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Ticks and Deer

"Understanding what regulates Lyme disease, predicting its spread, and ultimately reducing its transmission to humans, depends on our ability to look at the disease from an ecological perspective. Knowing the players is of limited usefulness unless we also know how they interact with one another." —Dr. Richard S. Ostfeld, PhD

Tick-Borne Diseases
Some people have speculated that Lyme disease and other tick borne illnesses are directly related to deer numbers. The issue is complex and has been made difficult by conflicting scientific views on the subject. It's important to keep in mind how tick-borne illnesses are transmitted so that a long-term, effective strategy for dealing with these diseases can be planned.

There are many kinds of ticks, which are arachnids like mites and spiders, and they have existed for thousands of years. Since the end of the 19th century, scientists have understood the important potential for ticks to carry diseases but several tick-borne diseases have been recognized in humans only recently, including Lyme disease, Ehrlichiosis and Babesiosis. One family of ticks is called Ixodidae and includes American Dog or Wood ticks, eastern and western Black-legged ticks, Rocky Mountain and lone star ticks. Ehrlichiosis is carried by dog ticks, lone star and black-legged ticks. Lyme disease and Babesiosis are carried by eastern and western black-legged ticks.

Ticks feed only on animal blood, which they suck out after biting an animal. They need to feed on 3 different animals in order to change from one stage of their life cycle to the next. After being attached for a while, they spew some of the contents of their gut back into the animal. If the animal is infected with bacteria or parasites, some of them are carried into the tick along with the blood meal, remain in the tick’s gut, and may be regurgitated into the next host when the tick eats, thus spreading the infection. 

In between blood meals, eggs develop into larvae, larvae develop into nymphs and nymphs develop into adults. The ticks are simply carriers of disease and do not become ill. In the larval stage, black-legged ticks will feed on reptiles such as lizards and snakes, on birds and on other small animals. Nymphs and adults feed on larger animals, such as cattle, deer, humans, bears, horses, cats and dogs, coyotes, bobcats and foxes. American dog ticks will feed on cattle, deer, raccoons, opossum, humans and dogs.  

Lyme disease is caused by a type of bacterium known as a spirochete, and was first diagnosed in 1975. Ehrlichiosis is caused by a bacterium known as a Rickettsia and was first diagnosed in 1986 and since then, several different species of the Rickettsia have been recognized causing different illness patterns. The illness can range from a mild bout with few or no symptoms to a life threatening illness which can cause serious sequelae or even death. Babesiosis is parasitic illness, commonly seen in wildlife and recently transmitted to humans. Its symptoms can range from very mild to a serious chronic, relapsing illness. Several other illnesses are also transmitted by ticks. Lyme disease, Ehrlichiosis and Babesiosis have all been present in wildlife for centuries. Lyme disease for example has been demonstrated by DNA studies to have been present in a white footed mouse preserved since 1894.

Infectious diseases in general have certain patterns of behavior in common and the incidence of each changes over time. When an illness first enters the human population, a few cases will be identified, but over time the numbers gradually increase and the illness may reach epidemic proportions. By natural selection, as the host defense mechanisms mutate and adapt, the incidence of the illness wanes.

In the middle ages, for example, the bacteria that cause the plague or black death were responsible for millions of deaths. Today plague infections still occur quite often but are generally mild, and no longer cause epidemics. Lyme disease has generated much publicity because of a rise in the number of cases, but this is most likely because humans are at this point in time very susceptible. Ehrlichiosis, babesiosis and other emerging diseases will most likely become more common over time, unless the underlying ecological problems that are responsible for their appearance in humans are addressed.

In the early history of the United States when the European settlers who had developed immunity to common European illnesses transmitted the illness to the native American population, the native people who had not had time to develop immunity, became seriously ill and often died.

Ticks and the illnesses that they cause are not static over time or space. The type of ticks and the kind of illnesses found in a particular geographic area can change quite radically, because of climate changes, population movement, migration of birds carrying tick borne illnesses (Journal of Medical Entomology Volume 38, Issue 4 (July 2001) and loss of animal habitat due to increased human presence. Ticks that are normally found in one area of the country have been found recently in far distant areas, and illnesses transmitted by one kind of tick can become transmitted by another kind.

