Myxozoans are intracellular parasites that have recently been reclassified from protozoa to metazoa (kingdom Animalia). Myxozoans are parasitic in poikilothermic vertebrates, especially fish, in which they may be associated with devastating diseases such as whirling disease (Myxobolus cerebralis) in trout and “hamburger” or proliferative gill disease (Henneguya ictaluri) in catfish. Myxozoans of fish often have alternative life stages (actinosporeans) in invertebrates such as oligochaetes, but the life cycles for many metazoans remain uncertain. Amphibians and chelonians are also known to be hosts for myxozoans.
Homeothermic vertebrates have only recently been reported to harbor myxozoan parasites. There are three reports in the literature of myxozoan spores being found in the diarrheic feces of humans including one AIDS patient. In most of the patients the spores were probably derived from ingested fish and were not parasitic, but in the AIDS patient the spores were passed over a period of several months. A report from Europe describes parasitic structures in the brains of moles that were considered to be myxozoan-like organisms. No reports of myxozoans in birds were found during an extensive literature search, however, during the course of routine postmortem histologic examinations, myxozoan parasites were identified in the bile ducts of waterfowl from 4 separate zoological collections.
To date, six cases have been identified and include both captive exotic and free-flying native anseriforms all of the family Anatidae, sub-family Anatinae. Species included: two South African yellow-billed ducks (Anas undulata undulata); a cape teal (Anas capensis); a baikal teal (Anas formosa); a wood duck (Aix sponsa), and a mallard (Anas platyrhynchos). All were housed or found in enclosures with open water that housed a number of zoo species as well as variable numbers of wild birds attracted to the open water and ample food. In addition to anseriform species, birds in the exhibits included flamingos, storks, and native and exotic herons and egrets. Some, but not all, of the exhibits also contained fish species.
Histologically, bile ducts were variably distended by a thin walled amorphous lightly basophilic to eosinophilic structure in which were embedded smaller bodies. This amorphous “sac” was presumed to be sporoplasm undergoing asexual division. The smaller bodies were ill-defined structures with eukaryotic nuclei in some cases; these were presumed to be sporoblasts. In other cases, the sporoplasm contained better-defined structures with paired internal refractive polar capsules typical of mature myxozoan spores. Inflammation varied from mild to severe. The more severe inflammatory lesions appeared to be centered on ruptured bile ducts with extrusion of the myxozoan spores into the hepatic parenchyma. When inflammation was mild, the organisms were considered to be of no clinical significance. Most of the cases had other more significant health problems including botulism and trauma.
Biliary myxozoanosis does not appear to be a significant problem in waterfowl, however, the organisms, particularly prior to spore formation, can easily be overlooked or “written-off” as sloughed epithelium or debris. Identification of additional cases is necessary to further our understanding of this infection. Currently, work is underway to classify the parasites from the cases reported above, both ultrastructurally and by characterization of the small ribosomal subunit gene sequences.
The authors thank Drs Mark Stetter and Gary West for submission of cases and AFIP Department of Veterinary Pathology secondary review of two of the cases.
1. Cavalier-Smith T. A revised six-kingdom system of life. Biol Rev Camb Philos Soc. 1998;73:203–266.
2. Friedrich C, Ingolic E, Freitag B, Kastberger G, Hohmann V, Skofitch G, et al. A myxozoan-like parasite causing xenomas in the brain of the mole, Talpa Europaea L, 1758 (Vertebrata, Mammalia). Parasitol. 2000;121;(Pt5):483–492.
3. Gardiner CH, Fayer R, Dubey JR. An Atlas of Protozoan Parasites in Animal Tissues. United States Department of Agriculture. Agricultural Research Service. Agriculture Handbook Number 651. 1988.
4. Kent ML, Andree KB, Bartholomew JL, El-Matbouli M, Cesser SS, Devlin RH, et al. Recent advances in our knowledge of the Myxozoa. J Eukaryot Microbiol. 2001;48:395–413.
5. Moncada LK, Lopez MC, Murcia MI, Nicholls S, Leon F, Guio OL, Corredor A. Myxobolus sp., another opportunistic parasite in immunosuppressed patients? J Clin Microbiol. 2001;39:1938–1940.
6. Siddall ME, Martin DS, Desser SS, Cone DK. The demise of a phylum of protists: phylogeny of Myxozoa and other parasitic cnidaria. J Parasitol. 1995;81:961–967.