Postweaning Multisystemic Wasting Syndrom (PMWS) became epizootic in Switzerland in 2003 to 2008 although infectious risk factors including Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) are missing or play a minor role since the systematic eradication of enzootic pneumonia and actinobazillosis occurred in Switzerland. A superstimulation of the immune system was also not the trigger as it is forbidden to immunize against PRRSV, M hyopneumoniae and A. pleuropneumoniae. Additionally, no changes took place in husbandry, feeding, management and genetic of the Swiss pigs before the epizooty. In a case-control study 30 problem and 30 control farms (“matched pairs” with similar farm size and in the immediate vicinity) were analyzed. Farm allocation was verified by PCV2 DNA measurements of 5 healthy weaned pigs in each control farm and 5 healthy and 5 PMWS affected weaners in each control farm. Diseased pigs showed in average 1.8x108 DNA templates per ml serum significantly higher than healthy pigs from control farms with 1x106 DNA templates per ml serum. Virus load in healthy pigs did not differ significantly between control- and PMWS affected farms. PMWS mainly emerged among the animals in the 5th to 8th week of live in the problem farms. In the „Full model“risk factors were identified such as high occupancy in the small weaning boxes (p=0.002), large groups in gestation stage (p=0.03) as well as reduced birth weight <1.3 kg (p=0.04). Surprisingly, we found that also flies pose a risk to PMWS disease in the monovariate analysis.
Chronic stress caused for example by high stocking density, large groups in the gestation stage and reduced birth weight may influence social interaction in pigs negatively and lead to disturbances in the ontogenesis of the immune system and / or in the defense against infection. Heavy fly infestation must be viewed not only as a vector for disease transmission, but also as a stress factor.