The disease occurs as two forms:
Epizootic: Infection follows the entrance of virus into a non-immune herd. Symptoms develop very quickly and the disease spreads very rapidly (explosive infection). Older animals show anorexia, lethargy, diarrhoea, weight loss and vomiting. They recover in 1 week but in very young animals, mortality approaches 100%.
Enzootic: The herd is persistently infected. Immune sows provide lactogenic immunity (IgA in milk) to the sucklings. Animals often have diarrhoea 1-2 weeks after weaning as maternally derived antibody declines. Clinical signs are similar but less severe than fully susceptible pigs. Similar pattern to rotavirus infection.