The symptoms of Ehrlichiosis, for example, resemble and can be confused with those of another rickettsial tick borne illness, Rocky Mountain spotted fever, once found only in western states but now also found in the northeast. Closely related illnesses can be carried by different species of ticks in different geographic areas, some diseases can be carried by more than one kind of tick, and ticks and bacteria can mutate. There are even a few cases where ticks that have not historically carried Lyme disease, the lone star and dog ticks, are thought to be implicated in the transmission of Lyme disease. (Texas A and M University Tick Pamphlet, Garland Mcllveen, Jr., Pete D. Teel and Philip J. Hamman, emedicine, Tick-Borne Diseases, Lyme, Jonathan A Edlow, MD, 12.10.08.) While at present black legged tick bites seem to cause the majority of cases, this could change in the future. With ongoing habitat disruption, as animals and the ticks they carry migrate, we simply cannot predict what the future holds.

It is important to consider the natural world as a whole ecosystem, rather than try to understand complex, multi-host illnesses in terms of one factor, such as deer numbers. The axiom that nature abhors a vacuum applies to biology as much as it does to physics. The fact is that if deer were to be reduced greatly in number, they would leave a niche (a vacant territory with a food source) that would be filled either by deer from surrounding areas or by other species. During the period before the void was filled, ticks would be likely to feed on other mammals, including humans and companion animals, and congregate in greater numbers on the remaining deer.  

A good example of this phenomenon is what has happened historically to coyotes, which have enlarged their range to fill the niche left by wolves after humans virtually eliminated the wolf population. In western States, the range of elk has changed radically with human development and elk have been introduced into Eastern states. Moose are already found in Connecticut. Moose and elk, both related to deer, are only two species that might expand their range to fit a niche left by deer were one to occur.

We have no way of knowing at present which ticks found in this area would prefer to feed off elk or deer, what new ticks other animals might introduce into the area, or what new illnesses might arise as a result of such a transposition, but even if none of them did, we would still have ehrlichiosis, transmitted by dog ticks, in our midst as we do at present, no matter how many deer were killed.

Thirty two years ago Lyme disease was unheard of and 21 years ago Ehrlichiosis was unheard of in humans. We cannot predict what new diseases will be in our midst 21 and 32 years in the future. Killing deer in an attempt to gain control over Lyme disease is not an effective strategy. Not only will it not reduce Lyme disease incidence as studies have shown (Med. Entomol. 44(5): 752Ð757 (2007), but it may cause much more serious problems.

Deer and ticks
Deer are hosts, not carriers. Deer do not become sick with Lyme disease, borreliosis or ehrlichiosis. They are dead-end hosts for the bacterium and cannot infect another animal directly. Deer simply supply ticks with a place to mate and a blood meal. Eliminating or drastically reducing the deer population has little or no effect on the disease, because the tick has many other suitable hosts, which include over 40 bird species and all mammals.

Ticks spend 95% of their life in the leaf litter on the forest floor during their 2 year life cycle, and the number of ticks on the forest floor is not directly related to the number of deer in the area. (Environ. Entomol. 34(4): 801Ð806, 2005.) 23% of the ticks on the forest floor were found to be infected with Lyme disease while only 1% of those on deer were infected. (Am J Trop Med Hyg. 1988 Jul;39(1):105-9.)

The incidence of Lyme disease is determined by white footed mice and tick population density rather than by deer. (Vector Borne Zoonotic Dis 2001 Summer;1(2):129-38)

White footed mice are the most competent reservoir of Lyme disease and over 90% of ticks feeding on wild mice become infected with the Lyme bacterium (PLoS Medicine June 6, 2006). White footed mice attract more ticks per animal and have a higher rate of Lyme disease than other small rodents. Thus, as biodiversity has been lost, Lyme disease has increased (Proc Natl Acad Sci. 2003 Jan 21;100(2):567-71.).

White footed mouse populations, which can survive in small territories, are thriving while their larger predators (such as fox, coyotes, hawks, owls and bobcats) who require large territories, are excluded from smaller territories. White footed mice are able to survive in greater numbers than other small mammals who might otherwise attract more ticks and dilute the Lyme disease reservoir.

In Weapons of Mouse Destruction?, Animal Ecologist Dr.Richard S. Ostfeld explains "Understanding what regulates Lyme disease, predicting its spread, and ultimately reducing its transmission to humans, depends on our ability to look at the disease from an ecological perspective. Knowing the players is of limited usefulness unless we also know how they interact with one another. We know predators have a role and are working to more clearly define it."

