Abstract
Atoxoplasma sp. are coccidian parasites that exhibit a prolonged life cycle, involving both the reticuloendothelial system and intestinal epithelium. Merogony (asexual reproduction) occurs in both lymphoid-macrophage cells and intestinal cells, resulting in the presence of merozoites in the mononuclear leukocytes of the peripheral blood. Gametogenesis (sexual reproduction) occurs in the intestinal cells of the same host.1-3 Infection with Atoxoplasma sp. has been documented in a variety of passerine species.5-8 The death of several tanagers (family Emberizidae) from confirmed and suspected Atoxoplasma infections prompted this study to determine prevalence of infection within the tanager collection at a large zoological park.
Diagnosis of atoxoplasmosis has been historically difficult, relying primarily on identification of the organism in a buffy-coat or organ impression smear. For this study, a polymerase chain reaction (PCR) assay was used to test blood, feces, or tissue samples from 55 birds, representing 15 tanager species (Genera Dacnis, Thraupis, Tangara, Cyanerpes, Habia, Ramphocelus, Piranga, Tersina, Euphonia, and Tachyphonus).4 From clinically healthy birds, 17 of 33 (51.5%) blood samples and one of eight (12.5%) fecal samples were positive for Atoxoplasma by PCR. Frozen tissues (preferentially liver, spleen, and intestine) from 25 deceased birds were tested using PCR and 16 of 25 (64.0%) were positive for the parasite.
A survey was then sent to 57 institutions to qualitatively determine the extent of Atoxoplasma infections in other tanager collections and establish that the infections were not an incidental finding in one institution. Twenty-two institutions responded to the survey and four of those (18.8%) provided necropsy reports that were strongly indicative of Atoxoplasma infections. Of those responding institutions, only one other was using PCR to identify the organism.
The high percentage of Atoxoplasma-positive results from clinically healthy birds suggests that Atoxoplasma is prevalent subclinically within captive tanager collections. Young birds and stressed adults are the most likely to develop clinical disease. The prevalence suggests that changes in husbandry and breeding should be undertaken to decrease the risk of transmission within a collection. Thorough disinfection of enclosures, changes in enclosure design, and the prevention of fecal contamination of food and water are the most important methods for decreasing transmission. Selective breeding programs and regular screening of breeding pairs will facilitate the reduction of the Atoxoplasma organism within a collection and minimize transmission to the more susceptible offspring. Care should also be taken when housing tanagers in outdoor enclosures, as the transmission risk of Atoxoplasma infection from wild passerines has not been established.
Literature Cited
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