Merck Manual

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Lyme Disease


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jul 2019| Content last modified Jul 2019
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Lyme disease is a tick-transmitted infection caused by Borrelia species, primarily Borrelia burgdorferi in the United States, which are spiral-shaped bacteria called spirochetes (see figure How Bacteria Shape Up).

  • Most people are infected when they go outdoors in wooded areas where Lyme disease is common and are bitten by ticks infected with the bacteria Borrelia burgdorferi.

  • Typically, a large, red spot appears at the site of the bite and slowly enlarges, often surrounded by several red rings.

  • Untreated, Lyme disease can cause fever, muscle aches, swollen joints, abnormalities of the electrical conduction system of the heart, and eventually problems related to brain and nerve malfunction.

  • The diagnosis is based on the typical rash and symptoms, opportunity for exposure to ticks, and blood tests to detect antibodies to the bacteria.

  • Taking antibiotics usually cures the disease, but some symptoms, such as joint pain, may persist.

(See also Overview of Bacteria.)

Lyme disease was recognized and named in 1976 when a cluster of cases occurred in Lyme, Connecticut. It is now the most common tick-borne infection in the United States. It occurs in 49 states. More than 90% of the cases occur along the northeastern coast from Maine to Virginia and in Wisconsin, Minnesota, and Michigan. On the West Coast, most cases occur in northern California and Oregon. Lyme disease also occurs in Europe, China, Japan, and the former Soviet Union.

Usually, people get Lyme disease in the summer and early fall. Children and young adults who live in wooded areas are most often infected.

The bacteria that cause Lyme disease are transmitted by the deer tick (Ixodes), so named because the adult ticks often feed on the blood of deer. Young deer ticks (larvae and nymphs) feed on the blood of rodents, particularly the white-footed mouse, which is a carrier of Lyme disease bacteria in the United States. Ticks are usually in the nymph stage when they infect people. Deer do not carry or transmit Lyme disease bacteria. They are only a source of blood for adult ticks. In Europe, sheep are hosts for the adult tick.

Did You Know...

  • Lyme disease is the most common tick-borne infection in the United States.

  • Deer do not carry or transmit Lyme disease bacteria and are only a source of blood for adult ticks.

Deer ticks also carry other infections (such as babesiosis and anaplasmosis). These infections occur mainly in the same places as Lyme disease in the United States—the Northeast and upper Midwest. So when ticks transmit Lyme disease, they may also transmit these infections, and people may have more than one infection at a time.

The bacteria that cause Lyme disease are transmitted to people when an infected tick bites and stays attached for more than 36 hours. Brief periods of attachment rarely transmit disease.

At first, the bacteria multiply at the site of the tick bite. After 3 to 32 days, the bacteria migrate from the site of the bite into the surrounding skin, causing a rash called erythema migrans. The bacteria may enter the lymphatic system and infect lymph nodes. Or the bacteria may enter the bloodstream and spread to other organs, such as the skin in other areas of the body and the heart, nervous system, and joints.

Did You Know...

  • Usually, people get Lyme disease only if a tick remains attached to them for at least a day and a half.


Lyme disease has three stages:

  • Early localized

  • Early disseminated (widespread)

  • Late

The early and late stages of Lyme disease are usually separated by a period without symptoms.

Early localized–stage Lyme disease

Typically, a large, raised, red spot (erythema migrans) appears at the site of the bite, usually on the thigh, buttock, or trunk or in the armpit. The spot typically appears 3 to 32 days after the tick bite, which most people do not remember.

Usually, the spot slowly expands to a diameter of up to 20 inches (50 centimeters), often clearing in the center. But the appearance can vary. For example, the center may remain red, or several rings may appear around the red center (looking like a bull's eye or target). Although erythema migrans does not itch or hurt, it may be warm to the touch. The spot usually disappears after about 3 to 4 weeks.

About 25% of infected people never develop—or at least never notice—the characteristic red spot.

Images of the Rash of Lyme Disease

Early disseminated–stage Lyme disease

This stage begins when the bacteria spread through the body. This stage can begin days to weeks after the spot first appears.

Fatigue, chills, fever, headaches, stiff neck, muscle aches, and painful, swollen joints are common. These symptoms of Lyme disease may last for weeks. In nearly half of people who are not treated, more, usually smaller erythema migrans spots appear on other parts of the body. Less commonly, people have a backache, nausea, vomiting, sore throat, swollen lymph nodes, and an enlarged spleen.

Although most symptoms come and go, feelings of illness and fatigue may last for weeks. These symptoms are often mistaken for influenza or common viral infections, especially if erythema migrans is not present.

Sometimes more serious symptoms develop. The nervous system is affected in about 15% of people. Common problems are meningitis (which causes headache and a stiff neck) and Bell palsy (which causes weakness on one or occasionally both sides of the face).

These problems may last for months. Nerve pain and weakness may develop in other areas and last longer.

Up to 8% of infected people develop heart problems. These problems include

  • Delayed movement of electrical signals through the heart, resulting in an abnormal heart rhythm (arrhythmia) called heart block

  • Inflammation of heart tissue (myocarditis) and the sac around the heart (pericarditis) with chest pain

The arrhythmias may cause palpitations, light-headedness, or fainting.

Late-stage Lyme disease

If the initial infection is untreated, other problems develop months to years later.

Arthritis develops in more than half of people, usually within several months. Swelling and pain typically recur in a few large joints, especially the knee, for several years. The knees are commonly more swollen than painful, often hot to the touch, and, rarely, red. Cysts may develop and rupture behind the knees, suddenly increasing the pain. In about 10% of people with arthritis, knee problems last longer than 6 months.

