Besnoitia tarandi in caribou: “I‘ve got you under my skin”

“I’ve got you under my skin” a catchy song from Frank Sinatra is also an accurate way to describe Besnoitia tarandi in caribou. This microscopic parasite forms small cysts that look like bumps in and under the skin and other tissues. Sometimes they can lead to hair loss and crusty skin, especially on the face and legs(1). Although we routinely see this parasite in caribou, we know very little about it or its relationship with its host.

We don’t know how this parasite is spread. There is evidence it can be spread by biting insects, which were suspected as the cause of an outbreak in captive caribou at the Assiniboine Park Zoo in Manitoba in 1983 (2). For other Besnoitia species, the life cycle involves a predator, like cats, which eat the infected tissue and then shed the parasite’s eggs in their feces. These eggs can contaminate food and water, which caribou then ingest, continuing the cycle.

We also don’t know the impact this infection has on individuals and populations of caribou. From the few studies available on caribou, infections tend to be more common in the fall and they are often found in animals that are already in poor condition(3); However, the exact relationship between these factors is still unclear. Disease does not always appear in cattle but when it does it presents as fevers, lameness, reddened skin, lymph node enlargement, and swelling of the testicles caused by inflammation in blood vessels. Tissue cysts appear in later stages of disease with thickened skin and degeneration of testicles sometimes resulting in sterility. Once infected, cattle remain so for life. Based on the disease in cattle, concerns for caribou include fertility, poor condition, and poor coat quality which may affect the survival of an arctic species.

Cases of besnoitiosis are generally submitted to diagnostic labs to confirm the presence of a parasite or from hunter harvests where there is concern over whether the meat is edible. Although pronouncing besnoitia can be quite a mouthful it is perfectly safe to eat.

Besnoitia tarandi is a parasite that poses a hidden threat to caribou, causing various health issues that can impact their survival. More research is needed to fully understand this parasite.

Figure 1 Hide from a caribou. Pieces of hide submitted after a field necropsy in May of 2022. Cause of death was likely predation but scavenging after death was also a possibility. During the necropsy small bumps were noticed on the inside of the skin and on the surface of the hones of the hindleg.  A) Inner surface of the skin showing small bumps (arrows). B) Microscopic view of besnoitia organisms in the skin from A. Asterix – besnoitia cyst. Arrow – inflammation.

1. Wobeser G. Besnoitiosis in a woodland caribou. J Wildl Dis. 1976 Oct;12(4):566–71. `
2. Glover GJ, Swendrowski M, Cawthorn RJ. An epizootic of besnoitiosis in captive caribou (rangifer tarandus caribou), reindeer (rangifer tarandus tarandus) and mule deer (odocoileus hemionus hemionus). J Wildl Dis. 1990 Apr 1;26(2):186–95.
3. Ducrocq J, Beauchamp G, Kutz S, Simard M, Taillon J, Côté SD, et al. Variables associated with besnoitia tarandi prevalence and cyst density in barren-ground caribou (rangifer tarandus) populations. J Wildl Dis. 2013 Jan 1;49(1):29–38.