Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans by the bite of infected blacklegged ticks. Most cases of Lyme disease can be treated successfully with a few weeks of antibiotics. Steps to prevent Lyme disease include using insect repellent, removing ticks promptly, landscaping, and integrated pest management. The ticks that transmit Lyme disease can occasionally transmit other tick-borne diseases as well.

Causes

The Lyme disease bacterium, Borrelia burgdorferi, normally lives in mice, squirrels and other small animals. It is transmitted among these animals – and to humans -- through the bites of certain species of ticks.

Risk Factors

In the northeastern and north-central United States, the blacklegged tick (or deer tick, Ixodes scapularis) transmits Lyme disease. In the Pacific coastal United States, the disease is spread by the western blacklegged tick (Ixodes pacificus). Other tick species found in the United States have not been shown to transmit Borrelia burgdorferi. Blacklegged ticks live for two years and have three feeding stages: larvae, nymph, and adult. When a young tick feeds on an infected animal, the tick takes the bacterium into its body along with the blood meal.

The bacterium then lives in the gut of the tick. If the tick feeds again, it can transmit the bacterium to its new host. Usually the new host is another small rodent, but sometimes the new host is a human.

Most cases of human illness occur in the late spring and summer when the tiny nymphs are most active and human outdoor activity is greatest.

Although adult ticks often feed on deer, these animals do not become infected. Deer are nevertheless important in transporting ticks and maintaining tick populations.  

Other Modes of Transmission

Person-to-Person
There is no evidence that Lyme disease is transmitted from person-to-person. For example, a person cannot get infected from touching, kissing or having sex with a person who has Lyme disease.

During Pregnancy & While Breastfeeding
Lyme disease acquired during pregnancy may lead to infection of the placenta and possible stillbirth, however, no negative effects on the fetus have been found when the mother receives appropriate antibiotic treatment. There are no reports of Lyme disease transmission from breast milk.

From Blood
Although no cases of Lyme disease have been linked to blood transfusion, scientists have found that the Lyme disease bacteria can live in blood that is stored for donation. Individuals being treated for Lyme disease with an antibiotic should not donate blood. Individuals who have completed antibiotic treatment for Lyme disease may be considered as potential blood donors

From Pets
Although dogs and cats can get Lyme disease, there is no evidence that they spread the disease directly to their owners. However, pets can bring infected ticks into your home or yard. Consider protecting your pet, and possibly yourself, through the use of tick control products for animals.

Other Transmission
You will not get Lyme disease from eating venison or squirrel meat, but in keeping with general food safety principles meat should always be cooked thoroughly. Note that hunting and dressing deer or squirrels may bring you into close contact with infected ticks.

There is no credible evidence that Lyme disease can be transmitted through air, food, water, or from the bites of mosquitoes, flies, fleas, or lice.

Symptoms

The Lyme disease bacterium can infect several parts of the body, producing different symptoms at different times.  Not all patients with Lyme disease will have all symptoms, and many of the symptoms can occur with other diseases as well.  If you believe you may have Lyme disease, it is important that you consult your health care provider for proper diagnosis. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system.

The first sign of infection is usually a circular rash called erythema migrans or EM.  This rash occurs in approximately 70-80% of infected persons and begins at the site of a tick bite after a delay of 3-30 days.  A distinctive feature of the rash is that it gradually expands over a period of several days, reaching up to 12 inches (30 cm) across. The center of the rash may clear as it enlarges, resulting in a bull’s-eye appearance.  It may be warm but is not usually painful.  Some patients develop additional EM lesions in other areas of the body after several days. Patients also experience symptoms of fatigue, chills, fever, headache, and muscle and joint aches, and swollen lymph nodes.  In some cases, these may be the only symptoms of infection. 

Untreated, the infection may spread to other parts of the body within a few days to weeks, producing an array of discrete symptoms.  These include loss of muscle tone on one or both sides of the face (called facial or “Bell’s palsy), severe headaches and neck stiffness due to meningitis, shooting pains that may interfere with sleep, heart palpitations and dizziness due to changes in heartbeat, and pain that moves from joint to joint.  Many of these symptoms will resolve, even without treatment.

After several months, approximately 60% of patients with untreated infection will begin to have intermittent bouts of arthritis, with severe joint pain and swelling.  Large joints are most often effected, particularly the knees. In addition, up to 5% of untreated patients may develop chronic neurological complaints months to years after infection.  These include shooting pains, numbness or tingling in the hands or feet, and problems with concentration and short term memory.  

Diagnosis

Lyme disease is diagnosed based on symptoms, physical findings (e.g., rash), and the possibility of exposure to infected ticks; laboratory testing is helpful in the later stages of disease.  Validated laboratory tests can be very helpful but are not generally recommended when a patient has erythema migrans.  For detailed recommendations on serologic testing, click here.

When making a diagnosis of Lyme disease, health care providers should consider other diseases that may cause similar illness.  Not all patients with Lyme disease will develop the characteristic bulls-eye rash, and many may not recall a tick bite.  Laboratory testing is not recommended for persons who do not have symptoms of Lyme disease.

Treatment

Most cases of Lyme disease can be cured with antibiotics, especially if treatment is begun early in the course of illness.  However, a small percentage of patients with Lyme disease have symptoms that last months to years after treatment with antibiotics.  These symptoms can include muscle and joint pains, arthritis, cognitive defects, sleep disturbance, or fatigue.  The cause of these symptoms is not known.  There is some evidence that they result from an autoimmune response, in which a person’s immune system continues to respond even after the infection has been cleared.

Prevention

Use repellent, tick checks, and other simple measures to prevent tick bites

Ask your doctor if taking antibiotics after tick bite is right for you

Learn the early signs of tick-borne illness

Control ticks around your home and in your community

A Lyme disease vaccine is no longer available. The vaccine manufacturer discontinued production in 2002, citing insufficient consumer demand. Protection provided by this vaccine diminishes over time. Therefore, if you received the Lyme disease vaccine before 2002, you are probably no longer protected against Lyme disease.

References:

CDC