Systemic Besnoitiosis Suspected in Two Virginia Opossums from Ontario

This spring, the CWHC received a female and male Virginia opossum (Didelphis virginiana) from Wings Rehabilitation Centre in Amherstburg, Ontario. The female opossum was found injured, while the male opossum was found emaciated and remained at the facility for 77 days. Both were euthanized due to their respective conditions. Our preliminary necropsy findings of both opossums indicated innumerable pinpoint, raised, tan nodules across many of their major organs (liver, heart, kidney, lung, and spleen). Nodules were also present in the subcutaneous tissues and skeletal muscles, which were suggestive of underlying Besnoitia darlingi infections. Upon microscopic examination, the injured female opossum was also found to have a systemic bacterial infection, which may have initially invaded via its injuries. Microscopic examination of both opossums revealed that the internal nodules were parasitic cysts, which were consistent with a systemic Besnoitia infection.

Besnoitia spp. are coccidian endoparasites that infect a variety of wild and domestic mammals, as well as some reptiles 2. The organisms produce thick-walled, white, round to ovoid tissue cysts that are approximately 2 to 4 millimeters in diameter 1. In chronic infections, cysts often develop in the subcutis of the skin, mucous membranes of the upper respiratory tract, genital mucosa, and the conjunctiva and sclera 4. These can appear grossly as small white nodules, and diagnosis is typically confirmed through cytology or histopathology 4. There are still gaps in explaining the life cycle of several Besnoitia species, including the range of intermediate hosts and the identity of the definitive host3; however, for Besnoitia darlingi, domestic cats are the definitive hosts and opossums serve as intermediate hosts 4.

Although Besnoitia infections in Virginia opossums are often a non-pathogenic incidental finding, systemic infections can lead to debilitation and death 2. The parasitic load of these urban-adapted scavengers can reflect environmental health or transmission dynamics with other wild or domesticated hosts 2. These findings highlight the importance of wildlife disease surveillance in understanding emerging and overlooked parasitic threats. While the zoonotic potential of Besnoitia remains unconfirmed, its presence in Ontario wildlife expands our epidemiological map and contributes to a more complete understanding of parasite behavior and transmission dynamics. Further pathology and molecular research will help clarify the distribution and potential health significance of this parasite within a One Health framework.

Written by Kaitlyn Young, research student, CWHC Ontario/Nunavut

References

Edwards, E. (2024). Systemic besnoitiosis in a Virginia opossum. Texas A&M Veterinary Medical Diagnostic Laboratory https://tvmdl.tamu.edu/case-studies/systemic-besnoitiosis-in-a-virginia-opossum/

Ellis, A. E., Mackey, E., Moore, P. A., Divers, S. J., VetMed, B., ZooMed, D., … & Yabsley, M. J. (2012). Debilitation and mortality associated with besnoitiosis in four Virginia opossums (Didelphis virginiana). Journal of Zoo and Wildlife Medicine, 43(2), 367-374.

Malatji, M. P., Tembe, D., & Mukaratirwa, S. (2023). An update on epidemiology and clinical aspects of besnoitiosis in livestock and wildlife in sub-Saharan Africa: A systematic review. Parasite epidemiology and control, 21, e00284. https://doi.org/10.1016/j.parepi.2023.e00284

Olias, P., Schade, B., & Mehlhorn, H. (2011). Molecular pathology, taxonomy and epidemiology of Besnoitia species (Protozoa: Sarcocystidae). Infection, genetics and evolution : journal of molecular epidemiology and evolutionary genetics in infectious diseases, 11(7), 1564–1576. https://doi.org/10.1016/j.meegid.2011.08.006

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