Patterns of Pathogen Exposure in Gray Wolves

A biologist transfers wolf serum into vials for disease screening.

NPS/Jillian Richie

Pathogens (that is, bacteria, viruses, and other microorganisms that can cause disease) tend to be related to latitude, being more prevalent toward the equator and less toward the poles. We investigated to see if this trend held for pathogens infecting grey wolf populations across North America.

We compiled a large dataset of blood serum samples from nearly 2,000 wolves in 17 study areas, spanning 80° longitude and 50° latitude. This included wolf populations from Mexican wolves in New Mexico and Arizona; wolves in Ontario, Canada and Isle Royale National Park in Michigan; wolves from the Rocky Mountains and north into Canada’s Northwest Territories; and Alaska—from the Alaska Peninsula to Yukon-Charley Rivers National Preserve. The wolf monitoring program contributed samples from 105 wolves from Yukon-Charley Rivers National Preserve collected between 1993 and 2019 and 154 samples from Denali National Park and Preserve collected between 2003 and 2019. Samples also came from several other parks across the country.

We identified human density as a major driver of pathogen exposure at a continental scale. We also found that the prevalence of pathogens increased as wolves aged, and denser wolf populations had a greater risk of exposure. Only one disease, N. caninum (a parasite), was linked to latitude due to its high prevalence in white-tailed deer, an important prey species of wolves that occurs in more southernly latitudes.

The study also found that the prevalence of all pathogens in wolves from Yukon-Charley Rivers National Preserve and Denali National Park and Preserve was either lower than or equal to the average of all wolf populations, suggesting they are a relatively healthy population. This is likely due to the remoteness and higher latitude of the parks. The lower human density within and adjacent to the parks likely reduces transmission between dogs (or other pets) and wolves since human density may be a proxy for density of unvaccinated dogs that act as reservoirs for infectious diseases that spillover into wolves. Since white-tail deer do not occur in northern latitudes, including Alaska, that also likely contributed to the low level of occurrence of N. caninum in the parks' wolves. Potential outbreaks of canine distemper were identified in the Denali wolf population in the early and late 2000s and future work will help clarify the potential relationship between the incidence of these diseases and demographic rates.

Eleven members of the Yukon Fork wolf pack rest along a ridge top.

NPS/Matt Cameron

Patterns and processes of pathogen exposure in gray wolves across North America

Abstract

The presence of many pathogens varies in a predictable manner with latitude, with infections decreasing from the equator towards the poles. We investigated the geographic trends of pathogens infecting a widely distributed carnivore: the gray wolf (Canis lupus). Specifically, we investigated which variables best explain and predict geographic trends in seroprevalence across North American wolf populations and the implications of the underlying mechanisms. We compiled a large serological dataset of nearly 2,000 wolves from 17 study areas, spanning 80° longitude and 50° latitude. Generalized linear mixed models were constructed to predict the probability of seropositivity of four important pathogens: canine adenovirus, herpesvirus, parvovirus, and distemper virus—and two parasites: Neospora caninum and Toxoplasma gondii. Canine adenovirus and herpesvirus were the most widely distributed pathogens, whereas N. caninum was relatively uncommon. Canine parvovirus and distemper had high annual variation, with western populations experiencing more frequent outbreaks than eastern populations. Seroprevalence of all infections increased as wolves aged, and denser wolf populations had a greater risk of exposure. Probability of exposure was positively correlated with human density, suggesting that dogs and synanthropic animals may be important pathogen reservoirs. Pathogen exposure did not appear to follow a latitudinal gradient, with the exception of N. caninum. Instead, clustered study areas were more similar: wolves from the Great Lakes region had lower odds of exposure to the viruses, but higher odds of exposure to N. caninum and T. gondii; the opposite was true for wolves from the central Rocky Mountains. Overall, mechanistic predictors were more informative of seroprevalence trends than latitude and longitude. Individual host characteristics as well as inherent features of ecosystems determined pathogen exposure risk on a large scale. This work emphasizes the importance of biogeographic wildlife surveillance, and we expound upon avenues of future research of cross-species transmission, spillover, and spatial variation in pathogen infection.

Brandell, E. E., P. C. Cross, M. E. Craft, D. W. Smith, E. J. Dubovi, ...B. L. Borg, M. Sorum, ... et al. 2021. Patterns and processes of pathogen exposure in gray wolves across North America. Scientific Reports 11: 3722.