Pathogenesis: Virus enters oral pharynx and replicates in palatine tonsil before viraemia at 2-6 days post infection. Pyrexia, anorexia, leucopenia and generalised petechial haemorrhage are typical signs. Haemorrhagic lymphadenitis and death following profound thrombocytopenia are not uncommon in young piglets. Intestinal lesions over lymphoid tissue eg Peyer's patches showing as 'button ulcers' and haemorrhage in serosal membranes are characterisitic. CNS lesions can occur with tremors & incordination. Once the virus has crossed the placenta, there can be foetal death, abortion and congenital damage.
This cross-uterine congenital infection with cerebellar hypoplasia or growth retardation can occur without clinical symptoms in the sow.