The taxonomic position of the Eimeriidae coccidia genera Atoxoplasma and Isospora has been subject to debate. It is currently suggested that Atoxoplasma be considered synonymous with Isosopora and unified in a single group of coccidia with intra- and extraintestinal forms.
Isosporiasis is a disease of passerine birds. Canaries, finches, sparrows, and species of the Sturnidae family (starlings, mynahs) are most often affected. Very rare cases of infection have also been reported in raptors. Poultry are not known to be affected.
Isospora has a direct life cycle that includes an intestinal and an extra-intestinal, systemic phase. Schizogony 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 seen in young birds. In susceptible species or weakened birds, mortality can be high (as much as 80%) and rapid, especially in fledglings. Isospora infection complicates management and is a threat to successful captive breeding of some species such as the endangered Bali mynahs. Clinical signs include listlessness, diarrhea, anorexia, and weight loss. 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. Parasite-infected mononuclear cells are present in blood and organ impression smears or aspirates, especially in the liver and spleen. Oocysts are present in droppings. Isospora are pale-staining, nonpigmented, oval, intracytoplasmic bodies within mononuclear cells 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
Diagnosis is difficult in chronically infected older birds. Very few parasites are present in blood and tissues, and oocysts are shed intermittently, sometimes in high numbers. 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 to determine 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 Isospora, however. Buffy coat and organ smears are other but less sensitive diagnostic methods. Negative findings should not be interpreted to mean that infection is not present. 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 successfully reduced mortality and 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 October 2013 by Arnaud J. Van Wettere, DVM, MS, PhD, DACVP