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But deer cause Lyme disease, right?

“It’s commonly believed the Lyme disease risk is tied to the presence of deer ticks and white-tailed deer. But this simply isn’t correct.” - The Cary Institute


Over 100 peer-reviewed studies on Lyme disease are analyzed in the book "Lyme Disease, The Ecology of a Complex System." It's complicated, but in sum: deer aren't responsible for Lyme disease. And the Cary Institute, one of the leading research centers working on Lyme disease, says deer are taking the fall, but they're not to blame for Lyme disease

Notably, the American Lyme Disease Foundation does not advocate for deer culls. Nor does the Lyme Disease Association. Nor does the CDC.

Truth #1: Deer don't carry Lyme disease. Ticks do.


Ticks do not actually get Lyme disease from deer, as is commonly believed—rather, ticks contract it as larvae when they feed on infected mice. Adult female ticks need a host animal (e.g., deer) to lay their eggs and for food, but the host doesn't become infected. As the Michigan DNR says, "Infected ticks that drop from deer present little risk to humans or other animals since the ticks are now at the end of their life cycle and will not feed again."


Ticks are only dangerous if they're infected with Lyme disease, and deer play no role in infecting them.

Find out more about the tick life cycle here.


In a Harvard study conducted in Massachusetts, they reduced their deer from around 400 in 1983 to just over 100 in 1991, yet Lyme disease kept growing. Why? As the researcher says, "The deer do not carry the bacteria. They [or another host] are needed to continue the life cycle of the tick, but they are not infected. So as you killed deer, you would simply have more ticks per deer because the surface area of each is enough to support many ticks. Just killing deer won’t do the job."

Truth #2: Killing deer doesn't kill ticks


2007 study focused on an area in New Jersey that was doing deer culls. Researchers found that the number of ticks actually increased after deer culls, and the Lyme disease incidence rate remained steady, despite the drop in deer population from the deer cull.






Truth #3: Decreasing deer doesn''t decrease Lyme disease


In the first 15 years after Lyme disease was uncovered, researchers thought deer were critical to the life of ticks because many adult ticks like to feed on deer. They were excited to find that after deer were eliminated from New England islands, tick populations plummeted, and Lyme disease was wiped out. Unfortunately, nature is more complicated than that.


First, adult ticks like to feed on other animals, too-- raccoons, skunks, opossums and more. 


The only evidence of a relationship between deer numbers and Lyme disease risk took place on islands where there were no other suitable animal hosts for adult ticks. 



As the Michigan Department of Natural Resources writes,  "As a management tool for Lyme Disease, there is still debate in the scientific community as to whether reducing the number of deer present in an area will effectively or dramatically reduce Lyme Disease 'risk'."


The paper "Will Culling White-Tailed Deer Prevent Lyme Disease?" published in Zoonoses and Public Health in December 2015 summarizes and evaluates the evidence and concludes, "To date, most studies have been conducted in ecologic situations that are not representative of the vast majority of areas with high human Lyme disease risk. Robust evidence linking deer control to reduced human Lyme disease risk is lacking."


A growing body of evidence suggests Lyme disease risk is linked to the numbers of small-mammal hosts that infect the ticks. "In addition, the continuing and rapid increase in Lyme disease over the past two decades -- long after the recolonization of deer -- suggests that other factors, including changes in the ecology of small-mammal hosts may be responsible for the continuing emergence of Lyme disease... and increases in Lyme disease in the northeastern and midwestern United States over the past three decades are frequently uncorrelated with deer abundance."


Harvard's Andy Spielman showed that getting rid of the deer in a region did not eliminate Lyme disease. And there was a seasonal relationship with Lyme outbreaks that corresponded with the life cycle of the tick, but not necessarily with that of the deer. 


Ultimately, reducing the infected tick population can reduce the risk and incidents of Lyme disease.

Lyme disease is rare or absent in parts of the US
where deer are abundant.

Lyme Disease information from the Centers for Disease Control. Deer population estimates compiled from multiple sources.


Truth #4: There is a correlation between ticks and Lyme disease. There is no correlation between deer and ticks.


Studies done over a decade ago on an island and peninsula found tick populations plummeted when they nearly eradicated deer. However, not only are mainlands are different in terms of animal populations, but also more recent studies have found something else.


When deer numbers drop, ticks can either crowd in on the remaining deer or find other hosts. As S.R. Telford in Ecology and environmental management of Lyme Disease says, "Eradication... is not achievable for any vector-borne infection."


