Cause

Lyme disease is caused by infection with Borrelia burgdorferi (B. burgdorferi) bacteria.

How the disease is spread

Lyme disease is spread when you are bitten by a tick that is infected with B. burgdorferi bacteria. When an infected tick bites you, bacteria travel to the tick's salivary glands and then into your body through your skin. It takes about 24 hours for a tick to attach itself to the skin and begin to feed. The tick generally must be attached to you for about 36 hours in order for it to transmit the Lyme disease bacteria.

In the United States, the two types of ticks that carry Lyme disease bacteria are:

• Deer ticks, which spread Lyme disease in the northeastern and upper Midwestern U.S.
• Western black-legged ticks, which spread Lyme disease along the Pacific coast—mostly northern California and Oregon.

Dogs, cats, and horses can become infected with Lyme disease bacteria, but they can't pass the illness to humans. But infected ticks may fall off the animals and then bite and infect humans. Animals may develop symptoms similar to those seen in people, including fever and swollen joints. A vaccine for dogs is available in some states.

There is no convincing proof that Lyme disease can be spread to humans by insects such as mosquitoes, flies, or fleas.

Is Lyme disease contagious?

Lyme disease is not contagious and cannot be spread from person to person. But certain precautions should be taken to prevent spread of the illness through ways such as blood transfusions.

• People with active Lyme disease should not donate blood, because the bacteria that cause the illness can be transmitted this way. People who were previously treated for Lyme disease may be able to donate blood, but they should check with the blood bank first.
• A pregnant woman may be able to pass Lyme disease to her unborn child, but proven cases are rare. Lyme disease has not been shown to cause birth defects or fetal death.
• There is no proof that breast-feeding mothers can pass the illness to their babies through breast milk. But a nursing mother who is suspected of having Lyme disease may be asked to stop nursing until she has completed a course of antibiotic therapy. The baby is also watched closely for symptoms of Lyme disease.