Retrospektiv studie av porcint circovirus typ 2 och postweaning multisystemic wasting syndrome i Sverige

Abstract: Porcine circovirus type 2 (PCV2) is a common virus present in most parts of the world. PCV2 has been pointed out as the major causative agent to post weaning multisystemic wasting syndrome (PMWS) that affects pigs after weaning. The clinical symptoms of PMWS are impaired growth, diarrhoea, respiratory problems and increased mortality. At autopsy enlarged lymph nodes with depletion of lymphocytes are commonly found. Since 1991 PMWS has spread all over the world and was first described among Swedish pigs in 2003. Although PCV2 is widespread, only some pigs develop PCV2-related diseases. Why this is and what other factors that are required in order for the disease to develop is yet unknown. The main theories involve other infections, for example PRRS, and management issues but also genetic differences between pigs and/or virus strains have been suggested. The PCV2 virus genome has four open reading frames (ORF ), ORF1 and ORF2 are connected by two short introns. ORF1 codes for proteins involved in virus replication whereas ORF 2 codes for the capsid protein that most antibodies are directed to. Today the nature of the proteins coded for by ORF 3 and 4 are unknown. Consequently, most studies of genomic differences between various PCV2 isolates from different years and parts of the world have focused on ORF2. A serological survey in Sweden revealed that more than 90% of Swedish pigs have antibodies to PCV2. It was also demonstrated that pigs in a SPF-herd seroconverted to PCV2 during 1993, possibly via introduction of contaminated semen. Pigs in this herd are still healthy (<0,1% mortality) although PCV2 isolated from a pig in this herd repeatedly has caused PMWS when used in experimental infection studies. The purpose of the present study was partly to find out if PCV2 has been present in Swedish pig farms prior to 1993 and, if so, how similar older PCV2 isolates are to virus recently isolated from diseased pigs. In particular, isolates from the years just prior to the outbreak of PMWS in Sweden are being studied. The archived material analysed comprise formalin fixed and paraffin embedded tissue from 15 pigs, primarily lymphoid tissue. Three samples originated from 2001, 2002 and 2003, the rest of the samples were from the years 1985-87. All samples were analysed for presence of PCV2 by immunhistochemical labelling. Selected samples were deparaffinized and DNA was extracted and processed by end-point PCR in order to detect PCV2. Samples from 1985-87 were also tested for PCV2 by real-time PCR. PCV2 was found in the samples from 2002 and 2003 but not in older samples. Sequence analyses of ORF2 revealed a great similarity between the isolates from 2002 and 1993 whereas the isolate from 2003 was more closely related to PCV2 isolated from Swedish pigs with PMWS. These preliminary results warrant extended analyses of PCV2 isolated from diseased and non-diseased Swedish pigs, raised in various management systems.