Lyme Disease

Lyme disease, first identified in 1975 in Lyme, Connecticut, is a bacterial illness transmitted by a tick bite. Although the disease is found more frequently in the North and the upper Midwest, cases have been reported in Virginia since 1982. For the past 5 years 77 cases have been reported annually on average, mainly from the eastern and central parts of the state.

The Pathogen

The causative agent of Lyme disease is a spirochete called Borrelia burgdorferi. The type of Borrelia infecting humans in the U.S. is designated B. burgdorferi sensu stricto. B. burgdorferi sensu strico and two related Borrelia, B. garinii and B. afzelii also cause Lyme disease in Europe. In Asia, only B. garinii and B. afzelii cause Lyme disease in humans. Evidence is accumulating that these closely related, but different, spirochetes are associated with somewhat different disease expressions. Arthritis appears to occur more frequently following infection with B. burgdorferi sensu stricto; neurologic manifestations are more common in infections with B. garinii; and cutaneous manifestations occur more frequently in association with B. afzelii infection

Borrelia burgdorferi, the spirochetal bacteria that cause Lyme disease, seen at 400x magnification.
Photo courtesy of CDC

Black-legged ticks (Ixodes scapularis), also known as deer ticks, are responsible for transmitting Lyme disease to humans in the northeastern and north-central United States. Ixodes ticks are much smaller than common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Ticks feed by inserting their mouths into the skin of a host and slowly take in blood. Both deer and rodent hosts must be abundant to maintain the enzootic cycle of B. burgdorferi. Ixodes ticks are most likely to transmit Lyme disease after feeding for two or more days.

From left to right: The deer tick (Ixodes scapularis) adult female, adult male, nymph, and larva on a centimeter scale. Photo courtesy of CDC.

Symptoms

Within 2 days to a few weeks after being bitten by an infected tick, 60 percent of people develop a circular or oblong rash, called erythema migrans, or EM, at the site of the bite. The EM rash usually increases to 2 to 3 inches in diameter and sometimes to as large as 20 inches. As it enlarges, the center clears giving it a "bull's-eye" appearance. Sometimes multiple rashes occur. Because it does not itch or hurt, EM may be hard to detect. EM is often more pronounced after a warm bath.

In addition to, or instead of the EM, a feeling of tiredness, headache, fever, stiff neck, muscle aches, joint pain and swollen lymph nodes can occur. If left untreated, Lyme disease can cause chronic arthritis or heart and nervous system complications in a small percentage of infected people several weeks to several months after exposure.

Because the deer tick is so small in its nymph stage, many people are not aware of its bite. If you have been in an area that may contain ticks and experience any of these symptoms, contact your doctor. State that you may have been bitten by a tick. This is extremely important because the diagnosis of Lyme disease is usually based on symptoms and history of tick exposure. The available blood tests are often negative. On the other hand, recent studies have shown that many people who think they may have the late stages of Lyme disease, actually suffer from other maladies and have blood tests that are falsely positive.

Transmission Cycle

Knowing the complex life cycle of Ixodes ticks is important in understanding the risk of acquiring Lyme disease and in finding ways to reduce this risk. The life cycle of the deer tick requires 2 years to complete. Adult female ticks lay eggs on the ground in early spring. By summer, eggs hatch into larvae. Larvae feed on mice, other small mammals, deer, and birds in the late summer and early fall, molt into nymphs, and then are dormant (inactive) until the next spring. Nymphs feed on rodents, small mammals, birds and humans in the late spring and summer and molt into adults in the fall. In the fall and early spring, adult ticks feed and mate on large mammals (especially deer) and bite humans. The adult female ticks then drop off these animals and lay eggs in spring, completing a 2-year life cycle.

Ticks, small rodents, and other non-human vertebrate animals all serve as natural reservoirs for B. burgdorferi. This means that the Lyme disease bacteria can live and grow within these hosts without causing them to die. Larvae and nymph ticks typically become infected with the Lyme disease spirochete, B. burgdorferi, when they feed on small animals that carry the bacteria in spring and summer. The bacteria remain in a tick as it changes from larva to nymph or from nymph to adult in late summer or early fall. Infected nymphs bite and transmit B. burgdorferi bacteria to other small rodents, mammals, and humans, all in the course of their normal feeding behavior.

Image courtesy of the American Lyme Disease Foundation.

The Risk

In the United States, Lyme disease is mostly localized to states in the northeastern, mid-Atlantic, and upper north-central regions, and to several counties in northwestern California. The majority of cases reported to the CDC are from northeastern states.

Individuals who live or work in residential areas surrounded by tick-infested woods or overgrown brush are at risk of getting Lyme disease. Persons who work or play in their yard, participate in recreational activities away from home such as hiking, camping, fishing and hunting, or engage in outdoor occupations, such as landscaping, brush clearing, forestry, and wildlife and parks management in endemic areas may also be at risk of getting Lyme disease.

Treatment

The transmission of the bacteria that causes Lyme disease from an infected tick is unlikely to occur before 36 hours of tick attachment. For this reason, daily checks for ticks and promptly removing any attached tick that you find will help prevent infection. Individuals who are bitten by a deer tick should remove the tick promptly, and may wish to consult with their health care provider. Persons should promptly seek medical attention if they develop any signs and symptoms of early Lyme disease. When detected early, Lyme disease can be mild and easily treated with oral antibiotics such as tetracycline or penicillin. Even in the late stages, Lyme disease usually can be treated successfully with antibiotics, but the treatment lasts longer and is more involved.

As of February 25, 2002 the manufacturer announced that the LYMErix™ Lyme disease vaccine will no longer be commercially available.