Several recent studies (e.g., Jordan and Schulze, 2005; Ostfeld et al., 2006; Jordan et al., 2007) on mainland sites in New York and New Jersey found no correlation between deer and ticks. 




Truth #5: Ticks are complicated.

The latest science says infected black-legged ticks that bite us are the ones to blame for Lyme disease. They got the infection from mice. And then they spread it around by hopping on host animals.


It's important to understand the complicated lifecycle of ticks. In a process that takes about 2 years, ticks develop from an egg to a larva, larva to nymph, and finally from a nymph to an adult. Larvae and nymphs need blood to get to the next stage, and adult females need blood to lay eggs. At each stage, the tick seeks an animal host for a single blood meal and then drops off the host. They're usually on their hosts only about two and a half weeks. The rest of the time is spent off the host, developing into the next stage and waiting for another host to come along. Hosts can be lots of different animals including deer, squirrels, chipmunks, mice -- and over 40 species of birds and songbirds. In the spring, the eggs hatch into larvae which feed on mice, raccoons, squirrels, rabbits and other animals. In the fall, the larvae turn into

The Life Cycle of Black-Legged Ticks,

from the Centers for Disease Control

nymphs which hibernate over winter. And then in the spring, the nymphs wake up and munch, typically on mice.


Most of the ticks that bite and infect us are nymphs, which are most active in the summer months when people and pets are also most active. With climate change, these ticks are projected to be more widespread than ever before.


Richard Ostfeld, disease ecologist at the Cary Institute says, "While deer usually gets the blame for spreading tick-borne disease, rodents are actually the primary threat... Our research suggests that white-footed mice are more important numerically."


Tamara Awerbuch, instructor in the Department of Global Health and Population at the Harvard School of Public Health and a specialist in emerging epidemics, has done extensive research on deer ticks. Based on her studies, she says killing deer won’t effectively combat Lyme disease because ticks also depend on another key host animal: white-footed mice.


The Institute of Ecosystem Studies concurs, concluding that "The incidence of Lyme disease is determined by white footed mice and tick population density rather than by deer." 


Truth #6: Washtenaw County is at low risk for Lyme disease.


According to the Washtenaw County Health Department, even though there may be one case of Lyme disease here, "the likelihood of infection is probably still low in our area." by the city’s own report, “Washtenaw County is not identified, at this time, as a county with a potential risk of Lyme disease.”


According to the Michigan Department of Natural Resources, "The risk of developing the illness is minimal in Michigan and even if infection occurs the disease can be diagnosed and treated with antibiotics."

According to our state government, "The relationship between deer and the disease is complex. Deer show no symptoms of the disease. Deer may carry small numbers of the spirochete that causes Lyme disease but they are dead-end hosts for the bacterium. Deer cannot infect another animal directly and no deer hunter has acquired the disease from dressing out a deer. Infected ticks that drop from deer present little risk to humans or other animals since the ticks are now at the end of their life cycle and will not feed again. There is no evidence that humans can become infected by eating venison from an infected deer."



Truth #7: A cull could put us at risk for disease transmission.


The USDA National Wildlife Research Center says "Because disease transmission is often a function of population density, culling is sometimes used to slow or prevent the spread of a disease. This technique also has disadvantages, because it can lead to an influx of new diseased animals, and because populations generally rebound quickly." (This is why culls must be done year after year after year.) Washtenaw County is not at risk now, but Lyme disease is endemic in other counties. Killing our healthy deer leaves room for other animals -- possibly diseased ones -- to move in. 


If we want to reduce a population, but are concerned about wildlife disease transmission, the USDA National Wildlife Research Center suggests wildlife fertility control.



According to The New England Journal of Medicine, "Lyme disease is rarely, if ever, fatal, and is not contagious." And scientists are working on an anti-tick vaccine




Truth #8: There are lots of zoonotic diseases... some much worse than Lyme disease.


Humans and animals can pass many diseases back and forth, some very serious, even deadly; here is a long list from the CDC. The question is: If there's a potential risk of disease from a species, should we destroy that species? Because our dogs can transmit rabies, should we eradicate them? Or should we research vaccines and other ways to prevent transmission and contraction -- as well as methods of treatment? Should our dollars be going to hire sharpshooters to kill deer, or should they be going to help places like Columbia University Lyme and Tick-Borne Diseases Research Center?


According to The New England Journal of Medicine, "Lyme disease is rarely, if ever, fatal, and is not contagious." Scientists are working on an anti-tick vaccine, but until then, look to the CDC for tips on preventing tick bites, removing ticks, and the latest on Lyme disease.

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