Congenital tremor and splay leg in Swedish piglets : a study of the viral causes and routes of transmission

Congenital tremor (CT) and splay leg (SL) are diseases regularly seen in new-born piglets in Sweden. They are diagnosed based on the clinical picture, which in CT are tremor and sometimes ataxia, and in SL, a lack of adduction of the hind legs. The cause of the clinical signs is unknown, but impaired nerve function due to myelin damage is a theory. However, the syndromes are transient, and piglets that can nurse often recover within some months. For decades, there has been speculation about the causes of these diseases. Infection in utero with certain viruses, hereditary factors, and drug treatment of pregnant sows may induce the syndromes in the offspring. In Sweden, one type of CT has been described: CT type A-II, a variant that for a long time was suspected to be caused by an unknown virus. Then, in 2015, a new virus named atypical porcine pestivirus (APPV) was discovered, and in 2016, APPV was associated with CT type A-II. There were also indications of an association between APPV and SL. This thesis aimed to investigate viral causes of CT type A-II and SL in Swedish piglets. APPV was detected in brain tissue from piglets with signs of CT type A-II, both in new and historical material. The virus could also be detected in commercial boar semen from a Swedish boar stud and in serum samples from Swedish wild boars. However, the virus was not found in piglets with splay leg or in healthy piglets, and high-throughput sequencing of the material did not reveal any other relevant viruses. Consequently, it seems unlikely that APPV, or any other virus, would be the primary cause of SL in Sweden