January 24, 2017

Vector-Borne Diseases in Gray Wolves in Wisconsin

A report published in PLOS One describes the prevalence of four vector-borne diseases in gray wolves in Wisconsin. 
By Laurie Anne Walden, DVM, ELS
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.



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