A congenital persistent swine fever infection. I. Clinical and virological observations

A congenital swine fever infection is described. The infection is characterized by persistent viraemia, continuous virus excretion and late onset of disease, with death occurring 2–11 months after birth.A “carrier sow” from a field outbreak gave birth to one dead, one slightly abnormal and nine normal piglets, six of which were allowed to live. Disease symptoms were not observed before the age of 9–28 weeks (average 20 weeks) and the average survival time was over 6 months. Virus titres in blood plasma fluctuated between 10^5?0 and 10^6?9 plaque forming units/ml and virus was excreted throughout the lives of the pigs. Leucocyte counts showed normal to subnormal values, falling below 10 000/mm³ at about the onset of illness. Lesions characteristics of swine fever were not observed, but severe atrophy of the thymus was seen in all animals. Cryostat sections revealed the presence of viral antigen in lymphoidal, reticulo-endothelial and epithelial tissues. The difference between this persistent congenital infection and the chronic swine fever described by other authors, and the importance of “healthy” virus carriers for the epizootiology of the disease are discussed.