The objective of these studies was to investigate if porcine postweaning multisystemic wasting syndrome (PMWS) could be induced in healthy pigs following contact with air from pigs with clinical signs of PMWS. The pigs were housed in different units. Either 31 (study I) or 25 (study II) pigs with clinical symptoms of PMWS from a PMWS-affected herd and 25 healthy pigs from a PMWS-free, but PCV2-positive, herd were housed in unit A. Fifty pigs from a PMWS-free herd were housed in unit B, which were connected by pipes to unit A. In unit C, 30 pigs from a PMWS-free herd were housed as controls. In study II, the pigs in units A and B from the PMWS-free herd developed clinical signs of PMWS 2-3 weeks after arrival. PMWS was confirmed at necropsy and the diseased pigs had increased PCV2 load and increased antibody titers against PCV2 in serum that coincided with the development of clinical signs typical of PMWS. Sequence analysis revealed that the PCV2 isolate belonged to genotype 2b. In conclusion, the present study showed that PMWS can be induced in pigs from a PMWS-free herd by airborne contact with pigs from a PMWS-affected herd. 1. Introduction Postweaning multisystemic wasting syndrome (PMWS) is an important disease in weaned pigs worldwide. PMWS was first described in Canada in 1991 as a chronic disease with progressive weight loss in pigs from 4–16 weeks of age [1]. Since then, the disease has been diagnosed in many countries in North America, Asia, and Europe including Denmark [2, 3]. The clinical signs of PMWS comprise unthriftiness/wasting, paleness of the skin, enlarged lymph nodes, and occasionally jaundice, respiratory symptoms, or diarrhoea [1, 3, 4]. Affected pigs have lesions in lymphoid organs characterized by lymphoid depletion and the presence of giant cells and inclusion bodies [4–7]. PCV2 has proved to be necessary but not sufficient for development of PMWS, since the virus is present in both affected and PMWS-free pigs and herds [4, 8]. The PCV2 virus is transmitted between pigs by the oro-fecal and/or respiratory routes [9, 10] and vertical transmission has also been documented [4, 11]. The high prevalence of PCV2 in almost all herds of all pig-producing countries indicates that the transmission of PCV2 is very effective [12–15]. In contrast, only a few studies have been performed on the “transmission” of the PCV2-associated disease complexes (PCVDs), that is, whether PMWS can be “transmitted” from PMWS-affected to PMWS-free pigs. A study performed in New Zealand demonstrated disease development in healthy pigs in direct or indirect
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