The number of black legged tick nymphs is strongly related to the number of acorns, which are eaten by white footed mice, the preceding year, rather than to the number of deer. (PLoS Biology, June 2006, vol 4, issue 6).

Because the number of infected ticks is dependent on the numbers of white footed mice and not the number of deer, reducing deer numbers does not result in less cases of human Lyme disease, because the percentage of infected ticks (and therefore the percentage probability of infection) remains the same. Thus even if the number of tick bites is reduced, the probability of contracting Lyme disease from each bite remains the same. 

In the past 30-50 years, many human processes have changed the ecosystem and resulted in loss of biodiversity than at any other time in history. These include climate change, pollution and habitat loss and fragmentation, as we have systematically turned farmland into residential subdivision to keep pace with burgeoning population growth. As forest and other habitats have become subdivided and degraded, some species of plants and animals can no longer live where they formerly did. This has resulted in reduced biodiversity.

The incidence of Lyme and other tick borne diseases is not simply due to deer. While the fact that we have a lot of cases of Lyme and other tick borne diseases may intuitively seem to be related to seeing what appears like an increasing number of deer, a plan that proposes to kill deer to solve the problems of Lyme disease is ill conceived, ineffective and is a disservice to anyone suffering from Lyme or anyone yet to contract this serious disease. Solutions to the problems of tick borne disease have to take into consideration habitat degradation and find strategies that work in the long term for the safety and security of future generations.

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Why Hunting Won’t Reduce Human Risk of Lyme Disease
by Laura Simon Field Director, The Humane Society of the United States

Lyme disease is spread by Ixodes scapularis, the Black-legged tick. The actual disease- carrying agent is a bacterium (Borrelia burgdorferi) which is carried in the bloodstream of its host. The tick transforms from a larvae into a nymph and then an adult over a 2 year span. At each stage, the tick takes a blood meal from a host and then drops off and molts into the next life stage.

Lyme disease has proven difficult to control largely because the tick (Ixodes scapularis) is carried by many hosts including 49 bird species and all mammals (Anderson and Magnarelli, 1984). Deer seem to the preferred host for the adult stage of the tick. For unknown reasons, the tick seems to prefer a progressively larger host. Certain small rodent species, namely the white-footed mouse, serve as the primary host for immature ticks. In addition, birds can transport the disease to new areas (Anderson, 1988, Battaly and Fish, 1993).

At one time, the Black-legged tick was called a “deer tick.” This common name was a misnomer due to tick’s multiple hosts. Hosts for the immature tick stages may actually play a primary role in carrying infectious ticks and perpetuating the disease cycle. A recently published study (Ostfeld et al, 2006) found that human risk of exposure to Lyme disease seems to be correlated with the abundance of immature (rodent) hosts and their food resources, not deer numbers.

Deer Hunting and Lyme Disease
The reason that hunting is not efficacious in controlling Lyme disease is because hunting does not significantly reduce the tick population. For example, in one study where as many as 70% if the deer were removed from an island, there was “no marked reduction in the abundance of the tick.” (Wilson et al, 1984, p.697)

Another study conducted at Crane’s Beach in Ipswitch, MA found that a gradual reduction of deer density (from 350 to 60 deer between 1985 to 1991) did not produce a rapid, precipitous decline in immature tick abundance. Instead, immature ticks declined 5-7 years after the depopulation effort while adult tick numbers actually increased throughout the study period. In the final 2 years of depopulation, the nymph tick population rose to about the same level as witnessed in 1983, when sampling began (Wilson and Deblinger, 1993).

When the deer population was reduced as much as 83%, the authors concluded that “the reduction in tick numbers was insufficient to reduce the number of female ticks that reproduced.” (Deblinger et al, 1993, p.148)

Most hunting seasons are also poorly timed to affect tick reproduction. By the time regular hunting season occurs in November, a good portion of adult ticks have already mated and dropped off the deer to lay eggs. This issue was discussed by researchers who stated, “As Wilson and Deblinger noted, deer reduction practices carried out when adults are relatively inactive at the end of fall will have minimal impact on the tick population.” (Falco and Daniels 1993)

In addition, the ticks seem to confound deer reduction efforts by taking advantage of other hosts (Duffy et al, 1994) or congregating at higher densities on the remaining deer (Deblinger et al, 1993).

It remains unclear how far a deer population needs to be reduced to impact the transmission dynamics of Lyme disease and tick-human transmission of the disease (Ginsberg and Stafford, 2005). Research suggests that the critical threshold must be extremely low.

