“Some of that decline can be attributed to a reduction in trapping activity, but clearly the muskrat population is significantly smaller than it used to be,” he said. “A number of theories to explain the widespread muskrat declines have been proposed, including habitat loss, predation, environmental contamination and diseases. In this study, we examine a number of those possibilities.”
To analyze trends in muskrat mortality, researchers pored over 131 articles, published from 1915 to 2019, from 27 U.S. states and nine Canadian provinces that contained information about muskrat exposure to diseases and contaminants and mortality events. Information collected from articles included; year of survey; location of survey; methodology; number of animals surveyed; pathogen or contaminant identities; and the presence or absence of associated disease, as evidenced by reported clinical signs or lesions.
Among the common factors reported associated with muskrat infections or mortality in some cases were: viruses including canine distemper virus, rabies and Aleutian mink disease virus; a variety of fungal infections; ailments such as tularemia and Tyzzer’s disease; cyanobacteria, possibly indicating the presence of toxic algae; parasites including protozoans, trematodes, cestodes, nematodes and ectoparasites such as ticks; toxins, including heavy metals from industrial discharges and lead from ammunition deposits; and agricultural-related contaminants including pesticides, herbicides and insecticides.