The parasite has a tissue phase, in which macrophages throughout the body are distended with schizonts, as well as a red cell phase. Infection of red cells results in hemolytic anemia, but many of the clinical signs and death are apparently due to vascular disease caused by the tissue phase and are not related to the degree of parasitemia.
Cases occur usually in the months of May to June. Clinical signs include lethargy, anorexia, pallor, icterus, fever, and dyspnea with death occurring within one week of onset of signs. Cats have a moderate normocytic normochromic non-regenerative anemia, a mild to moderate leukopenia (a degenerative left shift and moderate toxic change are commonly observed), and a moderate to severe thrombocytopenia (average, 60,000 platelets/uL). The organism is recognized as small, ring or safety pin-shaped bodies in red cells. Less than 5% of red cells contain the parasite. Schizonts within macrophages may be observed in aspirates or imprints of spleen, liver, lymph node and bone marrow. Rarely, circulating macrophages with schizonts may be seen at the feathered edge of peripheral blood smears. Widespread hemorrhage, edema, and thrombosis, probably caused by the tissue phase, are usual post-mortem findings. The disease is diagnosed by identification of the organism in erythrocytes, of schizonts in tissues, and fluorescent antibody testing of frozen tissue. No effective treatment is known.