Tularemia Outbreak in Muskrats at Long Point, ON

On April 13, the Canadian Wildlife Health Cooperative Ontario/Nunavut Region (CWHC ON/NU) was alerted to a mass mortality event involving muskrats in the Crown Marsh area of Long Point. The initial report indicated that over 35 muskrats were found either sick (weak and barely responsive when approached) or dead in the area.

Dead muskrat found in the Crown Marsh area of Long Point. Photo credit: Scott Gillingwater.

Samples were collected from the area on April 16 and 18 and delivered to the CWHC at the Ontario Veterinary College for post-mortem examination. Post-mortem examination on collected muskrat carcasses was performed on April 17 and 20. Multiple muskrats were found to have enlarged internal lymph nodes as well as numerous pinpoint white foci throughout the spleen.Microscopic examination of these tissues revealed random regions of necrosis (cell death) scattered throughout both the liver and spleen with numerous Gram-negative bacteria noted on Gram stain.

Spleen with numerous white foci scattered throughout, which correspond with areas of necrosis. Photo credit: Christina McKenzie.

Based on these findings, spleen and liver samples from three affected muskrats were sent to the Public Health Agency of Canada, National Microbiology Laboratory, in Winnipeg, for further testing. PCR testing revealed that all samples from the three submitted muskrats were positive for Francisella tularensis (the causative agent of tularemia).

Histology of liver necrosis (asterisk) secondary to infection with Francisella tularensis. Photo credit: Brian Stevens.

Tularemia is known to be endemic in Ontario, but mass mortality events in wildlife due to tularemia are rarely reported. In the CWHC database, only 3 other cases were reported in Ontario. There were 2 cases of beavers diagnosed with tularemia in 1994 with one case from the Willowbank area on the St. Lawrence and the second case from the Sioux Lookout area. The most recent reported case was a beaver in 2003 from the Pembroke area.

Francisella tularensis causes disease in many lagomorphs (Eastern cottontail rabbits in this region) and rodents (including mice, beavers, and muskrats). The bacteria can be passed by direct contact, via arthropod vectors (ticks, deer flies), ingestion of contaminated water, and inhalation. Death commonly occurs within 2 weeks of infection and the typical findings on post-mortem examination are enlarged liver, spleen, and lymph nodes with numerous white foci scattered throughout.

Francisella tularensis can also infect humans and there are rare reports of tularemia in people from Ontario. Humans can be exposed by direct contact with infected animals (which most commonly occurs when trappers are skinning infected animals), inhaling airborne bacteria in contaminated environments, eating/drinking contaminated food or water, or a bite from an infected arthropod (most commonly ticks and deer flies). The symptoms in people can vary greatly but nonspecific, flu-like symptoms (fever, chills, headache, muscle aches, and joint pain) are common. Other symptoms include skin ulcers, swollen and painful lymph nodes, diarrhea (if infected meat or water is consumed), and, if the bacteria is inhaled, pneumonia.

During an outbreak situation, it is presumed that bacterial levels would be higher in the surrounding environment, so caution is warranted for anyone who is traversing through the area or wading into the water in the Crown Marsh area of Long Point. There is also a danger to off-leash dogs as they can become infected and develop similar symptoms to humans, especially if they consume infected meat. It is recommended that dogs are kept on leash and monitored closely while in this area. It is recommended that people do not handle wildlife found dead unless they are wearing protective gloves (or a similar protective barrier) to prevent direct contact of the animal with the skin. Anyone who handles dead wildlife (even while wearing the appropriate protective gear) should wash their hands thoroughly to minimize the chances of exposure. If anyone encounters dead wildlife in Ontario, please report it to the CWHC ON/NU region at 1-866-673-4781 or e-mail on-nu@cwhc-rcsf.ca.


Submitted by:

Brian Stevens – CWHC ON/NU

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