Although the term "deer tick" is often used, in actuality there is no such thing. The tick that carries Lyme disease feeds on approximately 50 species of mammals and about 70 species of birds. It is the blacklegged tick (Ixodes scapularis) that causes Lyme disease when the tick is infected with the bacteria Borrelia burgdorferi.
When an infected tick attachs itself to an animal to feed (including a human), it can pass Lyme disease to the host. Deer however, do not become infected with Lyme disease even when infected ticks feed on them, nor do deer pass the disease onto other animals. What deer can do is provide the tick with a blood meal typically during the tick's last stage of life (the adult stage) before the tick drops off and lays eggs for the next generation. The misnomer "deer tick" came into use because early studies of Lyme disease did not adequately understand its complex ecology and incorrectly associated the blacklegged tick primarily with deer. Studies have since shown there is a myriad of other animals that provide blood meals for the blacklegged tick and an abundance of deer does not correlate to a high incidence of Lyme disease ("Lyme Disease: the ecology of a complex system" by Richard S. Ostfeld, Ph.D., Oxford University Press, 2011).
The disease was originally named after the town Lyme, Connecticut where a resident suffered from the illness possibly as early as the 1950's. Early on Lyme disease was confused with other illnesses sharing common symptoms like juvenile rheumatoid arthritis. Research on Lyme disease began in earnest in the 1970's focusing on the Connecticut area however it was researchers on Long Island not Connecticut who identified the Lyme disease bacteria while conducting research on another tick borne disease, Rocky Mountain spotted fever. But it remained unclear at the time whether the specific species of tick that spread Lyme disease was the same or different than the ticks that spread other tick borne illnesses.
The basic life cycle of ticks was and is well understood. First is the larvae stage followed by the nymph stage and finally the adult stage (Lyme disease is usually transmitted during the nymph stage). In early studies on Nantucket in 1976 and 1977, all three stages of tick life were found on deer so the tick was given the name Ixodes dammini or "deer tick". Deer were thought to be the keystone host for the Lyme disease tick and from that point on deer reduction became the focus of much research.
But a comprehensive study published in 1993 clarified this issue and established that there was a lack of evidence for a DNA differentiation between the blacklegged tick and the supposed "deer tick". It was discovered that the Lyme disease carrying tick was in fact the blacklegged tick and it lived successfully on as many as 125 vertebrates such as squirrel, fox, voles, opossum, raccoon, skunk, mice and other rodents. The fact that all three stages of tick life were found on deer in Nantucket in the early studies was later attributed to a dearth of alternative hosts on the island at the time (Ostfeld, 2011). The theory that deer were the keystone hosts for Lyme disease ticks was rejected and the notion of a "deer tick" was scientifically invalidated, although use of the term persists today.
More recent studies have found that deer reduction often results in tick populations either remaining the same or sometimes increasing. Furthermore it has been found that after deer reduction, there is often no corresponding decrease in Lyme disease (Ostfeld, 2011); ("Effects of Reduced Deer Density on the Abundance of Ixodes scapularis and Lyme Disease Incidence in a Northern New Jersey Endemic Area" by Robert A. Jordon et al., Journal of Medical Entomology, 2007).
Researchers have been exploring the implications of the existence of multiple tick host animals besides deer. An interesting study found the elimination of deer actually caused tick "hot spots" or high concentrations of ticks on small areas of land (much the size of residential properties). This was because deer served as "dilution hosts" carrying ticks away from the smaller area and dispersing them. Once deer were eliminated, other wildlife such as rodents brought infected ticks into the area resulting in tick concentrations. ("Localized Deer Absence Leads to Tick Amplification" by Sarah E. Perkins et. al., Ecology, 2006). In Richard Ostfeld's recent and exhaustive book on the ecology of Lyme disease (see book reference above), study after study is cited showing that deer reduction has not led to Lyme disease elimination. The theory that ticks will simply shift to other hosts in the absence of deer is becoming well established.
In many parts of the eastern and central United States there are abundant tick populations but relatively few cases of Lyme disease. An interesting nuance in the Lyme disease transmission chain is the fact that not all hosts become infected with Lyme disease even though an infected tick feeds on it (humans and mice do become infected for example but deer and gray squirrels do not). The reason for this is still unclear but it has led scientists to focus research on not only how to control total tick population size but more importantly, how to contain the spread of the Lyme disease bacteria with or without regard to total tick numbers.
Researchers are trying to determine which hosts are better "reservoirs" for the bacteria, in other words which animals easily contract and pass on the Lyme disease bacteria. And the white footed mouse appears to be at the top of the list. Mice are consistently shown to be very efficient reservoirs for Lyme disease because 75% to 95% of the ticks that feed on mice become infected with the Lyme disease bacteria and then the ticks in turn, infect other animals (Ostfeld, 2011). This is why it probably makes more sense to encourage people to clean up leaf litter and dry out basements, rather than shoot deer if they want to try to keep Lyme disease at bay. (A researcher at Harvard agrees, "...hunting deer won't effectively combat Lyme disease because ticks also depend on another key host animal: white-footed mice." Tamara Awerbuch, Ph.D., Harvard School of Public Health).
It isn't entirely clear why the dogma blaming deer for the spread of Lyme disease continues today. The disease can be confusing which creates fear and contributes to the perpetuation of misinformation. Unlike most other infectious diseases, Lyme disease can be difficult to diagnose because it shares common and general symptoms that can be mistaken for other sickness. Compounding this is a high rate of both false positives and false negatives from the diagnostic blood test. And since a vaccine to prevent the disease is not available, and since the body cannot naturally fight the disease without the help of antibiotics, people panic at the mere thought of potential exposure.
Public fear has a way of taking on a life of its own and persistent falsehoods can endure even in the face of contradictory truths. This may be the case with Lyme disease much the same way the fear of autism led many to turn away from childhood vaccines. Just as the early studies linking autism to childhood vaccines have recently been unequivocally renounced and discredited, the hope is that deer will stop being vilified for crimes they aren't committing. But I have my doubts...my neighbor just told me she isn't vaccinating her new baby-she is worried about the vaccine's connection to autism.
Prior Blog Posts:
The Structure of our State Wildlife Agency System
9/25/09
Teaching Children to Kill
10/02/09
The Supreme Court and the Culpability of a Child
11/18/09
The Federal Gun Tax
12/09/09
Lead Ammunition
1/14/10
Connecting the Dots of Violence
2/01/10
Man as "Super-Predator"
3/17/10
U.S. v Stevens: What interests are being protected?
4/27/10
Predation Control: to what end?
9/14/10
The Paradox of Killing Wildlife as Population Control
10/24/10
Who Does Wildlife Belong to? (The Public Trust Doctrine)
11/22/10
