed to verify the presence of PCV2 (2). A microarray approach was applied to
assess the changes induced by the current UK PCV2 isolate on different immune cell subsets as well as bone-marrow cells.
The information derived in part 1 (Epidemiology) and part 2 (Immunology) of the project then informed part 3 (reproduction
of disease under controlled conditions).
Results to date: There is no doubt that PCV2 is endemic in the UK, with 99.8% of farms being tested positive for PCV2 (2).
After developing a new scoring scheme for PMWS-affected farms (3) as well as identifying risk factors on farms for the
occurrence of PMWS (4), we were able to show that the virus is not inducing a pathogen specific signature in infected cell
types. Indeed, PCV2 is inducing very cell specific changes, all mounting in a suppression of immune responses, pot6entially
pathing the field for secondary infections (Schmidt et al., in preparation). The first trials to reproduce the disease under
controlled conditions, using the risk-factor identified are currently on their way, and the first results indicate the successful
reproduction of PMWS. Available results will be presented and discussed at the meeting.
Acknowledgements This project is funded by the BBSRC CEDFAS Initiative, with industrial support from Pfizer Animal
Health, BioBest Ltd. And BPEX.
References
1. Porcine circovirus type 2 (PCV2) infection before and during an outbreak of post-weaning multi-systemic wasting
syndrome (PMWS) on a pig farm in the UK. Wieland, B., Werling, D., Nevel, A.,
19
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T.G., Cook A., Done S.H., Armstrong D., and Wathes, C.M. Vet Record, in press
2. Prevalence of endemic pig diseases in England: an overview six months into a large-scale cross-sectional study on Post-
Weaning Multi-systemic Wasting Syndrome (PMWS). B. Wieland, P. Alarcon, M. Velasova, A. Nevel, H. Towrie, D.
Pfeiffer, C. Wathes, D. Werling. The Pig Journal 2010; 63: 20-23
3. Assessment and quantification of post-weaning multi-systemic wasting syndrome severity at farm level. Alarcon P,
Velasova M, Werling D, Stärk KD, Chang YM, Nevel A, Pfeiffer DU, Wieland B. Prev Vet Med. 2011 Jan 1;98(1):19-28.
4. Farm level risk factors associated with severity of post-weaning multi-systemic wasting syndrome. Alarcon P, Velasova
M, Mastin A, Nevel A, Stärk KD, Wieland B. Prev Vet Med. 2011 Sep 1;101(3-4):182-91.