Safety issues
Recent research (S. Perkins et al, 2006) suggests that a local absence of deer may actually increase tick feeding on rodents, which can then lead to the potential for disease “hot spots.”

In addition, researchers warn that hunting may actually increase the public safety risk in the short-term because any remaining ticks who are still “questing” for a large host are more likely to end up on large hosts like humans after deer numbers have been reduced (Ginsberg and Zhioua, 1999).

Deer reduction is not synonymous with disease reduction
The issue of infectivity comes into play when understanding why fewer deer does not mean less human disease.

Research indicates that approximately 50% of ticks are infectious for Lyme disease. If a person is bitten by 12 ticks a year, and half of those ticks are infected, then the probability of that person being bitten by at least one infected tick is 99.98%. An intervention which cuts the number of tick bites by 90% will not lower the probability of transmission by the same factor (90%). This is because even if the person is bitten by only one tick, half are infected, so that person will still have a 56.5% probability of becoming infected with Lyme disease. So it is not just the number of ticks, but their infectivity rate and probability of being bitten, that comes into play when looking at disease transmission risks (Mather et al, 1996).

Tools for tick control
Some of the best ways to control human Lyme disease involve doing a combination of the following: checking oneself and family members for tick after being outdoors, taking precautions like wearing light-colored clothing, tucking in sleeves and socks, using tick-repelling products on your skin and insecticidal sprays on properties, doing habitat alteration to reduce tick and tick-host habitat, and consulting a doctor immediately when signs of Lyme disease or the characteristic rash occur.

There are two relatively new devices on the market that target ticks exclusively and have shown promising results in terms of significantly reducing the tick population.

One is called the Maxforce system which is a bait box that attracts mice and applies fipronil (the active ingredient in Frontline) to their bodies when they enter. In a study done by Connecticut Agricultural Station, there was an 80% and 96% reduction in nymphs by the first and second years of the study, respectively, and infectivity was lowered 67% by the second year. They also found a 77% reduction in questing adults on the treated properties and lowered infectivity rates (Dolan et al, 2004). This device is best suited to a property-level approach.

A similar baiting device exists for deer, called the “4-Poster.” The 4-Poster is a device that uses the deer to kill the ticks (Pound, 2000). This device has been tested by the USDA in a 5 state, 7 year research program and has proven extremely effective in reducing tick numbers (the study results are being written up currently, pers comm, Dr. Mathew Pound, also McGraw and McBride, 1991). It contains a corn bait, which attracts deer, and when they eat the corn, a chemical (10% permethrin) is applied to their necks and shoulders which kills 95%-98% of the adult ticks. A study done at the Goddard Flight Center found that by using the 4-Poster system, adult ticks were completely eliminated by the 2nd year of the study; all stages were reduced 91-100% by year 3 (Solberg et al, 2003).

Summary: Based on current scientific knowledge, attempting to control human risk of Lyme disease by reducing deer numbers would not be a viable or efficacious strategy.

BIO: Laura Simon is the Field Director for the Urban Wildlife Program for the Humane Society of the United States (HSUS). The Humane Society of the United States is the nation’s largest animal protection organization representing more than 12 million members and constituents. Ms. Simon has 20 experience in the urban wildlife field and received a Master's Degree of Environmental Management from the Yale School of Forestry and Environmental Studies. She authored the booklet Living with Deer. Her office is located in CT, the phone number is: 203-389-4411.

Lyme Disease and Deer Citations
Anderson, J.A. 1988. Mammalian and avian reservoirs for Borrellia burgdorferi. Lyme Disease and Related Disorders, Eds J.L. Benach and E.M. Bosler. Vol 539. NY: Annals New York Academy of Sciences.

Anderson, J.F. and L.A Magnarelli. 1984. Avian and mammalian hosts for spirochete –infected ticks and insects on a Lyme disease focus in Connecticut. Yale J. of Biology and Medicine 57:627-641.

Battaly, G. R. and D.Fish. 1993. Relative importance of bird species as hosts for immature Ixodes dammini (Acari: Ixodidae) in a suburban residential landscape of Southern New York State. J. Med. Entomol. 30: 740-747.

Deblinger, R.D., M.L. Wilson, D.W Rimmer, and A. Spielman. 1993. Reduced abundance of immature Ixodes dammini (Acari: Ixodidae) following incremental removal of deer. J. Med. Entomol. 30: 144-150.

