Lyme disease was first recognized in the United States in 1975, after an unusual 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.
In 2005, 23,305 cases of Lyme disease were reported yielding a national average of 7.9 cases for every 100,000 persons. In the ten states where Lyme disease is most common, the average was 31.6 cases for every 100,000 persons.
The Lyme disease bacterium, Borrelia burgdorferi, is spread through the bite of infected ticks. The black-legged tick (or deer tick,Ixodes scapularis) spreads the disease in the northeastern and north-central United States, and the western black-legged tick (Ixodes pacificus) spreads the disease on the Pacific Coast. These ticks are usually found in wooded areas and have complex life cycles. In some regions, black-legged ticks can spread other diseases in addition to Lyme disease, including babesiosis and anaplasmosis (formerly known as ehrlichiosis). In general, ticks need to be attached for 36 to 48 hours before they can transmit the Lyme disease bacterium.
Lyme disease is an infection caused by the corkscrew shaped bacterium Borrelia burgdorferi, a member of the family of spirochetes.
Ticks can attach to any part of the human body but are often found in hard-to-see areas such as the groin, armpits, and scalp. In most cases, the tick must be attached for 36-48 hours or more before the Lyme disease bacterium can be transmitted.
Adult ticks can also transmit Lyme disease bacteria, but they are much larger and may be more likely to be discovered and removed before they have had time to transmit the bacteria. Adult Ixodes ticks are more active in the spring and fall.
Ixodes ticks are much smaller than the common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Adult Ixodes ticks are larger, about the size of a small apple seed. Left to right: Adult Female, Adult male, nymph, larva
Ixodes ticks search for host animals from the leaf litter of the forest floor or from the tips of grasses and shrubs. Ticks crawl on to animals or persons as they brush against them; ticks cannot jump or fly. 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 person or animal. Ixodes ticks are slow feeders: a complete blood meal can take several days. As they feed, their bodies slowly enlarge.
Although in theory Lyme disease could be spread through blood transfusions or other contact with infected blood, there are no known cases of this happening. There is no definitive evidence that a person can get Lyme disease from the air, food, or water; from sexual contact; from insects such as mosquitoes, flies or fleas; or directly from wild or domestic animals.
The risk of exposure to ticks is greatest in the woods and in the edge area between lawns and woods. But ticks can also be carried by animals into lawns and gardens and into houses by pets. Campers, hikers, outdoor workers, and others may be exposed to infected ticks in wooded, brushy, and grassy places. People living in houses built in heavily wooded areas where infected ticks are common may also be at higher risk for exposure.
Lyme disease is distributed over a wide geographic area in northern temperate regions of the world. In the United States, most infections occur in the following areas:
For Lyme disease to exist in an area, three elements must be present in the natural environment: 1) animals that are infected with Lyme disease bacteria, 2) ticks that can transmit the bacteria, and 3) animal hosts (such as mice and deer) that can provide food for the ticks in their various life stages. Ticks that transmit Lyme disease bacteria can be found in temperate regions that have a constant high relative humidity at ground level.