Epizootiology of Lyme Disease
Lyme disease was first recognized as a distinct clinical illness in 1975, when 51 residents from Old Lyme, Lyme and East Haddam, Connecticut were diagnosed as having a unique form of oligoarticular arthritis. Since its description, Lyme disease has emerged as a significant threat to the public's health in the northeastern United States. Nationally, Lyme disease increased from 523 reported cases in 1982 to 4,507 reported cases in 1988, 8,552 cases in 1989, and 13,043 cases in 1994. The Centers for Disease Control reported that Lyme disease accounted for 81% of all reported cases of arthropod-transmitted diseases in the United States between 1986-1991. In 1982, a treponema-like spirochete was isolated from the midgut of adult I. scapularis suggesting that this organism may be involved in the etiology of Lyme disease. Shortly thereafter, spirochetes were isolated from the blood of Lyme disease patients and from adult I. scapularis. The following year the I. scapularis spirochete was recognized as a new species and named Borrelia burgdorferi. Furthermore, the blacklegged tick, I. scapularis, was considered to be the most important vector of the spirochete.
Acquisition of Spirochetes
Typically, immature I. scapularis acquires B. burgdorferi during its initial bloodmeal from infected reservoir hosts, primarily from white-footed mice. After molting, the following life stage is transstadially infected. Transstadially infected nymphs and adults can then transmit the spirochete to non-infected hosts. Nymphal blacklegged ticks have had a single previous blood meal (in the larval stage) and subsequently approximately 25% of the nymphs can be found infected. On the other hand, adult blacklegged ticks have had the benefit of two previous blood feedings (as larvae and nymphs) and therefore have a natural infection rate of nearly 50%. Because nymphal feeding precedes larval feeding, the enzootic transmission of B. burgdorferi is highly efficient. Thus, the majority of mice become infected with Lyme disease spirochetes in the spring before serving as hosts to the larvae later that summer. Spirochetes overwinter in the fed larvae, in the unfed nymphs or in the host animal. Ixodes scapularis larvae can also acquire B. burgdorferi transovarially from infected females; however, transovarial transmission seems to play at best a minimal role in the maintenance of B. burgdorferi in the tick population.
Symptoms and signs of Lyme disease
Early Lyme Disease:
The early stages of Lyme disease is usually marked by one or more of the following symptoms:
- chills and fever
- muscle and joint pain
- swollen lymph nodes
- a characteristic skin rash, called erythema migrans (EM)
Erythema migrans (EM) is a red circular patch that appears usually 3 days to 1 month after the bite of an infected tick at the site of the bite. The patch then expands, often to a large size. Sometimes many patches appear, varying in shape, depending on their location. Common sites are the thigh, groin, trunk, and the armpits. The center of the rash may clear as it enlarges, resulting in a bulls-eye appearance. The rash may be warm, but it usually is not painful. Not all rashes that occur at the site of a tick bite are due to Lyme disease (i.e. an allergic reaction to tick saliva at the site of the bite which can be confused with the rash of Lyme disease). Allergic reactions to tick saliva usually occur within a few hours to a few days following the tick bite, but usually do not expand and normally disappear within a few days.
Late Lyme Disease:
Some symptoms and signs of Lyme disease may not appear until weeks, months, or years after a tick bite:
- Arthritis is most likely to appear as brief bouts of pain and swelling, usually in one or more large joints, especially the knees.
- Nervous system abnormalities can include numbness, pain, Bell's palsy (facial paralysis which usually occurs on one side), and meningitis (fever, stiff neck, and severe headache).
- Less frequently, irregularities of the heart rhythm occur.
- In some persons the rash never forms; in some, the first and only sign of Lyme disease is arthritis, and in others, nervous system problems are the only evidence of the disease.
In rare cases, Lyme disease acquired during pregnancy may have possibly lead to infection of the fetus and to stillbirth, but adverse effects to the fetus have not been conclusively documented.
