Atoxoplasmosis: Beyond Bali Mynahs
American Association of Zoo Veterinarians Conference 2001
Denise McAloose, VMD, DACVP; Laura Keener, MT (ASCP); Mark Schrenzel, DVM, PhD, DACVP; Bruce Rideout, DVM, PhD, DACVP
Pathology Division, Center for Reproduction of Endangered Species, Zoological Society of San Diego, San Diego, CA, USA


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


Common name

Scientific name


Bali mynah

Leucopsar rothschildi

Sulawesi mynah

Basilornis celebensis

Robin chats

Western white crowned robin chat

Cossypha albicapilla albicapilla

Western snowy crowned robin chat

Cossypha niveicapilla niveicapilla


Northern house sparrow

Passer domesticus domesticus

Southern cape sparrow

Passer melanurus melanurus

Sudan golden sparrow

Auripaser luteus


Eastern golden-breasted starling

Cosmopsarus regius magnificus

Emerald starling

Lamprotornis iris

New Guinea metallic starling

Alponis metallica metallica

Northern superb starling

Spreo superbus superbus

Wattled starling

Creatophora cinerea


Northern paradise tanager

Tangara chilensis paradisea

Northern silver throated tanager

Tangara icterocephala frantzii

Northern spotted tanager

Tangara punctata punctata

Northern swallow tanager

Tersina viridis occidentalis

Guianan turquoise tanager

Tangara mexicana mexicana

Western bay headed tanager

Tangara gyrola catharinae

Western golden masked tanager

Tangara larvata fransicsae


Eastern white-throated laughing thrush

Garrulax albogularis eous

Nepalese greater necklaced laughing thrush

Garrulax pectoralis picticollis

Other species

Blue-backed fairy bluebird

Irena puella sikkimensis

Javan Ruby throated bulbul

Pycnonotus melanicterus dispar

Himalayan grey-headed bullfinch

Pyrrhula erythaca erythaca

Siberian goldfinch

Carduelis carduelis major

Red throated parrot finch

Erythrura psittacea

Tien shan red mantled rosefinch

Carpodacus rhodochlamys rhodochlamys

Fukien niltava

Niltava davidi

Surinam crested oropendola

Psarocolium decumanus decumanus

Red fronted serin

Serinus pusillus

Indochinese white-rumped shama

Copsychus malabaricus interpositus

Northern white headed buffalo weaver

Dinemellia dinemelli dinemelli

Southern capped wheateater

Oenanthe pileata pileata

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.

Literature Cited

1.  Box, E.D. 1996. Blood and tissue protozoa of the English sparrow (Passer domesticus domesticus) in Galveston, Texas. J. Protozool.13:204–208.

2.  Box, E.D. 1981. Isospora as an extraintestinal parasite of passerine birds. J. Protozool. 28:244–246.

3.  Box, E.D. 1975. Exogenous stages of Isospora serini (Aragao) and Isospora canaria sp. n. in the canary (Serinus canarius Linnaeus). J. Protozool. 22:165–169.

4.  Cooper, J.E., S. Gschmeissner, and A.G. Greenwood. 1989. Atoxoplasma in greenfinches (Carduelis chloris) as a possible cause of ‘going light.’ Vet. Rec. 124:343–344.

5.  Giacomao, R., P. Stefania, T. Ennio, V.C. Giorgina, B. Giovanni, and R. Giacomo. 1997. Mortality in black siskins (Carduelis atrata) with systemic coccidiosis. J. Wildl. Dis. 33:152–157.

6.  Harvey, C.J., B.A. Rideout, R.E. Papendick, M.R. Sutherland-Smith, I.H. Stalis, and C.H. Gardiner. 1997. Atoxoplasmosis in Indochinese white-rumped shamas (Copsychus malabaricus interpositus). Proc. Am. Assoc. Vet. Lab. Diagn. P. 21.

7.  Middleton, A.L. 1983. Lymphoproliferative disease in the American goldfinch, Carduelis tristis. J. Wildl. Dis. 19:280–285.

8.  Page, C.D. and K. Haddad. 1995. Coccidial infection in birds. Sems. Av. Ex. Pet Med. 4:138–144.

9.  Partington, C.J., C.H. Gardiner, D. Fritz, L.G. Phillips, and R.J. Montali. 1989. Atoxoplasmosis in Bali mynahs (Leucopsar rothschildi). J. Zoo Wildl. Med. 20:328–335.

10.  Quiroga, M.I., N. Aleman, S. Vazquez, and J.M. Nieto. 2000. Diagnosis of Atoxoplasmosis in a canary (Serinus canarius) by histopathologic and ultrastructural examination. Av. Dis. 44:465–469.

11.  Swayne, D.E., D. Getzy, R.D. Siemons, C. Bocetti, and L. Kramer. 1991. Coccidiosis as a cause of transmural lymphocytic enteritis and mortality in captive Nashville warblers (Vermivora ruficapilla). J. Wildl. Dis. 27:615–620.


Speaker Information
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Denise McAloose, VMD, DACVP
Zoological Society of San Diego
Center for Reproduction of Endangered Species, Pathology Division
San Diego, CA, USA

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