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Pathology Case of the Month - Muskrats

January 31, 2020

Case History: Over a two-week period in November 2017, approximately 20 muskrats (Ondatra zibethicus) were found dead in a managed wetland in Ohio, USA. 

Gross Findings: Five muskrats were examined at necropsy. Body condition varied from good to poor. All five muskrats had pinpoint to 1-mm diameter white or red foci throughout the liver (Fig. 1). 

Photograph of a muskrat liver with pinpoint white and red foci.
Figure 1. Photograph from a muskrat (Ondatra zibethicus) found dead in Ohio, USA. The liver contained disseminated pinpoint to 1-mm diameter white foci (arrows). (Credit: Julia Lankton, USGS National Wildlife Health Center. Public domain.)

Histopathological Findings: Multifocally, the livers contained areas of random to massive coagulative necrosis (Fig. 2A). Adjacent hepatocytes frequently contained filamentous argyrophilic (silver-positive) bacterial rods arranged in packets (Fig. 2B).

Photographs from a muskrat liver with random areas of hepatocellular necrosis indicated.
Figure 2. Photographs from a muskrat (Ondatra zibethicus) found dead in Ohio, USA. (A) Throughout the liver are random areas of hepatocellular necrosis (star). H&E stain. (B) Hepatocytes at the periphery of necrotic areas multifocally contain packets of argyrophilic bacterial rods (arrows). Modified Steiner’s stain. (Credit: Julia Lankton, USGS National Wildlife Health Center. Public domain.)

Morphologic Diagnosis/es:

  1. Hepatocellular necrosis, random to massive, acute, severe, with intralesional silver-positive bacterial rods

Disease: Tyzzer’s disease; in muskrats, Tyzzer’s disease has historically been known as Errington’s disease or hemorrhagic disease

Etiology: Clostridium piliforme, a spore-forming, rod-shaped, gram-positive obligate intracellular bacterium; may stain as gram-negative unless grown under hypoxic conditions

Distribution: Worldwide

Host range: Tyzzer’s disease is most often documented in rodents, rabbits, and foals, although it can affect a wide range of domestic species. Reports in free-ranging wildlife are rare; affected species include muskrat, cottontail rabbit, snowshoe hare, and raccoon. Young and immunocompromised animals are usually most affected, but many factors including genetics, age, and bacterial virulence have been linked to susceptibility and disease progression.

Transmission: Transmission is through oral exposure to environmental spores shed in feces of infected animals. While the vegetative phase is labile, spores can survive in the environment for over a year.

Clinical signs: Clinically affected animals may be found dead or present with depression, rough haircoat, anorexia, jaundice, or diarrhea. Infections can also be subclinical and asymptomatic.

Pathology: A triad of lesions is typically seen affecting the liver, intestine, and heart. Grossly, pinpoint white foci in the liver are characteristic and correspond with areas of coagulative necrosis on histopathology; at the periphery of these areas, hepatocytes multifocally contain silver-positive bacterial rods arranged in haphazard clusters, referred to as a “pick-up sticks” formation. An ulcerative, necrotizing, or hemorrhagic typhlocolitis is common. The heart may contain white streaks corresponding with areas of myocardial necrosis.

Diagnosis: Characteristic gross and microscopic lesions with detection of C. piliforme in affected tissues. This bacterium cannot be cultured on cell-free media, therefore molecular-based tests such as PCR may be necessary to confirm diagnoses based on gross and microscopic examination. In the animals of this report, lesions in the heart or cecum were not noted, although freeze-thaw artifact and autolysis may have interfered with full microscopic evaluation of the intestinal tract.

Public health concerns: Gross lesions of Tyzzer’s disease can resemble those of tularemia, a zoonotic disease. Infection with Francisella tularensis should be ruled out by special histologic stains, culture, and/or molecular techniques in cases where both Tyzzer’s disease and tularemia are differential diagnoses.

Wildlife population impacts: While long-term muskrat declines have been documented, the potential contribution of Tyzzer’s disease or other infectious diseases to this decline is unknown.

Management: Managers should be aware that infectious diseases such as Tyzzer’s disease can cause large-scale mortality events in wild rodents.


Ahlers AA, Heske EJ. 2017. Empirical evidence for declines in muskrat populations across the United States. J Wildl Manage 81:1408–1416.

Grear DA, Lankton JS, Zaleski S, Witt M, Lorch JM. 2019. Mortality due to Tyzzer’s disease of muskrats (Ondrata zibethicus) in northern Ohio, USA. J Wild Dis 55(4):982-985

Merck Veterinary Manual. 2020. Overview of Tyzzer disease, Accessed January 2020. 

Wobeser G. 2001. Tyzzer’s disease. In: Infectious diseases of wild mammals, Williams ES, Barker IK, editors. Iowa State University Press, Ames, Iowa, pp. 510–513.

Wobeser G, Barnes HJ, Pierce K. 1979. Tyzzer’s disease in muskrats: Re-examination of specimens of hemorrhagic disease collected by Paul Errington. J Wild Dis 15(4):525-527

Wobeser G, Campbell GD, Dallaire A, McBurney S. 2009. Tularemia, plague, yersiniosis, and Tyzzer’s disease in wild rodents and lagomorphs in Canada: A review. Can Vet J 50:1251–1256.

Wobeser G, Hunter DB, Daoust PY. 1978. Tyzzer’s disease in muskrats: Occurrence in free-living animals. J Wildl Dis 14:325–328.

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