Risk factors causing postweaning multisystemic wasting syndrome (PMWS) onset in Swiss pig farms.

Postweaning multisystemic wasting syndrome (PMWS) was epizoozic between 2003 and 2008 in Switzerland. Nevertheless, infectious risk factors including porcine reproductive and respiratory syndrome virus (PRRSV) were missing at all or were seen only sporadically (enzootic pneumonia and actinobazillosis). In a case-control study, 30 farms with PMWS affected pigs were compared to 30 inconspicious farms ("matched pairs"). The case-control allocation was verified by PCV2 DNA measurements of 5 healthy weaned pigs in each control farm, 5 healthy and 5 PMWS affected weaners in each PMWS affected farm. Diseased pigs showed in average 1.8x10(8) DNA templates per ml serum significantly higher than healthy pigs from control farms with 1x10(6) DNA templates per ml serum. Virus load in healthy pigs did not differ between control- and PMWS affected farms. PMWS mainly emerged among affected pigs in the 5th to 8th week of age. In a logistic regression model risk factors were identified such as high occupancy in weaning pens (p = 0.002), large groups in gestation facilities (p = 0.03) as well as reduced birth weight < 1.3 kg (p = 0.04). We suggest these factors might have lead to chronic stress e.g. through influencing negatively social interaction in pigs or disturbances of the maturing immune system. Heavy fly and rodent infestation might not only be viewed as a vector for disease transmission, but, also as a stress factor.