Even before clinical signs develop, infected pigs shed virus in all their excretions and secretions. The normal means of spread is by direct contact between infected and susceptible animals, but airborne infection can occur over short distances. In chronic disease, virus persistence is characteristic. Infection of pregnant sows with low virulent strains often results in congenital infection in their piglets. Such infected piglets are immunotolerant virus excreters and are often responsible for maintaining the disease in a herd.
Humans can spread infection by syringes, instruments and the like.
Flies may transmit infection mechanically to nearby piggeries. With virulent strains of virus in naive pigs, the disease takes an acute course. Fever, anorexia, depression, conjunctivitis, nasal discharge, and huddling together with reluctance to move are first seen. Erythema or cyanosis of the skin of the ears, incoordination, and weakness of hind quarters are often observed. Constipation followed by diarrhoea develops.