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Description and Distribution
Lyme disease is an illness caused by a spirochete bacterium (Borrelia burgdorferi). This disease is transmitted to people and animals primarily by the bite of the black-legged tick, Ixodes scapularis.
In 1975, Lyme disease was first recognized in the United States in children from Lyme, Connecticut. However, the bacterium that causes Lyme disease was not identified until 1982. Since then, Lyme disease has been reported with increasing frequency. The majority of cases occur along the east coast from Delaware to Massachusetts and the upper midwest in Wisconsin and Minnesota. Lyme disease has now been reported from 43 states, including Michigan.
In Michigan, the first official reported human case of Lyme disease was in 1985. Cases have now been reported in both the upper and lower peninsula and in most of Michigan's 83 counties. It is anticipated that the number of cases reported will continue to increase.
Transmission and Development
The black-legged tick, Ixodes scapularis, is the most common carrier of Lyme disease in the mid-western and eastern states. I. pacificus is known to be the vector in the west. Other species of ticks such as the dog tick or wood tick, the lone-star tick and the rabbit tick, and biting insects such as mosquitoes, deer flies and horse flies have been shown to carry the Lyme disease bacterium. However, their ability to transmit the disease is not known at this time. Studies are continuing in Michigan to determine the extent of the black-legged tick population.
The black-legged tick has a rather complex life cycle involving development from an egg to a larva, larva to a nymph, and finally from a nymph to an adult. This process usually takes two years. Larvae and nymphs require blood to proceed to the next development stage, and adult females need blood to lay their eggs. At each of these stages, the tick seeks an animal host for a single blood meal and then drops off the host.
In their two years of life, black-legged ticks spend very little time (only about two and a half weeks) on hosts. The rest of the time is spent off the host, developing into the next stage and waiting for another host to come along.
In the spring, the eggs hatch into larvae. During the summer, the larvae feed on mice, squirrel, raccoon, rabbit and other animals. In the fall, the larvae mature into nymphs, which then hibernate over winter. In the spring and summer these nymphs become active again, preferring to feed on mice. It is during the time the tick is in the nymphal stage that it is most likely to infect humans. At the end of its life cycle the female tick lays eggs and dies.
Clinical Signs and Pathology
Lyme disease in humans is usually not a life-threatening illness and one should regard the health risks it does pose with concern rather than alarm. It is most often a mild illness mimicking a summer flu, but serious problems involving the heart, joints and nervous system may develop in some individuals.
Lyme disease in humans may progress through three stages, depending upon the individual. In stage 1, people may have any combination of the following signs and symptoms: headache, nausea, fever, spreading rash, aching joints and muscles and fatigue. These signs and symptoms may disappear altogether, or they may reoccur intermittently for several months. A characteristic red rash, called erythema migrans (EM) may appear within 3 to 32 days after a person is bitten by an infected tick. The rash is circular in shape and can attain a diameter of 2 to 20 inches. EM is not restricted to the bite site and more than one lesion may occur on the body. Up to 30% of the people who have Lyme disease do not develop EM lesions, making diagnosis more difficult.
In stage 2 (weeks to months after initial exposure to the bacterium or after the first symptoms appear), some people may develop complications involving the heart and/or nervous system. Specific disorders may include various degrees of heart block, nervous system abnormalities such as meningitis, encephalitis and facial paralysis (Bell's palsy), and other conditions involving peripheral nerves. Painful joints, tendons, or muscles may also be noted during this stage of the disease.
Arthritis is the most commonly recognized long-term sign of Lyme disease (stage 3). From one month to several years after their first symptoms appear, people may experience repeated attacks of arthritis.
Dogs, cats, cattle, horses and other domestic animals may also exhibit a variety of signs, including fever and lameness. Wild animals such as deer, raccoon and mice show no signs and apparently suffer no ill effects from the disease.
Lyme disease is difficult to diagnose because the disease mimics many other diseases and there is no definitive test for it at this time. A diagnosis should be based on a history of tick bite, the presence of a circular rash, an examination by a physician for other symptoms, and laboratory tests. The most reliable indication of Lyme disease is a large circular rash (erythema migrans). If you develop any of the symptoms or recall being bitten by a tick, discuss your suspicions of Lyme disease with your physician.
Treatment and Prevention
Prompt diagnosis and treatment with antibiotics can cure the infection and prevent later complications in both humans and domestic animals. Treatment during later stages of the disease often requires more intensive antibiotic therapy.
While there is no sure way to completely eliminate the chance of contracting Lyme disease, there are several specific preventative measures one can take:
Be aware of Lyme disease, but do not be so concerned that you cannot enjoy the outdoors. The risk of developing the illness is minimal in Michigan and even if infection occurs the disease can be diagnosed and treated with antibiotics.
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. In addition, the Lyme organism is killed by the high temperatures that would be reached when venison is cooked or smoked. Deer supply the tick that transmits the bacterium with a place to mate and provides a blood meal for the female tick prior to production of eggs. Research has shown that white-tailed deer are important to the reproductive success of the black-legged tick. In the absence of deer, this tick will opportunistically feed on other medium sized mammals and humans. 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".
There is very little risk of hunters contracting Lyme disease when pursuing game. This is because hunters are in the woods from October through March when the nymphal stage of the tick is inactive. Even though the adult stage of the tick is active in the fall (when temperatures are above 40º F), the heavier clothing that hunters wear makes it difficult for ticks to find and attach to bare skin. In addition, the risk of picking up ticks from game animals is insignificant compared with that from the environment (meadows, brushland or woods).
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.
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