Lyme disease is caused by infection with the bacteria Borrelia burgdorferi. It was not until the 1970s that the association of these bacteria with what came to be known as Lyme disease was discovered. Borrelia belong to a category of bacteria known as spirochetes, so named because of their corkscrew-like appearance. Borrelia burgdorferi survive in the guts of certain ticks in the genus Ixodes, and enter host organisms in the saliva of the feeding ticks. In the eastern U. S. white-footed mice, Peromyscus leucopus, are the principal reservoir hosts for B. burgdorferi , and it is from feeding on infected mice that ticks become infected. In susceptible animals these bacteria can reside in many types of tissue (e. g., muscle, nervous).

Vector. Persons become infected with Borrelia burgdorferi when they are bitten by an infected tick. In the eastern and central U. S. the blacklegged tick, often referred to as the deer tick, Ixodes scapularis, is the species responsible for transmitting B. burgdorferi to humans and other vertebrate hosts. In the Pacific coast states the western blacklegged tick, I. pacificus, fills the same role as I. scapularis as vector of B. burgdorferi. Both I. scapularis and I. pacificus are similar in appearance and biology. The blacklegged tick has 3 active life stages (larva, nymph and adult) that feed on a wide variety of vertebrate hosts, including birds and lizards, but most adults feed on white-tailed deer. Having found a host, larvae and nymphs feed 3-5 days, and adult females about 1 week then drop off the host. Although the life cycle of I. scapularis is about 2 years, each tick spends only about 2 weeks feeding on hosts.

According to the Centers for Disease Control (CDC), Lyme disease is now the most common vector-borne infection in humans in the United States. More than 16,000 cases of Lyme disease were reported to the CDC in 1999. Ninety-two percent of these were from the New England states. Some years the numbers are not as high, but according to the CDC Lyme disease is under-reported. Since its discovery in 1975, cases have been reported from Canada, Europe, Australia, Asia, and the USSR. Lyme disease has been reported in domestic animals, including dogs, horses, and more recently, cattle.

Persons at risk for getting Lyme disease are those who live, work, or visit parks in endemic areas and frequent sites where infected ticks are common, such as wooded areas inhabited by white-tailed deer.
 

Prevention
Tick bite prevention can include personal protection and reduction of tick populations. Most cases of Lyme disease are the result of persons being bitten by nymphal I. scapularis. The vast majority of host-seeking nymphs and larvae of I. scapularis are found in leaf litter in and adjacent to woods. Nymphs are also frequently found on and about stone fences and fallen logs. Therefore, it is best to limit one's contact with leaf litter, and do not sit on stone fences and fallen logs. Wear light colored clothing so that it is easy to see ticks on it. Tuck pants legs into socks. Carefully tape junctures of shoes, socks and pants legs with wide masking tape Entirely cover shoe laces with tape, because some species of ticks will enter shoes through eyelets and around tongue of shoe. Some larvae and occasionally nymphs can penetrate through the weave of some socks.

Use repellents. Permethrin-based repellents should be used only on clothing. Deet-based repellents can be used on skin. PBESL in conjunction with the Chemicals Affecting Insect Behavior Laboratory, BARC and Walter Reed Army Institute of Research, Silver Spring, MD are evaluating the efficacy of tick repellents that can be used on human skin. Be careful about long hair that contacts shirt or coat collars. Ticks can crawl from collar to hair undetected. Examine yourself and children for ticks repeatedly after spending time in tick infested areas. You cannot depend on showering to remove ticks, especially attached ticks. Place clothing worn in tick infested areas in a closed plastic bag until washing it. Ticks can survive machine washing with cool water. Unlike mosquitoes, the process by which ixodid ticks feed is slow, lasting a few days. In the case of Lyme disease, 24-36 hours of attachment are needed before transmission of B. burgdorferi starts. Other disease causing agents can be transmitted more quickly.

Attached ticks should be removed as soon as possible. Tick removal: Attached ticks are best removed using forceps (tweezers) with slender points. The tick should be grasped with the forceps as close to the person's skin as possible. Grasping the tick farther from its mouthparts increases the chance of squeezing its gut contents into the person. Pull the tick from the skin slowly and firmly. Wash the bite area and apply a topical antibiotic to it.

Because the early symptoms of many tick-borne illnesses are like those experienced with other ailments, such as the flu, it is prudent to see a physician, if you notice headache, fever, muscle pains, or other symptoms after having had a tick bite. Lyme disease and other tick-borne illnesses caused by bacteria are generally respond to antibiotics, when treatments are started early. It is important that your physician knows about your tick bite.
--From USDA Animal Parasitic Diseases

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