Abstract

Atoxoplasma sp. are intracellular, coccidian parasites that have been associated with morbidity and mortality in several species of passerine birds.^1,3-5,7,9,10 Infection begins with ingestion of sporulated oocysts and release of sporozoites within the intestinal tract.^2,9 Sporozoites subsequently invade intestinal epithelial cells, lymphocytes and macrophages. Dissemination occurs when infected cells gain access to the blood. Merogony (asexual replication) occurs both in the cells of the reticuloendothelial system and intestinal epithelial cells while gametogony (sexual replication) is limited to intestinal epithelial cells. Clinical disease is not always apparent but can include inappetence, lethargy, diarrhea and weight loss.⁸ Splenomegaly and hepatomegaly⁹ can be seen at necropsy and lymphohistiocytic inflammation is often associated with the presence of intracellular organisms. Antemortem diagnosis can be challenging, but the diagnosis can be made if characteristic 3–5-micron diameter, intraleukocytic sporozoites are identified in peripheral blood or buffy coat smears. Supportive evidence is obtained if feces are found to contain oocysts that show typical Isospora morphology (two sporocysts, each with four sporozoites) when sporulated.

The threat of atoxoplasmosis to birds in zoologic collections or captive breeding programs was probably underappreciated prior to the deaths of several Bali mynahs in 1988.⁹ Birds housed in mixed species exhibits or naturalistic enclosures that allow exposure to wild birds may be at increased risk of infection. Losses due to infection may have devastating effects in collections of endangered birds or in captive breeding programs. The Zoological Society of San Diego presently maintains 169 species of passerine birds, but prevalence and distribution of atoxoplasmosis in the collection and the pattern of disease associated with the organism have not been described. The goals of this retrospective study were therefore to identify the host range of Atoxoplasma sp. in our collection, document the distribution of organisms and histologic lesions associated with infection in individuals within species, and compare the pattern of infection and associated disease between species.

The Department of Pathology at the Zoological Society of San Diego maintains its necropsy and biopsy records in a computerized database. All of the records from 1983–2000 were searched for a diagnosis of atoxoplasmosis. Eighty-one of the 93 cases in which this diagnosis was made were available for review at the time of manuscript preparation. Six of the cases were excluded from the study because they did not contain organisms that were histologically or cytologically consistent with descriptions of Atoxoplasma sp.^2,9 The remaining 75 birds represented 33 passerine species (Table 1). Sparrows, starlings, tanagers and thrushes represented 60% of the total number of cases.

Table 1. Passerine species with atoxoplasmosis

Fifty-four of the affected birds (28 species) were housed at the San Diego Zoo (SDZ) and 19 (four species) were housed at the Wild Animal Park (WAP). Two of the affected birds were wild northern house sparrows (one dead, one euthanatized for Atoxoplasma sp. screening) found on grounds at the SDZ. Thirty-four of the total number of birds were male, 40 were female and the sex of one bird was undetermined. Sixty-two animals died naturally. Thirty-six of these were found dead and 26 were clinically ill but died despite medical treatment. Humane euthanasia was performed in the remaining 13 cases. Excluding the wild sparrows in which age was not known, the overall mean age of the birds in this study was 25.5 mo (range: 0.17–215.2 mo). Birds (n=32) in which atoxoplasmosis was implicated as the primary cause of death had a mean age of 23.8 mo. Birds in which atoxoplasmosis was considered a secondary diagnosis or an incidental finding (n=41) had a mean age of 27.1 mo. Forty-four percent of birds in the former group and 37% of birds in the latter group were less than 6 mo old at the time of death.

The diagnosis of atoxoplasmosis was based on identification of protozoa that were morphologically consistent with Atoxoplasma sp.^2,9 in peripheral blood from impression smears of postmortem tissue samples, in histologic sections or after sporulation of oocyst-containing feces. Both sexual (gamonts) and asexual (zoites) stages of the organism were found in 21 (64%) different species of birds. Zoites were more commonly identified in the cytoplasm of mononuclear leukocytes from impression smears of lung, spleen, and liver than in histologic sections. Infected monocytes in cytologic preparations (modified Wright-Giemsa stain) generally contained a single 2–4 micron diameter, intracytoplasmic, clear to lightly basophilic zoite with central granular, magenta material. The presence of the organism caused indentation of the nucleus in most cells.

In histologic tissue sections, gamonts were present in epithelial cells lining the villi of the small intestines. They were basophilic and varied from 6–12 microns in diameter. Intraleukocytic zoites were generally difficult to find but were most often seen in the lung, liver and spleen. Zoites appeared smaller (1–2 microns in diameter) in tissue sections than in cytologic preparations, which was probably an artifact due to different preparation methods. Inflammation was more often associated with intraleukocytic zoites than with gamonts, and was generally lymphohistiocytic. When intestinal inflammation was present, it was primarily associated with intraleukocytic zoites in the lamina propria rather than with intraepithelial gamonts. The most severe cases had the appearance of an infiltrating leukocytic neoplasm in which small numbers of cells contained intracytoplasmic zoites. Similar descriptions of severe lymphoproliferative disease associated with intracellular protozoa have been reported in goldfinches⁷ and warblers¹¹.

In contrast to the general pattern of lymphohistiocytic inflammation, all of the Indochinese white-rumped shamas contained aggregates or infiltrates of extremely large, histiocytic cells with abundant eosinophilic cytoplasm and one or two intracytoplasmic, 1–2 micron in diameter, eosinophilic organisms. The organisms were identified as apicomplexan protozoa with transmission electron microscopy and the diagnosis of atoxoplasmosis was based on their presence and the production of oocysts with Isospora morphology after fecal sporulation. A description of these cases has been published elsewhere.⁶

The results of this retrospective study expand the list of susceptible passerine species known to become infected with, develop clinical illness and die from atoxoplasmosis. Important questions such as whether the protozoal infections in the present cases are due to a single or multiple species of Atoxoplasma and whether interspecies transmission can occur remain to be answered. Polymerase chain reaction (PCR) for Atoxoplasma sp. has recently been implemented in the Molecular Diagnostics Laboratory at the Zoological Society of San Diego. Examination of archived tissue and antemortem tissue and fecal samples will be important tools with which to answer these questions.

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