Lyme disease is often difficult to diagnose because its symptoms and signs mimic those of many other diseases. The fever, muscle aches, and fatigue of Lyme disease can easily be mistaken for viral infections, such as influenza, infectious mononucleosis or chronic fatigue syndrome. Joint pain can be mistaken for other types of arthritis, such as rheumatoid arthritis, and neurologic signs can mimic those caused by other conditions, such as multiple sclerosis. At the same time, other types of arthritis or neurologic diseases can be misdiagnosed as Lyme disease.
Diagnosis of Lyme disease depends upon:
- Exposure to ticks, especially in areas where Lyme disease is known to occur. If you are bitten by a tick, always save it - correct identification and testing can confirm the presence or absence of the Lyme disease spirochete within the tick.
- Symptoms and signs as described above.
- The results of blood tests used to determine whether the patient has antibodies to Lyme disease bacteria. These tests are most useful in later stages of illness, but even then they may give inaccurate results, because laboratory tests for Lyme disease have not yet been standardized nationally.
- Consultation with a health care provider.
Treatment and prognosis
Lyme disease is treated with antibiotics under the supervision of a physician. Several antibiotics are effective. Usually they are given by mouth but may be given intravenously in more severe cases. Patients treated in the early stages with antibiotics usually recover rapidly and completely. Most patients who are treated in later stages of the disease also respond well to antibiotics. In a few patients who are treated for Lyme disease, symptoms of persisting infection may continue, making additional antibiotic treatment necessary. Varying degrees of permanent damage to joints or the nervous system can develop in patients with late chronic Lyme disease. Typically these are patients in whom Lyme disease was unrecognized in the early stages or for whom the initial treatment was unsuccessful. Rare, indirect deaths from Lyme disease have been reported.
Removing leaves and clearing brush and tall grass around houses and at the edges of gardens may reduce the numbers of immature ticks. This is particularly important in the eastern United States, where most transmission of Lyme disease is thought to occur near the home.
A relationship has been observed between the abundance of deer and the abundance of deer ticks in the eastern United States. Consequently, removing vegetation that attracts deer and constructing physical barriers may help discourage deer and attached ticks from coming near the house.
Applying acaricides (chemicals that are toxic to ticks) to gardens, lawns, and the edge of woodlands near homes is being done in some areas, but questions remain regarding its effectiveness and environmental safety. Application to residential properties should be supervised by a licensed professional pest control expert.
Personal protection from tick bites:
The chances of being bitten by a tick can be decreased with a few precautions.
- Avoid tick-infested areas, especially in May, June, and July (many local health departments and park or extension services have information on the local distribution of ticks).
- Wear light-colored clothing so that ticks can be spotted more easily.
- Wear long pants and tuck the pant legs into your socks or boots; wear a long-sleeved shirt and tuck it into your pants; and use a hat for added protection.
- Tape the area where pants and socks meet so that ticks cannot crawl under clothing.
- Spray insect repellent containing DEET (products shouldn't contain any more than 30% DEET) on clothes, or treat clothes (especially pants, socks, and shoes) with permethrin, which kills ticks on contact. Remember that these products should be used with caution.
- Walk in the center of trails to avoid overhanging grass and brush.
- After being outdoors, remove your clothing and wash and dry it at a high temperature.
- Inspect yourself carefully and remove any attached ticks. For tick removal: grasp the tick with fine tweezers as close to the skin surface as possible, pull straight up with a slow, steady force and avoid crushing the tick or slipping off the body. Ultimately you do not want to force any material from the tick into your skin. Clean the area of tick attachment with disinfectant. Ticks (saved in a sealed container) can be submitted to the Mosquito Commission laboratory or certain local health departments for identification.
Preventive Antibiotic Treatment:
Antibiotic treatment to prevent Lyme disease after a known tick bite may not be warranted. Physicians must determine whether the advantages of using antibiotics outweigh the disadvantages in any particular instance.
As of February 2002 the human vaccine for Lyme disease (LYMErix) has been discontinued. GlaxoSmithKline stated that sales of LYMErix were insufficient to justify the continued investment in manufacturing, distribution and marketing.
Page Last Updated: 7/24/2008 4:28:00 PM