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Lyme Disease

Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Vector-Borne Infectious Diseases

Publication date: 01/01/1991

Table of Contents

How the disease is spread
Geographic distribution
Life cycle of Lyme disease ticks
Lyme disease in domestic animals
Symptoms and signs of Lyme disease
Lyme disease and pregnancy
Treatment and prognosis
Personal protection from tick bites
Lyme disease research



Lyme disease was first recognized in the United States in 1975, after a mysterious outbreak of arthritis near Lyme, Connecticut. Since then, reports of Lyme disease have increased dramatically, and the disease has become an important public health problem in some areas of the United States.

Lyme disease is an infection caused by Borrelia burgdorferi, a member of the family of spirochetes, or corkscrew-shaped bacteria.

How the disease is spread

Lyme disease is spread by the bite of ticks of the genus Ixodes that are infected with Borrelia burgdorferi. The deer (or bear) tick, Ixodes dammini, which normally feeds on the white-footed mouse, the white-tailed deer, other mammals, and birds, is responsible for transmitting Lyme disease bacteria to humans in the northeastern and north-central United States. (In these regions, this tick is also responsible for the spreading of babesiosis, a disease caused by a malaria-like parasite.) On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick, Ixodes pacificus, and in the southeastern states possibly by the black-legged tick, Ixodes scapularis.

Ixodes ticks are much smaller than common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Adult ticks are slightly larger.

Ticks can attach to any part of the human body but often attach to the more hidden and hairy areas such as the groin, armpits, and scalp.

Research in the eastern United States has indicated that, for the most part, ticks transmit Lyme disease to humans during the nymph stage, probably because nymphs are more likely to feed on a person and are rarely noticed because of their small size (less than 2 mm). Thus, the nymphs typically have ample time to feed and transmit the infection (ticks are most likely to transmit infection after approximately 2 or more days of feeding).

Tick larvae are smaller than the nymphs, but they rarely carry the infection at the time of feeding and are probably not important in the transmission of Lyme disease to humans.

Adult ticks can transmit the disease, but since they are larger and more likely to be removed from a person's body within a few hours, they are less likely than the nymphs to have sufficient time to transmit the infection. Moreover, adult Ixodes ticks are most active during the cooler months of the year, when outdoor activity is limited.

Ticks search for host animals from the tips of grasses and shrubs (not from trees) and transfer to animals or persons that brush against vegetation. Ticks only crawl; they do not fly or jump. Ticks found on the scalp usually have crawled there from lower parts of the body. Ticks feed on blood by inserting their mouth parts (not their whole bodies) into the skin of a host animal. They are slow feeders: a complete blood meal can take several days. As they feed, their bodies slowly enlarge.

Although in theory Lyme disease could spread through blood transfusions or other contact with infected blood or urine, no such transmission has been documented. There is no evidence that a person can get Lyme disease from the air, food or water, from sexual contact, or directly from wild or domestic animals. There is no convincing evidence that Lyme disease can be transmitted by insects such as mosquitoes, flies, or fleas.

Campers, hikers, outdoor workers, and others who frequent wooded, brushy, and grassy places are commonly exposed to ticks, and this may be important in the transmission of Lyme disease in some areas. Because new homes are often built in wooded areas, transmission of Lyme disease near homes has become an important problem in some areas of the United States. The risk of exposure to ticks is greatest in the woods and garden fringe areas of properties, but ticks may also be carried by animals into lawns and gardens.

Geographic distribution

Lyme disease has a wide distribution in northern temperate regions of the world. In the United States, the highest incidence occurs in the

  • Northeast, from Massachusetts to Maryland.
  • North-central states, especially Wisconsin and Minnesota.
  • West Coast, particularly northern California.

For Lyme disease to exist in an area, at least three closely interrelated elements must be present in nature: the Lyme disease bacteria, ticks that can transmit them, and mammals (such as mice and deer) to provide food for the ticks in their various life stages. Ticks that transmit Lyme disease can be found in temperate regions that may have periods of very low or high temperature and a constant high relative humidity at ground level.

Life cycle of Lyme disease ticks

Knowing the complex life cycle of the ticks that transmit Lyme disease is important in understanding the risk of acquiring the disease and in finding ways to prevent it.

