Deer Impacts on Safety and Health

Deer Collisions with Vehicles Harm People, Hurt Deer, and Necessitate Costly Repairs

As a local deer herd increases in size, there is a corresponding increase in deer-vehicle collisions (DVC). This means damage to cars, pain and death to deer, and sometimes injuries or even death to humans. 

It has been well documented that DVC’s are correspondingly reduced when the deer population is reduced. In the study, in three cities, “local deer herds were reduced by 54%, 72%, and 76%, with resulting reductions in DVCs of 49%, 75%, and 78%, respectively.”

As the deer population swelled, Ann Arbor reached a peak of 90 reported deer-vehicle crashes within Ann Arbor in 2015,  before the deer management program started. We were culling with cars. In 2019, after four years of active deer management, the annual crash totals decreased to 50. 

DVC graph, courtesy The City of Ann Arbor

Here is a map of the deer involved/associated crashes in the City of Ann Arbor in 2019 from Michigan Traffic Crash Facts.

Map of DVC’s in Ann Arbor, from

Deer vehicle crashes from 2011-2019, from the City of Ann Arbor Deer Management GIS.  Individual years of DVC’s (and deer counts) can be plotted on the above link.

Michigan averages almost 50,000 car-deer crashes every year, including over 1,200 injuries and 14 deaths. This makes deer the most dangerous animal in the state, and the most significant road hazard after drunk driving.

Deer Help to Spread Tick-borne Illnesses

Deer are associated with tick-borne illnesses including Lyme disease. Deer are the typical reproductive host for the deer tick or black-legged tick species that carries the Lyme disease-causing bacteria. Small mammals such as mice are the reservoir of the bacteria, not the deer themselves.  Nymph-stage ticks feed off the mice and then bite humans, spreading the infection.  But it is deer that usually provide the blood meal that the adult female tick requires to be able to lay eggs, thereby beginning the life cycle of the deer tick anew. See University of Rhode Island Tick Encounter FAQ Question 01

Locally acquired Lyme disease has been documented in Washtenaw County.  It is quite possibly already in Ann Arbor but is not yet officially recorded by County Health or the CDC as of Feb. 1, 2021. See Washtenaw County Health Department, Lyme Disease in Washtenaw County and Emerging Disease Issues, Michigan and U.S. Lyme Disease Map 2020.

Controlling the density of the deer population limits the incidence of Lyme and other tick-borne illnesses. From the downloadable Connecticut Tick Management Handbook pp. 52-61, “The abundance and distribution of I. scapularis (the deer tick) has been related to the size of the deer population. It has been estimated that over 90% of adult ticks feed on deer, each laying ~3,000 eggs. Therefore, deer are the key to the reproductive success of the tick.” See also Michigan Department of Health and Human Services  Ticks and Your Health: Preventing tick-borne diseases in Michigan  and  Journal of Medical Entomology, “The Relationship Between Deer Density, Tick Abundance, and Human Cases of Lyme Disease in a Residential Community” and Journal of Integrated Pest Management,  “Deer Reduction is a Cornerstone of Integrated Deer Tick Management.”

The New York State Community Deer Management Handbook says “Reducing deer populations to very low levels can reduce tick densities and probably Lyme disease rates, because deer are the primary food source for adult female black-legged ticks.

The latest research provides some additional precision.  It appears that if the number of deer per square mile is above 13 there is indeed no relation between the density of deer and number of deer ticks. But if deer density is 13 per square mile or below, every deer you remove from that square can potentially reduce the presence of deer ticks and of Lyme disease.

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