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Pig Journal Volume: 51
Publication date: June 2003

Refereed Section

VACCINATION RAMIFICATION? AN OBJECTIVE LOOK AT HOW VACCINATION MIGHT AFFECT PMWS AND PDNS
J.D. Mackinnon

Abstract
It has been proposed that immune stimulation plays a pivotal role in the development of clinical post-weaning multisystemic wasting syndrome (PMWS) and porcine dermatitis and nephropathy syndrome (PDNS). More specifically, some researchers have suggested that the vaccination of piglets might act as the major event in this respect. However, in a physiological and clinical sense, it is difficult to single out vaccination as the one important factor because of the gamut of immune modulating agents encountered by piglets post-weaning at a time when their immune system is undergoing rapid expansion. When taken together, limited experiments using both positive and negative intervention in attempts to generate clinical PMWS have given equivocal results. There are distinct differences in the responses to immune stimulation of gnotobiotic pigs and conventional pigs infected with porcine circovirus Type 2 (PCV-2), the agent thought to be responsible for PMWS and PDNS. It has not been possible to reproduce the clinical entity consistently in conventional animals.
???? Field experience does not suggest temporal or epidemiological relationships between the use of vaccines in piglets and the development of PMWS or PDNS. The emergence of PMWS has taken from between one and nine years in countries where piglet vaccination is practised. At farm level, clinical PMWS can precede the introduction of vaccination, it can occur on farms that have never employed vaccination of piglets and it does not (or has not yet) emerged on farms that do use vaccination. Some of the highest mortality rates due to PMWS have in fact been experienced on farms that do not vaccinate, suggesting that there are many other factors to consider in relation to the emergence of clinical PMWS. Controlled studies in the field so far indicate that there are no differences in mortality rates between vaccinated and non-vaccinated piglets due to PMWS in PMWS-positive herds and clinical experience suggests that the control of endemic diseases by vaccination can mitigate the effects of concomitant PMWS.
???? Retrospective investigations have demonstrated that PCV-2 has been present for almost 30 years or more in commercial pig populations, so the question arises of whether there have been changes in PCV-2 virulence or changes in the pig population that have led to the emergence of clinical disease, or both. There is no evidence for significant changes to the virus. Convergence and globalisation of pig breeding may have facilitated genetic susceptibility, either as the removal of immune protection or as an enabling of the virus, or the true cause of PMWS has yet to be identified.

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