Excluding deer could be a counterproductive strategy for controlling tick-borne infections, because the absence of deer from small areas may lead to an increase in ticks, rapidly turning the area into a potential disease hotspot, according to a team of U.S. and Italian researchers.
"Deer are referred to as dilution hosts or dead-end hosts," says Sarah Perkins, a postdoctoral researcher at Penn State's Center for Infectious Disease Dynamics. "They get bitten by ticks but never get infected with tick-borne pathogens, such as the bacteria causing Lyme disease."
However, deer are critical to adult female ticks in the last stages of their three-part lifecycle. Ticks use them for a final blood meal before dropping off to produce thousands of eggs, Perkins explains. Currently, health officials believe that removing deer from the equation could disrupt the tick lifecycle and leave fewer ticks to feed on rodents, which, unlike deer, can transfer a range of tick-borne pathogens. Ultimately the tick-borne disease will fade out.
However, previous field studies show that removing deer sometimes leads to higher tick densities and sometimes lower, and the outcome seems dependent on the size of area from which deer are excluded.
"Very few studies have looked at how removing the deer affects the intensity of tick bites on rodents, and how it relates to the size of the area from where the deer are excluded," explains Perkins, whose findings are published in the current issue of the journal Ecology.
Researchers first collected data from published information on tick densities in deer excluded areas ranging in size from roughly 2.5 acres to 18 acres. Next, over a six-month period, they captured rodents from a 2.5-acre deer excluded area in the Italian Alps in a known hotspot for tick-borne encephalitis -- a disease passed to humans through the bite of an infected tick.
"From previous studies we found that tick densities decreased in (geographically) large areas and increased dramatically in smaller areas," suggesting that there is a threshold area -- from where deer are excluded -- for tick populations to either increase or decrease, notes the Penn State researcher.
Statistical analyses of ticks on the captured rodents indicated that compared to the control areas, the deer-excluded areas hosted a significantly higher number of nymph and adult female ticks, as well as a high prevalence of tick-borne encephalitis.
Because tick-borne encephalitis is transmitted only between ticks feeding on these rodents, the findings suggest how small deer-free areas could quickly turn into a disease hotspot.
"This goes somewhat against conventional wisdom. When you remove deer, it does not always reduce the tick population," says Perkins. "If you were to exclude deer from hundreds of acres, tick numbers will fall. But in an area less than 2.5 acres, you are more likely to increase tick density and probably create tick-borne hotspots." Researchers say the study demonstrates how the strategy of keeping deer away may work only for large areas but is likely to amplify tick populations in smaller areas. Fragmented patches of forest and small parks that are off-limits to deer could also turn into a disease reservoir, they caution.
"We need to be cautious about keeping deer away from small areas, even people's backyards, as it might only lead to more ticks that are infected with tick-borne pathogens," says Perkins. She adds that forest areas deer consistently avoid also have the potential of turning into a haven for tick-borne disease.
Other authors of the paper include Isabella M. Cattadori, postdoctoral scholar, and Peter J. Hudson, the Willaman Professor of Biology, both at Penn State University, and Valentina Tagliapietra and Annapaola P. Rizzoli, Centro di Ecologia Alpina, Italy.
A grant from the Autonomous Province of Trento, Italy, supported this work.
The Penn State Center for Infectious Disease Dynamics is at http://www.cidd.psu.edu/.
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