The life cycle of these ticks requires 2 years to complete. Adult ticks feed and mate on large animals, especially deer, in the fall and early spring. Female ticks then drop off these animals to lay eggs on the ground. By summer, eggs hatch into larvae.

Larvae feed on mice and other small mammals and birds in the summer and early fall and then are inactive until the next spring when they molt into nymphs.

Nymphs feed on small rodents and other small mammals and birds in the late spring and summer and molt into adults in the fall, completing the 2-year life cycle.

Larvae and nymphs typically become infected with Lyme disease bacteria when they feed on infected small animals, particularly the white-footed mouse. The bacteria remain in the tick as it changes from larva to nymph or from nymph to adult. Infected nymphs and adult ticks then bite and transmit Lyme disease bacteria to other small rodents, other animals, and humans, all in the course of their normal feeding behavior.

Lyme disease in domestic animals

Domestic animals may become infected with Lyme disease bacteria and some of these (dogs, for instance) may develop arthritis. Domestic animals can carry infected ticks into areas where humans live, but whether pet owners are more likely than others to get Lyme disease is unknown.

Symptoms and signs of Lyme disease

Early Lyme Disease: The early stage of Lyme disease is usually marked by one or more of the following symptoms and signs:

  • fatigue
  • chills and fever
  • headache
  • muscle and joint pain
  • swollen lymph nodes
  • a characteristic skin rash, called erythema migrans

Erythema migrans 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, however. For example, an allergic reaction to tick saliva often occurs at the site of a tick bite. The resulting rash can be confused with the rash of Lyme disease. Allergic reactions to tick saliva usually occur within hours to a few days after the tick bite, usually do not expand, and 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 (paralysis of the facial muscles, usually 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 Lyme disease.

Lyme disease and pregnancy

In rare cases, Lyme disease acquired during pregnancy may lead to infection of the fetus and possibly to stillbirth, but adverse effects to the fetus have not been conclusively documented. The Centers for Disease Control and Prevention (CDC) maintains a registry of pregnant women with Lyme disease to advance the understanding of the effects of Lyme disease on the developing fetus.


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 or infectious mononucleosis. 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 should take into account

  • History of possible exposure to ticks, especially in areas where Lyme disease is known to occur.
  • Symptoms and signs.
  • 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. Laboratory tests for Lyme disease have not yet been standardized nationally.

Treatment and prognosis

Lyme disease is treated with antibiotics under the supervision of a physician. Several antibiotics are effective. Antibiotics usually 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 or recur, 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 deaths from Lyme disease have been reported.


Tick Control: Removing leaves and clearing brush and tall grass around houses and at the edges of gardens may reduce the numbers of ticks that transmit Lyme disease. 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.

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.

Reducing and managing deer populations in geographic areas where Lyme disease occurs may reduce tick abundance. Removing plants that attract deer and constructing physical barriers may help discourage deer from coming near homes.

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. Tuck pant legs into socks or boots and shirt into pants.
  • Tape the area where pants and socks meet so that ticks cannot crawl under clothing.
  • Spray insect repellent containing DEET on clothes and on exposed skin other than the face, or treat clothes (especially pants, socks, and shoes) with permethrin, which kills ticks on contact.
  • Wear a hat and a long-sleeved shirt for added protection.
  • Walk in the center of trails to avoid overhanging grass and brush.

After being outdoors, remove clothing and wash and dry it at a high temperature; inspect body carefully and remove attached ticks with tweezers, grasping the tick as close to the skin surface as possible and pulling straight back with a slow steady force; aviod crushing the tick's body. In some areas, ticks (saved in a sealed container) can be submitted to the local health department 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. If antibiotics are not used, physicians should alert patients to the symptoms of early Lyme disease and advise them to return for reevaluation if symptoms occur.

Lyme disease research

Research continues to discover

  • Where ticks are most likely to be and how best to protect against them.
  • Which chemicals and other approaches are best for controlling ticks in each kind of habitat.
  • Better diagnostic tests.
  • Improved antibiotic treatment.
  • An effective vaccine.
  • Effects of mother's infection on the developing fetus.
  • How Lyme disease bacteria cause chronic infections of the joints and nervous system and how to prevent these complications.

For further information, contact the CDC Voice Information System at (404) 332-4555, your physician, or your local health department.


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This page last reviewed: Wednesday, January 27, 2016
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