Dolan, M.C. and G.O. Maupin, B.S. Schneider, C.Denatale, N.Hamon, C. Cole, N.S. Zeidner, and K. C. Stafford III, 2004. Control of immature Ixodes scapularis (Acari: Ixodidae) on rodent reservoirs of borrelia burgdorferi in a residential community of southeastern Connecticut. J. Med. Entomol.41 (6) pp. 1043-1054.

Duffy, D.C., S.R. Campbell, D. Clark, C. Dimotta, and S. Gurney. 1994. Ixodes scapularis (Acari: Ixodidae) deer tick mesoscale populations in natural areas: Effects of deer, area and location. Entomol. Soc. of America 31(1) 152-158.

Falco, R.C. and D. Fish, 1988. Prevalence of Ixodes dammini near the homes of Lyme disease patients in Westchester County, New York. Am. J. Epidemiol. 127; 826-830.

Ginsberg, H.S. and K.C. Stafford III, 2005. Forum: Management of Ticks and Tick-Borne Diseases. In Tick-Borne Diseases of Humans, edited by J.L. Goodman et al, 2005 Asm Press, Washington DC.

Ginsberg, H.S. and E. Zhioua. 1999. Influence of deer abundance on the abundance of questing adult Ixodes scapularis (Acari: Ixodidae). J. Med. Entomol. 36: 379-381. Ginsberg, H.S. 1993. Ecology and Environmental Management of Lyme Disease. Rutgers University Press. New Brunswick, NJ. 224 pp.

Jordan, R.A. and T. Schulze. 2005. Deer browsing and the distribution of Ixodes Scapularis (Acari: Ixodidae) in central New Jersey forests. Entomological Society of America. Vol. 34 (4) p. 801-806.

Kilpatrick, H.J., and W.D. Walter 1999. A controlled archery deer hunt in a residential community: cost, effectiveness, and deer recovery rates. Wildl. Soc. Bull. 27(1):115-123.

Mather, T.N, M.C. Nicholson; E.F. Donnelly, and B.T. Matyas. 1996. Entomologic index for human risk of Lyme disease. Am. J. Epidemiol. 144: 1066-1069. Mcgraw, L and J Mcbride. 1991. Tick Control Devices Reduce Lyme Disease. Agricultural Research, May 2001. pp 5-7

Ostfeld, R.and C. Canham, K. Oggenfuss, R. and F. Keesing. 2006. Climate, deer, rodents and acorns as determinants of Lyme disease risk. PLoS Biology.June 4 (6) p. 145.

Stafford, K.C. Ed, 2004. Tick Management Handbook. Published by the CT Agricultural Station, New Haven, CT.

Perkins, S.E. and I.Cattadori, V. Tagliapietra, A. Rizzoli, and P. Hudson. 2006. Localized deer absence leads to tick amplification. Ecology 87 (8), pl 1981-1986.

Pound, J.M., J.A. Miller, J.E. George and C.A. LeMeilleur. 2000. The “4-Poster” passive topical treatment device to apply acaricide for controlling ticks (Acari: Ixodidae) feeding on white-tailed deer. J. Med. Entomol. 37: 588-594.

Solberg, V.B. , J.A. Miller, T. Hadfield, R. Burge, J.M. Schech and J.M. Pound. 2003. Control of Ixodes scapularis (Acari: Ixodidae) with topical self-application of permethrin by white-tailed deer inhabiting NASA, Beltsville, Maryland. J. Vector. Ecol. 28: 117-134.

Telford, S.T. III. 1993. Forum: Management of Lyme disease p. 164-167 in H.S. Ginsberg (Ed), Ecology and Environmental Management of Lyme Disease, Rutgers Univ Press, New Brunswick, NJ.

Wilson, M.L. and R.D. Deblinger, 1993. Vector management to reduce the risk of Lyme Disease. p.126-156 in H.S. Ginsberg (ed), Ecology and Environmental Management of Lyme Disease, Rutgers Univ. Press, New Brunswick, NJ.

Wilson, M.L. , S.R. Telford III, J. Peisman, and A. Spielman, 1988. Reduced abundance of immature Ixodes dammini (Acari: Ixodidae) following elimination of deer. J. Med. Entomol. 25: 224-228.

Wilson, M.L, S.R. Telford III, J. Piesman, and A. Spielman. 1984. Effect of deer reduction on abundance of the deer tick (Ixodes dammini). Yale J. of Biol. and Med 57: 697-705.

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