Post-weaning Multisystemic Wasting Syndrome (PMWS) in pigs: an attempt to bring together the pieces of the puzzle

PMWS was first described in Canada (Harding 1996, Clark 1997) and soon after, the condition was recognized in several other countries: the USA (Daft et al 1996, Sorden et al 2000) and the EU where reports came from Spain (Segalès et al 1997), France (Le Cann et al 1997) and the UK-Northern-Ireland (Kennedy et al 1998). At the initial stage, a limited number of PMWS-affected herds were detected in each country but whilst their number increased, the disease was recognized in several other countries throughout the world like in 1998 in Korea and Japan (Lyoo et al 1999, Sato et al 2001), in 1999 in Taiwan (Chen Shihping et al 2001), in 2000 in Switzerland, Poland and Denmark (Borel et al 2001, Pejzak et al 2001, Vigre et al 2005). Later on, in 2003 it was diagnosed in other countries like NewZealand, Sweden and Norway (Stone 2004, Wallgren et al 2004, Brunborg et al 2004). During 2005 and 2006 numerous severe cases were reported in Canada (Ellis et al 2006). Within a few years, PMWS has become a worldwide animal health main concern to the pig industry and a real challenge to the veterinary science community. A small single stranded DNA virus (Porcine Type 2 Circovirus, PCV2) was rapidly found in the damaged tissues of affected pigs and suggested to play a pivotal role in disease onset through its impact on the immune system (Krakowka et al 2001). However as soon as the laboratory tools adequately tailored to PCV2 detection on large scale were available, it became evident that the virus was widespread and found as well in PMWS-affected as in non affected farms ; Additionally, retrospective studies consistently showed that it was so since decades far before PMWS was recognized and reported (Mesu et al 2000, Magar et al 2000). Despite differences were recently found in RFLP patterns of PCV2 isolates in Canada, to date there is no clear evidence of major genomic differences in PCV2 viruses recovered from pigs with and without PMWS that could explain expression or not of the disease (Larochelle et al 2003, Griesen et al 2004, De Boisseson et al 2004, Ellis et al 2006). Recently the role of retroviruses was assessed. Retroviramia was found associated to pigs sampled in PMWS-affected farms (Tucker et al 2006). However to date PCV2 still remains considered as the principal infectious cause of PMWS. In the meantime epidemiological studies were carried out trying to get more knowledge about the on-farm conditions leading-predisposing to clinical PMWS, whereas an important laboratory research effort was directed at a better understanding of the pathological mechanism incurred in PMWS with an emphasis placed on the immune system. The present paper is an attempt to bring together the main aspects of current available knowledge that could help the farm managers to avoid the most detrimental consequences of PMWS. Therefore it is restricted to those points assumed to have a practical interest to herd health maintenance regarding PMWS, in relation to management and husbandry.