Lyme disease is caused by a bacterium (a spirochete, similar to the syphilis bacterium) that is spread by black legged ticks from mice, shrews, chipmunks and birds to humans. The tick is a deer tick that depends on the existence of a high density of deer for its survival, around 10 to 12 deer per sq mile. When deer are kept in balance below that number, multiple studies have shown there are very few ticks and little or no Lyme disease.
How does this work so effectively?
Adult female deer ticks depend on a large 5 to 7 day blood meal on a large mammal that is present in large enough numbers for the ticks to reliably find one. Smaller mammals do not result in the level of fertility the tick needs for the tick species to go on and thrive. Fewer deer in an area reduce the chances of successful tick feeding and mating such that when deer numbers fall below a density of around 10 to 12 per square mile, ticks fail to succeed and the species dies out after two years. Lowering deer densities when they are high but still remaining above a certain level seems to have little effect on tick numbers as there is still a surplus of deer around that the ticks can thrive on. Hence the disappointing results of studies that only change deer densities in that upper range- from say 100 per sq mile down to 50 per sq mile as was done in the Bernards Township, NJ study- and then mistakenly reported as a failure of deer reduction to alter tick numbers or prevent Lyme disease! If the researchers went back there now and counted ticks after deer have been reduced from 110 per sq mile to near 10 per sq mile, they would find a very different result.
The Connecticut Agricultural Experiment Station's regular report, "Frontiers of Plant Science" has a summary paper in the Spring 2001 Vol 53 #2: "An increasing deer population is linked to the rising incidence of Lyme disease." by Dr Kirby Stafford PhD, says ..."A major problem related to the increasing population of white tailed deer is Lyme disease.” And "...the abundance and distribution of the tick is correlated with deer density, and the increase in Lyme disease is related to the resurging deer population."
Dr Stafford also states that "Deer are key to the reproductive success of deer ticks." (Tick Management Handbook 2007)
"Reducing deer densities to below 10 to 12 per sq mile has been shown to substantially reduce tick numbers and human Lyme disease." Kirby Stafford lll PhD, November 2007
"Experts agree that culling the deer herd would reduce the incidence of Lyme disease. But there are a number of impediments to achieving this goal. These include a general lack of awareness of the connection between deer and Lyme disease." Connecticut Post Dec 26 2007.
Furthermore the Connecticut state Department of Public Health openly deny that they have a role to play in informing the public of this most effective method of ending the Lyme disease epidemic. In a December 2007 interview Randall Nelson, a veterinarian and the "expert on zoonotic, or animal-borne, illnesses for the state Department of Health", said that in Connecticut, there were 1,788 reported cases in 2006, and 355 in Fairfield County. "There's no question — we have a lot of tick-borne illness in the state," he said, adding that the actual number of Lyme cases is much greater, owing to under-reporting of the disease.
But, he said, culling the deer population "is a wildlife-management issue" and doesn't fall under the area of the state health agency. This despite the now obvious connection between deer densities above a balanced level of 10 to 12 per square mile and the ability of the vector ticks to thrive and spread Lyme disease.
Simply reducing deer numbers to natural levels, without any other actions of any kind taken, can eradicate Lyme disease.
The evidence is clear and indisputable that as deer numbers go up in a region or state so do tick populations and human numbers of Lyme cases. All regions with high Lyme rates have high deer populations. Those with low deer populations have low or zero Lyme disease rates.
When deer are removed from an island (e.g. Monhegan, Maine) or have always been absent (e.g. 4 of the 6 Narraganset Bay Islands off Rhode Island), but all other animals such as white footed mice, chipmunks and birds are still present, there is no Lyme disease whatsoever.
reproduced from "Tick Management Handbook" revised edition 2007, page 10
The tick life cycle explains why this is so-- adult ticks depend on feeding on deer for fertility. The blacklegged deer tick species does not survive to start the next generation of potentially disease bearing ticks without deer.
Stafford et al 2003 Journal of Medical Entomology 40: 642-652
|Figure 1: This study demonstrates a dramatic reduction in tick numbers (purple line) in 1995, 2 years after deer populations were reduced in 1992-3. Tick numbers fell by 92% to almost zero, even though deer were only reduced by 74%. This dramatic reduction in tick numbers is typical when deer numbers are brought back to stable balanced numbers.|
Communities that have successfully reduced their deer numbers have dramatically reduced or eradicated ticks and Lyme disease (e.g. Monhegan, Maine; Mumford Cove and Groton Long Point, Connecticut)
CT DEP data
|Figure 2: At Mumford Cove an attempt to control deer numbers using contraceptives for 3 years failed to prevent a rising deer population. In 2000 the deer numbers were reduced by hunters down to 10.4 deer per sq mile and have been held at that level since then. There are now only 2 to 3 Lyme cases a year in this community compared to 30 new cases a year prior to deer reduction. The deer are now maintained at a steady 10 or so per square mile very easily by two pairs of hunters once a year. There are now virtually no ticks to be found in this community.|
Figure 3. Relationship of tick density to deer
|Figure 3: In a mainland situation where deer roam freely, Rand's study 5compared the abundance of adult deer ticks with the presence of deer along multiple 1000 ft transects within 8 study sites throughout southern Maine. Sampling included 74 transects, each examined from 1 to 3 years, for a total of 155 records (a total of 29 transect miles). Deer density, here in terms of ticks per square mile, was estimated from pellet group counts using a published conversion factor. As seen in the accompanying plot, deer density was highly positively correlated with tick abundance. In this study we found few ticks where deer densities dropped below 15/mi2.|
You do not have to live on an island to be successful at reducing deer numbers-- it has been done in Mumford Cove, CT, in Bridgeport, CT, and on the mainland of Maine and in several inland highly populated New Jersey towns. The main reason that islands or island-like sites are often cited as successful examples of deer reduction programs is that they are small communities where population-wide awareness and education on the role of deer in supporting ticks is easier to achieve and consensus on solutions is more easily reached. There is no truth to the rumor that "deer will move in to a mainland site just as quickly as they are removed". According to deer biologists deer are territorial and have a defined home range that they do not readily move out of. It can take several years for deer to realize there is vacant territory next door. This explains the success of the mainland deer reduction programs in New Jersey. Obviously a regional or state-wide approach will be most effective and will benefit the most people.
Role of high deer numbers revealed and confirmed by Connecticut Tick experts: Weston Forum article
Facts from the "CT DEP Booklet: Managing Urban Deer in Connecticut. A guide for Residents and Communities" 2nd Edition:
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