Infected horses have lifelong infection of their leucocytes until stressed (eg by pregnancy, corticosteroids, surgical operation, disease) or new virus variants arise. Viraemia then increases by 100-fold. The high levels of virus coats the rbc's which are then lysed by complement to cause jaundice, oedema, hemorrhagic diarrhoea, petechial hemorrhages and a high chance of mortality. Bouts of fever and jaundice can occur and worsen, from the increased immune response, every few months.