Infected birds are weak, depressed and anorexic. Abdominal protrusion due to splenomegaly may be present.
Central nervous signs may be present, and haemolytic anaemia and haemoglobinuria may occur.
Coma and death may occur quickly when the parasite burden is high. However, many birds, especially passerines, do not become ill and play an important role as asymptomatic carriers of the parasite.