Vector-Borne Diseases in Gray Wolves in Wisconsin

A report recently published in PLOS One describes the prevalence of four vector-borne diseases in gray wolves in Wisconsin, comparing disease distribution in wolves with distribution reported in domestic dogs and humans. Although wolf exposure was higher than domestic dog exposure for all four diseases studied (Lyme disease, anaplasmosis, ehrlichiosis, and heartworm disease), growth of the Wisconsin gray wolf population did not appear to be affected.

“World-wide concern over emerging vector-borne diseases has increased in recent years for both animal and human health,” write the authors. “It is useful to study vector-borne exposures in wolves to better understand health threats to wolf populations, but also because wolves can serve as a sentinel species for human and domestic dog health risk.”

The researchers used blood and serum samples collected from gray wolves in Wisconsin between 1985 and 2011. They tested the samples with the IDEXX SNAP 4Dx test, which is used to test domestic dogs for heartworm antigen and for antibodies to the causative agents of Lyme disease, anaplasmosis, and ehrlichiosis. A validation test showed that SNAP 4Dx could reliably diagnose Lyme disease in wolves, but it failed to yield a positive result for over one-third of wolf anaplasmosis and heartworm cases. Therefore, say the authors, this study may underestimate the prevalence of the latter two diseases among wolves.

Disease prevalences and geographic distributions in domestic dogs were gathered from a 2009 study and the IDEXX website. The incidence of Lyme disease in humans was provided by the Wisconsin Department of Health Services.

Lyme Disease

Lyme disease, caused by Borrelia burgdorferi, is vectored in Wisconsin by the black-legged tick, which in turn is hosted by white-tailed deer and other mammals. Over 65% of the wolf samples were positive for B. burgdorferi antibody, indicating previous exposure. The prevalence increased over time, with a 50% increase in the proportion of wolves exposed per decade.

Geographic analysis revealed a cluster of wolf exposure to B. burgdorferi in northwestern Wisconsin, where the exposure prevalence was 76%. The authors note that in this region the deer density is higher than in other areas wolves frequent and that tick density has been increasing. One of the geographic clusters of B. burgdorferi exposure in domestic dogs encompassed the wolf cluster. A human Lyme disease cluster that involved over half of the counties in Wisconsin included both the wolf and domestic dog clusters. Human cases of Lyme disease within this cluster increased 42 cases per decade between 1989 and 2011, say the authors. They suggest that the corresponding increases in prevalence among wolves and humans could be caused by ecological changes affecting ticks or the mammal hosts.

Anaplasmosis

Anaplasmosis, a rickettsial disease caused by Anaplasma phagocytophilum, is also vectored by the black-legged tick in Wisconsin, so coinfection of Borrelia and Anaplasma can occur. Almost half of the wolf samples were positive for Anaplasma antibody; most of these were also positive for B. burgdorferi. Wolf exposure did not significantly increase over time, in contrast to the increasing number of human cases during the same period. The authors theorize that the increase in human anaplasmosis diagnoses could have been caused by increased recognition or by humans spending more time in areas with ticks.

Ehrlichiosis and Heartworm Disease

Prevalence of both ehrlichiosis (caused by the bacterium Ehrlichia canis) and heartworm disease (caused by Dirofilaria immitis) in domestic dogs is higher in the southeastern United States than in Wisconsin. Only 5.7% of the wolf samples were positive for E. canis antibody, and 9.2% were positive for heartworm antigen. The proportion of wolves exposed to E. canis decreased 80% per decade during the period studied; the prevalence of heartworm disease did not significantly change over time.

Exposure Higher in Wolves Overall

For all four diseases, wolf exposure was higher than domestic dog exposure, probably because wolves have greater vector exposure and do not receive vaccines or tick preventives. Both wolves and domestic dogs had high rates of exposure to B. burgdorferi and A. phagocytophilum but much lower exposure to E. canis and heartworm disease. The estimated wolf population in Wisconsin increased from 1985 to 2011, so the authors conclude that vector-borne diseases did not appear to restrict population growth. They suggest that because environmental temperature can affect disease prevalence (for heartworm disease, for example), climate change could potentially affect disease risk. “Thus periodic assessment of prevalence and impact of vector-borne diseases will be an important part of future wolf conservation programs,” they write.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.