Post-weaning multisystemic wasting syndrome (PMWS) has been with us in the UK since 1999. The initial per-acute disease, causing high mortality in weaners and growers of 30% has subsided to the chronic form, which we see today, where mortality, associated with the disease is reported to range from 0-9% but has moved primarily into older pigs in the late grower or finisher stages (see Graph 1). The disease spread across the UK and the rest of Europe finally reaching Denmark in 2002. Although the initial reports of the disease came from Canada in the 1990's, it appeared to die down and it is only since 2004 it has become a severe problem in both the US and Canada.

Graph 1. Effects of PMWS (PCV-2) and PRRS mixed infection in a UK finishing shed

(from Burch and others, 2006)

The disease appeared to be caused by porcine circovirus type 2 (PCV-2), but the virus had been present in herds almost as far as we have records. The most likely reason for the change in the disease is due to a mutation of the virus and there is some evidence to support this but it is not completely conclusive. The disease has spread like a new virus infection and it has been shown to spread from direct pig-to-pig contact, via the faeces and urine and also by semen. The virus causes a viraemia so the virus spreads around the pig's body in the blood. It attacks the lymphoid immune system, especially destroying the lymph nodes and is found in profusion in the organs it has affected, including the lung, intestines, kidney and liver. The virus initiates its own damaging effect and may be complicated by other infectious agents. This has led to much confusion and controversy about the cause of the disease, as the PCV-2 viraemia may occur concurrently with other infections e.g. PRRS virus (see Graph 1), Mycoplasma hyopneumoniae, even Salmonella enterica Typhimurium and may exacerbate ZAP scores.

The UK national herd has basically been challenged with the PCV-2 infection and the level of disease has declined from the acute phase to a more chronic phase. It is therefore likely that the sows have now built up some immunity to the disease and this has been passed to the piglets via the maternally derived antibodies (MDA) in the colostrum. It has been shown that the PCV-2 virus challenge is reduced by even low levels of MDAs (AB titres >0.2) and so even have live vaccines (Thomas and others, 2005) which has impacted their introduction. Additionally, high levels of MDAs will protect pigs beyond 9 weeks of age (see Graph 2).

Graph 2. Effects of maternally derived antibodies on PCV-2 challenges at 3 & 9 weeks of age in three-week-old piglets
 

(after Thomas and others, 2005)

In Europe, Merial produced a killed PCV-2 vaccine for sows and has been provisionally licensed in a few countries. This product can be imported by Special Import Certificate now, into the UK. In N. America, they also have 3 additional vaccines for use in young pigs of 3-4 weeks of age (see Table 1).

Table 1. Table 1. PCV-2 Vaccines introduced (some under special license) in N. America

The sow vaccine is expected to increase protection in young pigs via MDAs but the duration of effect in the pig is unsure, especially once they are in the finishing shed. Potentially, if they receive a late challenge in finishing they will be susceptible. This is the possible situation in the UK, where we have a large number of straw-based solid-floor systems where there is a high level of faecal contamination, which appears to aid the transmission of the infectious challenge (Scott and others, 2006). On slatted-floor systems the challenge is less, as the faecal exposure is generally less but the challenge may be seen even later.

The PCV-2 vaccines for piglets are not expected in the UK for some time, although early trial reports from N. America suggest they offer outstanding protection for the whole life of the pig and are proving highly effective in reducing mortality and improving performance.

 

References:
Burch, D.G.S., Webster, G.I.A., Morgan, M., McDonald, M. and Klein, U. (2006) Comparative efficacy of Tiamutin and Lincospectin in the drinking water for the treatment of mixed enteric and respiratory infections in finishing pigs. Proceedings of the 19th International Pig Veterinary Society Congress, Copenhagen, Denmark, 2, p 343

Scott, K., Chennells, D.J., Campbell, F.M., Hunt, B., Armstrong, D., Taylor, L., Gill, B.P. and Edwards, S.A. (2006) The welfare of finishing pigs in two contrasting housing systems: fully slatted versus straw-bedded accommodation. Livestock Science, 103, 104-115

Thomas, P., Opriessnig, T., McKeown, N., Meng, X-J., and Halbur, P.G. (2005) Effect of PCV2 passive antibody levels on immunization with chimeric PCV1-2 vaccine and challenge with wild-type PCV2. Proceedings of the American Association of Swine Veterinarians Meeting, Toronto, Ontario, Canada, pp 23-25

 

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Pig Diseases & Medication:  Pig Technical Reviews

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