A few people develop abnormalities related to brain and nerve malfunction. Mood, speech, memory, and sleep may be affected. Some people have numbness or shooting pains in the back, legs, and arms.


  • Blood tests

  • Sometimes examination of a sample of joint fluid or cerebrospinal fluid (obtained by spinal tap)

The diagnosis of Lyme disease is usually based on all of the following:

  • Typical symptoms, including rash (particularly erythema migrans)

  • Opportunity for exposure (living in or visiting an area where Lyme disease is common)

  • Test results

Usually, doctors do tests that measure antibodies to the bacteria in blood. (Antibodies are proteins produced by the immune system to help defend the body against a particular attack, such as that by Borrelia burgdorferi.) However, antibodies may be absent if the test is done during the first several weeks of infection or if antibiotics are given before antibodies develop.

Antibodies develop in more than 95% of people who have had the infection for at least a month, particularly if they have not taken antibiotics. Once antibodies develop, they last for many years. Thus, antibodies may be present after Lyme disease has resolved or in people who have had an infection that did not cause symptoms.

Interpreting the results of blood tests is difficult. The uncertainty causes several problems. For example, in areas where Lyme disease is common, many people who have painful joints, trouble concentrating, or persistent fatigue worry that they have late-stage Lyme disease, even though they never had a rash or any other symptoms of early-stage Lyme disease. Usually, Lyme disease is not the cause. But they may have antibodies for the bacteria because they were infected years before and the antibodies last a long time. Thus, if a doctor treats people based solely on results of antibody tests, many people who do not have Lyme disease are treated with long, useless courses of antibiotics.

Cultures are not helpful because Borrelia burgdorferi is difficult to grow in the laboratory.

Sometimes doctors insert a needle into a joint to take a sample of joint fluid or do a spinal tap (lumbar puncture) to take a sample of the fluid that surrounds the brain and spinal cord (cerebrospinal fluid). Fragments of the bacteria’s genetic material may be present and detected using the polymerase chain reaction (PCR) technique. This technique produces many copies of a gene and enables doctors to identify Borrelia burgdorferi bacteria rapidly.


People should take precautions to avoid being bitten by a tick. If people may have been exposed to ticks, they should check their whole body very carefully after each possible exposure and remove any ticks they find. Checking is effective because ticks must usually be attached for more than a day and a half to transmit Lyme disease.

Preventing Tick Bites

People can reduce their chances of picking up or being bitten by a tick by doing the following:

  • Staying on paths and trails when walking in wooded areas

  • Walking in the center of trails to avoid brushing up against bushes and weeds

  • Not sitting on the ground or on stone walls

  • Wearing long-sleeved shirts

  • Wearing long pants and tucking them into boots or socks

  • Wearing light-colored clothing, which makes ticks easier to see

  • Applying an insect repellent containing diethyltoluamide (DEET) to the skin

  • Applying an insect repellent containing permethrin to clothing or wearing clothing commercially pretreated with permethrin

Usually, Lyme disease is transmitted by young deer ticks (nymphs), which are very small, much smaller than dog ticks. So people who may have been exposed to ticks should check the whole body very carefully, especially hairy areas, every day. Inspection is effective because ticks must be attached usually for more than a day and a half to transmit Lyme disease.

To remove a tick, people should use fine-pointed tweezers to grasp the tick by the head or mouthparts right where it enters the skin and should gradually pull the tick straight off. The tick's body should not be grasped or squeezed. Petroleum jelly, alcohol, lit matches, or any other irritants should not be used.

Preventing Tick Bites

If a person is bitten by a tick, doctors sometimes give the person a single dose of doxycycline by mouth to prevent Lyme disease from developing. Alternatively, doctors may have the person observe the area of the bite and give antibiotics only if the person develops the typical rash or other symptoms that suggest early Lyme disease.

A vaccine for Lyme disease used to be available but has been removed from the market. The vaccine was only moderately effective and had side effects similar to the symptoms of Lyme disease.


  • Antibiotics

Although all stages of Lyme disease respond to antibiotics, early treatment is more likely to prevent complications.

Antibiotics such as doxycycline, amoxicillin, or cefuroxime, taken by mouth for 2 to 3 weeks, are effective during the early stages of the disease. If early disease is localized, people may need treatment for only 10 days. If people cannot take any of these drugs, azithromycin is sometimes used but is less effective. Usually, doxycycline is not given to children under 8 years old or to pregnant or breastfeeding women.

Antibiotics can also help relieve many of the symptoms of Lyme disease.

For arthritis due to Lyme disease, antibiotics such as amoxicillin, cefuroxime, or doxycycline are given by mouth for 28 days, or ceftriaxone is given intravenously for 28 days.

For most neurologic abnormalities and for a type of abnormal heart rhythm (arrhythmia) called third-degree heart block (complete heart block), ceftriaxone or penicillin is given intravenously for 2 to 4 weeks. Sometimes a temporary pacemaker is needed for complete heart block. Doxycycline, given by mouth for 2 to 3 weeks, can help treat less severe first-degree heart block and Bell palsy.

Antibiotics eradicate the bacteria and, in most people, relieve arthritis. However, arthritis sometimes lasts even after all the bacteria are gone because inflammation continues. Even after successful antibiotic treatment, some people still have other symptoms such as fatigue, headache, and mental problems.

Nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen, may relieve the pain of swollen joints. Fluid that collects in affected joints may be drained. Using crutches may help.

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