A more integrated approach to pig research is needed if conditions such as post-weaning multi-systemic wasting syndrome (PMWS) are to be avoided in future, Francois Madec of France’s Food Safety Agency told delegates at the Alltech Symposium, Lexington, Kentucky.
“Despite a number of scientific reviews, PMWS and porcine circovirus type 2 (PCV2) remain puzzling problems. There is growing evidence that PCV2 is an associated cause of PMWS. However, additional circumstances are needed to bring about clearly-defined clinical disease.
“These circumstances are largely related to herd management, such as breeding and feeding strategies. Developments in these areas have been effective regarding their specific goals, but they may have collateral, detrimental consequences, sometimes delayed in time. This makes a co-ordinated approach to research in these areas essential.”
Explaining how PMWS occurred, Dr Madec said suddenness of outbreak was a common feature across the world. “In well-performing farrow-to-finish units, high losses suddenly occurred due to wasting in growing pigs without any perceivable premonitory clinical signs at any other stage of the herds.
“There are five basic hypothesis of where PMWS came from, each of them valid in some way,” he said.
First, PCV2 changed, so PMWS is induced by a new specific virulent strain. Second, PCV2 didn’t change, but there is another new or presently unknown pathogen involved.
Third, pigs were exposed to something new of a non-infectious nature, but which triggered PCV2 replication through immunomodulation and/or modification of the virus environment within the target cells. This new factor has been spreading through trade.
Fourth, management and husbandry have been changing and these changes in breeding herds may have also interfered with the routes of PCV2 spread. And finally, the genetic background of pigs changed, through reduced variability as a result of targeted breeding programmes and increased susceptibility of certain lines.
“Hypotheses one and two suppose the pathogen has only spread in countries where PMWS occurred. But, sudden onset is more compatible with hypothesis three, such an exposure would break the pre-existing more or less fragile balance involving PCV2.
“The quantitative clinical expression of PMWS, depending on farm and also on the batch in farrow-to-finish systems, is most in favour of hypothesis four. The immune system is affected in this disease and logically, poor hygiene and shortcomings in housing and husbandry contribute to the burden of secondary pathogens.”
But the most probable scenario in PMWS emergence still remains to be established, despite most of the pieces of the puzzle now being in place. That PCV2 plays a pivotal role is agreed, but the crucial question is why PCV2 suddenly became so influential on health.
“Additionally, it seems that PMWS clinical disease at both herd and pig level is a yes-or-no phenomenon, pigs or herds either get it or don’t, there is no middle ground. PCV2 is a necessary condition, but other factors are important at different stages.”