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
abundance (Rand et al. 2003).

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:

Lyme Disease Surveillance

"Connecticut reported 1,788 cases in 2006, mainly from doctors. But what if labs were still reporting? By assuming that labs report in the same proportion as 2002, we're looking at 5,902 cases for 2006." 
 Yvonne Bokhour and Peter Arno, Professor in the Department of Health Policy and Management, School of Public Health, New York Medical College, Valhalla, N.Y

The number of new cases of physician confirmed Lyme reported to the Connecticut DPH in 2006 was 1,788. But, as discussed in the attached article by Bokhour and Arno, the true number of physician confirmed cases is much higher, for two reasons.  First, until 2003 both doctors and labs were required to notify the state Department of Public Health when patients tested positive for Lyme, but since 2003 the state stopped requiring labs to report positive results.  In 2002, 4,631 cases were reported. But in 2003, cases fell to 1,403. By assuming that labs report in the same proportion as they did in 2002, the comparable number of cases for 2006 would be 5,902.  Second, the CDC tells us in their weekly MMWR report of May 2004 that studies show "that LD cases were underreported by six to 12-fold in some areas where LD is endemic (2,3)". Bokhour and Arno quote that case numbers represent only 10 percent to 20 percent of diagnosed cases.  The authors in the attached article write: "assuming conservatively that case reports represent 20 percent of diagnosed cases, Connecticut's 2006 case numbers jump to 29,509". The CDC suggest using a correction factor of 5 if lab reported cases are included in the raw data and as correction factor of between 6 and 12 if only physician reported cases are included.
So for 2006 the reported number of cases  is 1,788 and the estimated true number of cases is 29,509.

   1. CDC. Case definitions for infectious conditions under public health surveillance. MMWR 1997;46(No. RR-10):20--1.
   2. Meek JI, Roberts CL, Smith EV Jr, Cartter ML. Underreporting of Lyme disease by Connecticut physicians, 1992. J Public Health Manage Pract 1996;2:61--5.
   3. Coyle BS, Strickland GT, Liang YY, Pena C, McCarter R, Israel E. The public health impact of Lyme disease in Maryland. J Infect Dis 1996;173:1260--2. "
   4. Naleway AL et al. Lyme Disease Incidence in Wisconsin: A Comparison of State-reported Rates and Rates from a Population-based Cohort. Am. J. Epidemiol; 2002:155: 1120-1127.
   5. Rand, P. W., C. Lubelczyk, G. R. Lavagne, S. Elias, M. S. Holman, E. H. Lacombe, and R. P. Smith, Jr.: Deer Density and the Abundance of Ixodes scapularis (Acari: Ixodidae). J. Med. Entomol. 40:(2) 179-184, 2003.




Home | Evidence | Lyme Prevention | Environmental Impacts | Economic Impacts | Road Hazards | | Legislation | FAQ | Get Involved- Join CCELD | Scientific Advisors | Resources | Members