The taxonomic position of Atoxoplasma has been subject to debate and confusion. Presently, Atoxoplasma is a member of the Eimeriidae closely related to Isospora. Molecular techniques are necessary to distinguish Atoxoplasma from Isospora.

Atoxoplasma are pale-staining, nonpigmented, oval, intracytoplasmic bodies within mononuclear cells currently thought to be lymphocytes. Usually, cells contain a single parasite, but multiple organisms can be seen in severe, acute infections. Presence of the protozoan causes the nucleus to curve around it giving the appearance that the organism is located within an indentation of the nucleus. Passerine birds, especially canaries, finches, sparrows, and species of the Sturnidae family (starlings, mynahs) are affected. Very rare cases of infection have also been reported in raptors. Poultry are not known to be affected.

Atoxoplasma has a direct life cycle that includes an intestinal and an extra-intestinal, systemic phase. Merogony occurs in the intestinal epithelial cells as well as the circulating mononuclear cells. Gametogony and oocyst formation occur in the intestinal epithelial cells, and oocysts are passed in the feces. Transmission is fecal-oral.

Infection is most often not pathogenic and high parasitemia may be observed in young birds. In susceptible species or weakened birds, mortality can be high (up to 80%) and rapid, especially in fledglings. Atoxoplasma infection complicates the management and is a threat to successful breeding of some species such as the endangered Bali mynahs. Clinical signs include listlessness, diarrhea, and anorexia. In acutely affected birds, there is marked hepatomegaly and splenomegaly, often with multifocal necrosis. The enlarged liver and gallbladder can be seen through the abdominal wall, especially if it is moistened with alcohol, which provides the basis for the common name of black spot disease in passerine birds. High numbers of parasites infecting lymphocytes are present in blood and organ impression smears or aspirates, especially in the liver and spleen. Nearly spherical oocysts are present in droppings.

Diagnosis is difficult in chronically infected older birds. Very few parasites are present in blood and tissues, and oocysts are shed intermittently, although sometimes in high numbers. Distinction between oocysts of Atoxoplasma and Isospora spp is not possible by light microscopy, and systemic stages must be identified to confirm infection with Atoxoplasma. Buffy coat and organ smears are preferred. Negative findings should not be interpreted to mean that infection is not present. A PCR test is available and can be performed on blood, tissues, or feces, although its sensitivity is poor in fecal samples. PCR is useful in determining the incidence and prevalence in a collection, confirming the diagnosis, and possibly evaluating the efficacy of treatment. PCR cannot be used to ascertain that a bird is free of Atoxoplasma, however. In some chronically infected birds, hepatic and splenic enlargement persists because of infiltrations with high numbers of large lymphoid cells that serve as host cells for the parasite. Histopathologically, organisms are difficult to find and identify, and lesions may be mistaken for lymphoma.

Toltrazuril, sulfachlorpyridazine, and sulfachlorpyrazine have been successful in reducing mortality and reducing oocyst shedding. It is unlikely that these drugs clear the organism from the bird. Good management procedures, including isolation of age groups and scrupulous cleanliness (particularly daily cleaning before oocysts sporulate) help control the disease. Disinfectants have little effect on oocysts.

Last full review/revision March 2012 by Arnaud J. Van Wettere, DVM, MS, DACVP