With Deer Come Tick-Borne Diseases

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Wednesday, August 20, 2014, 9:33 am
Alice Dreger interview with Graham Hickling
A field team associated with Dr. Hickling collects and analyzes ticks

This is an interview between Alice Dreger for ELi and Dr. Graham Hickling, an ecologist at the University of Tennessee who specializes in emerging wildlife diseases and their interface with humans and domestic animals. Hickling is a former resident of East Lansing and he continues to collaborate with faculty and students from Michigan State University to conduct annual fieldwork in Michigan. This interview, conducted by email, has been lightly edited for clarity with the final version approved by the interviewee.

Question: What are we seeing in terms of the movement of deer-borne parasites in the state of Michigan, specifically with regard to diseases that affect the human population?

Answer: I'll answer this in terms of ectoparasites, specifically ticks that can transfer pathogens from wildlife to unsuspecting humans. In recent years, increased deer numbers in Michigan's Lower Peninsula forests have contributed to invasion and increase of blacklegged ticks (often called 'deer ticks’). This is the tick species responsible for spreading Lyme disease, which is already a serious problem in the Northeast and parts of the Upper Midwest. Michigan has been fortunate, however, to have a very low incidence of Lyme: only around 80 human cases are reported each year from this state.

It is important to understand that deer do not themselves carry the Lyme bacteria. Rather, deer provide the blood meal that each female tick uses to produce a clutch of up to 2000 eggs, which then hatches into the next generation of juvenile ticks. More deer means more ticks, and large tick populations then support cycles of the Lyme bacteria among smaller wildlife such as mice and birds.

Lyme disease ticks have been present for many years in Menominee County in the Upper Peninsula. What is new since the early 1990's is a focus of infected ticks that we have been watching develop in the southwestern corner of the Lower Peninsula. These ticks have spread rapidly north along the Lake Michigan coastline, and also inland at a somewhat slower rate. The State of Michigan has an article and map showing Lyme disease risk: Article with Map

Question: Are there other significant diseases spread directly (not through parasites) by deer?

Answer: Deer have the potential to spread diseases such as bovine tuberculosis, brucellosis, chlamydiosis, leptospirosis, campylobacterosis, salmonellosis, cryptosporidiosis, and giardiasis – but catching those from deer is rare, and the risk is primarily from directly handling deer carcasses.

Question: What might we expect, in terms of possible human diseases from deer-borne parasites, as deer increase in number in neighborhoods of East Lansing?

Answer: Blacklegged ticks are still largely absent from East Lansing, but given current trends their numbers may climb in the next 10-20 years. If that happens, we can expect to see locally acquired cases of human Lyme disease. Cases will also begin appearing among dogs, as they are similarly susceptible to the Lyme bacterium. Blacklegged ticks spread at least two other human diseases - anaplasmosis and babesiosis, and so it is possible that those will also begin to appear in mid-Michigan (although it's not likely that they would ever become as common as Lyme).

Question: Would reducing the number of deer decrease the risk of these problems?

Answer: This is a controversial question. On the one hand we have evidence from island eradications and small-scale experiments that completely removing deer will substantially reduce Lyme disease transmission. However, complete removal of deer from large areas is neither technically feasible nor socially acceptable at the present time (even in northeastern states that already suffer from very high Lyme disease burdens). 

So the relevant question is whether feasible, affordable reductions in deer numbers would benefit human health. And the potential success of that approach is unproven. Unfortunately, the blacklegged tick is a generalist species that feeds on a wide range of mammals, birds and even reptiles. Once ticks have successfully invaded an area, low numbers of deer may be all that are needed to support a tick population capable of maintaining Lyme transmission. I would speculate that, say, halving the local deer population would have very little beneficial effect on Lyme disease risk.

Question: What can East Lansing residents do now to try to prevent being infected with Lyme disease?

Answer: They can keep in mind that the risk of ticks can relatively easily be managed by taking sensible personal precautions. Wear long pants, long sleeves and repellents when venturing into tick-infested habitats. Carefully check yourself, your kids and your pets for ticks at the end of each day. And make a prompt visit to your doctor for a check-up, and if necessary a short course of antibiotics, if you begin to feel unwell after a tick bite.


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