I just received an email from a colleague. There is an ongoing email thread that I enjoy very much where colleagues share research, ideas, and generally debate about what is happening in the wildlife discipline. Bernd did such a great job summarizing the latest research and issues surrounding Deer and Lyme disease, I simply asked him if I could publish his message. There is so much misinformation and swirl around what is arguably a critical health issue and his conclusions were spot on. Thanks Bernd for the great wrap up. The studies he references are below.
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There are 3 important new papers out that shed light onto the never-ending discussion about deer, ticks and Lyme disease. For the longest time, the link between Lyme disease and deer has been in dispute in the scientific literature and obviously in the public. Despite some encouraging news from islands where deer population declines or eradication resulted in disappearance of Lyme disease, the debate centered on the role of mammals, particularly mice and chipmunks, and mesopredators (foxes, skunks, coyotes etc.) etc. With these 3 new papers (see references and abstracts below), I can now confidently stand up and tell everyone who wants to listen that the link between deer abundance, tick abundance, incidence of Borelliosis in the ticks, and Lyme disease incidence in humans has a direct link. These papers are from CT, Thousand Islands Region in Canada and Indiana. They use very different methods, but all come to the same conclusions.
While we do not understand the direct mechanisms, it is clear that deer management, if populations are dropped to low levels (5-8 per square kilometer?), results in human health benefits where ticks and Lyme disease are prevalent, and it is also expected to have all the ecological benefits that I am so concerned about. We certainly need to do more to nail down the mechanisms, the possibility to accomplish this in other areas, and the ways and means to actually do this over large areas. Regulated hunting appears to be able to contribute to some population declines in some cases, but there is no evidence to suggest we can get to <17 deer per square kilometer; not sufficient to affect Lyme disease.
I believe it is now up to wildlife managers and health departments to chart a course that will move us into the right direction. Regulated hunting, in the ways it has been used and practiced for the last century has not delivered what we need. My prediction is that even if we allow each hunter to take as many animals as possible, we will not see much of a change since most hunters are just out to take one for the wall, one for the freezer and maybe one for a friend. That is 2-3 deer per season. The few individuals, who do more as volunteers and stewards, will not change the overall picture significantly. I understand that this will not go over well. But I think we can't allow 15% of the population to determine what we see and experience in the landscapes around us (I am one of the 15%).
Lastly, particularly the Kilpatrick et al study in CT debunks the myth that areas with reduced deer densities are quickly refilled by immigration and increased fawning rates. Make no mistake, a cull, however effective, is no permanent solution, but as Jim Sterba pointed out, weeding a garden never stops, but once you have a low population it is much easier to maintain. I am not comparing shooting animals to weeding, but the population principles hold.
Dr. Bernd Blossey
Department of Natural Resources
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Please reach out if you’d like PDFs of the studies.
Geography, deer, and host biodiversity shape the pattern of Lyme disease emergence in the Thousand Islands archipelago of Ontario, Canada.
Werden L, IK Farker, J Boman, et al.
2014. PLos ONE. 2014 Jan9;9(1):e85640. doi:10.1371/journal.pone.0085640.
In the Thousand Islands region of eastern Ontario, Canada, Lyme disease is emerging as a serious health risk. The factors that influence Lyme disease risk, as measured by the number of blacklegged tick (Ixodes scapularis) vectors infected with Borrelia burgdorferi, are complex and vary across eastern North America. Despite study sites in the Thousand Islands being in close geographic proximity, host communities differed and both the abundance of ticks and the prevalence of B. burgdorferi infection in them varied among sites. Using this archipelago in a natural experiment, we examined the relative importance of various biotic and abiotic factors, including air temperature, vegetation, and host communities on Lyme disease risk in this zone of recent invasion. Deer abundance and temperature at ground level were positively associated with tick abundance, whereas the number of ticks in the environment, the prevalence of B. burgdorferi infection, and the number of infected nymphs all decreased with increasing distance from the United States, the presumed source of this new endemic population of ticks. Higher species richness was associated with a lower number of infected nymphs. However, the relative abundance of Peromyscus leucopus was an important factor in modulating the effects of species richness such that high biodiversity did not always reduce the number of nymphs or the prevalence of B. burgdorferi infection. Our study is one of the first to consider the interaction between the relative abundance of small mammal hosts and species richness in the analysis of the effects of biodiversity on disease risk, providing validation for theoretical models showing both dilution and amplification effects. Insights into the B. burgdorferi transmission cycle in this zone of recent invasion will also help in devising management strategies as this important vector-borne disease expands its range in North America.
White-tailed deer (Odocoileus virginianus) as a potential sentinel for human Lyme disease in Indiana.
Raizman, E. A., J. D. Holland, and J. T. Shukle.
2013. Zoonoses and Public Health 60:227-33.
We assessed the potential of white-tailed deer (WTD) (Odocoileus virginianus) to be a sentinel for human cases of Lyme disease (LD) in Indiana using location data from a 3-year survey of approximately 3400 hunted deer with associated tick Ixodes scapularis and Borrelia burgdorferi (Bb) data. Data on human LD cases at the county level were obtained from the Indiana Department of Health. All data were assigned to county centroids to match the resolution of the LD data before creating optimized trend surfaces for LD incidence, hunted deer count, Ixodes scapularis and Bb prevalence. To determine whether LD was spatially associated with the areas of high densities of deer, deer with Ixodes scapularis and deer with ticks infected with Bb, we used spatial analysis with distance indices (SADIE). The SADIE analysis found significant spatial association between LD and the distribution of three organismal predictor variables, that is, WTD, Ixodes ticks and Bb. Lyme disease incident rate varied between 0.08 cases per 10 000 habitants (Johnson county) and 5.9 cases per 10 000 habitants (Warren county). In conclusion, WTD can be used as an accurate and cost-effective sentinel for human LD. This method will permit public health workers to identify potentially endemic areas independently of human case reports.
The Relationship Between Deer Density, Tick Abundance, and Human Cases of Lyme Disease in a Residential Community.
Kilpatrick, Howard J.; Labonte, Andrew M.; Stafford, Kirby C.
Journal of Medical Entomology, July 2014
Here's a nice write up on this one in Science Daily.
White-tailed deer (Odocoileus virginianus Zimmerman), serve as the primary host for the adult blacklegged tick (Ixodes scapularis Say), the vector for Lyme disease, human babesiosis, and human granulocytic anaplasmosis. Our objective was to evaluate the degree of association between deer density, tick abundance, and human cases of Lyme disease in one Connecticut community over a 13-yr period. We surveyed 90‹98% of all permanent residents in the community six times from 1995 to 2008 to document resident's exposure to tick-related disease and frequency and abundance of deer observations. After hunts were initiated, number and frequency of deer observations in the community were greatly reduced as were resident-reported cases of Lyme disease. Number of resident-reported cases of Lyme disease per 100 households was strongly correlated to deer density in the community. Reducing deer density to 5.1 deer per square kilometer resulted in a 76% reduction in tick abundance, 70% reduction in the entomological risk index, and 80% reduction in resident-reported cases of Lyme disease in the community from before to after a hunt was initiated.