Trichinella infection, and other findings, in a grizzly bear autopsy.
In the summer of 2019 I received some preserved tissue samples from a grizzly bear that had been caught in a wolf neck-snare and was subsequently shot. Inadvertent snaring of non-target wildlife species is something we see in our diagnostic lab fairly frequently, but is an under-reported cause of death in wildlife as there is no systematic recording of these occurrences. Although the cause of death was known, this grizzly bear was autopsied and tissues were sent to our lab for microscopic investigation as part of an overall health assessment of the animal. There were a couple of noteworthy findings.
First, the bear had parasitic Trichinella larvae embedded in the musculature of the tongue (see photograph).These round worm parasites are difficult to detect without specialized techniques. In this case, they were of little or no significance to the bear. Their importance is that they can be transmitted to people through the consumption of improperly prepared meat and can cause significant illness, even death. For example in 2000, 78 individuals from 2 communities in northern Saskatchewan were exposed to Trichinella after eating infected bear meat. The exposure was discovered after 5 individuals were transferred to the hospital in Saskatoon for diagnosis and treatment after showing clinical signs of fever, skin rash, diarrhea, muscle pain and swelling of the limbs and around the eyes. Once the diagnosis of trichinellosis was made on these individuals, an outbreak investigation revealed the extent of the exposure. Thirty-one individuals of those exposed were confirmed to have been infected and the vast majority of these had eaten meat that had been dried over an open fire whereas the majority of those that didn’t become infected had eaten meat that had been boiled. The meat dried over the fire had not reached high enough temperatures to kill the parasite. Thorough cooking of infected meat is needed to kill the parasite.
The life cycle of this parasite depends on predation and scavenging of carcasses (see figure for details). There are at least 10 genotypes of Trichinella found worldwide of which 5 are found in North America, all of which can cause disease in people. The type most people are familiar with is Trichinella spiralis, which infects pigs and occasionally horses. Commercially raised pigs in Canada are considered free of this parasite based on surveillance and previous eradication programs; human infections do occur through the consumption of wild boar or poorly managed outdoor-reared, free-range, domestic pigs.
The bear from northern Saskatchewan was infected with Trichinella nativa, and I suspect so was the grizzly bear, based on the host and its geographic location. Additional molecular testing will hopefully confirm the species. Trichinella nativa has evolved for northern climates and can survive freezing for long periods of time, unlike Trichinella spiralis. Walruses, bears, foxes, and other carnivore/omnivore species can also harbor this parasite.
The Canadian Wildlife Health Cooperative (CWHC) collects samples from wild boar and other species for their own research on Trichinella, and for use by other researchers to better understand prevalence, genetic variants, and geographic distribution of this important zoonotic pathogen.
The other interesting microscopic finding in this grizzly bear was the presence of another parasite called Sarcocystis. Sarcocystis has a two-host, predator-prey life cycle. Within the prey host the parasite has a resting, cystic stage within muscle cells or occasionally the brain. The cyst contains numerous tiny, banana-shaped organisms. Once eaten and digested by a suitable prey host these organisms are released and continue their development in the predator’s intestine. Stages of the parasite that are infectious to prey are released into the environment via the feces of the predator. Prey become infected through ingestion of contaminated material. There are numerous species of Sarcocystis, each one adapted to a particular predator-prey cycle. What was interesting about this case was the presence of the resting cystic stage, which is typically found in prey, in the muscle of a predator. This parasite has been detected previously in the muscle of brown bears from Alaska. Although Sarcocystis infections are typically benign and a common incidental finding at autopsy, there was a recent report from British Columbia where a grizzly bear cub died of massive liver failure due to Sarcocystis infection. The Sarcocystis from the dead bear cub was genetically very similar to the parasite detected in Alaska and to other Sarcocystis identified as causing fatal liver disease in polar bears, dogs and other species. The relationship and life-cycle of these parasites remains to be determined, as does the circumstances by which fatal disease occurs.
Much can be learned by viewing with the light microscope the tissues collected from autopsies of wildlife. The tissues collected can also be used for genetic studies of infectious agents and can be stored indefinitely for future study. Systematic recording of autopsy findings of wildlife species in the CWHC National database (Wildlife Health Information Platform: WHIP) contributes to our understanding of diseases and causes of mortality in wildlife populations in Canada.
This article first appeared in the Western Canadian Game Warden magazine. Re-posted with permission.
Trent Bollinger, CWHC Western Northern Regional Director