Lyme Disease
How do the bacteria cause disease?
How is Lyme Disease diagnosed?
What is Lyme Disease?
Lyme Disease is an infectious illness that has different manifestations depending on the stage of the illness (i.e., early localized disease, early disseminated disease, and late disease).
What causes Lyme Disease?
A type of bacteria called a spirochete, which is named Borrelia burgdorferi, causes Lyme Disease. The bacteria is transmitted by ticks of the Ixodid species; in the United States, this is usually Ixodes scapularis, which is also known by its common name, the deer tick.
Who gets Lyme Disease?
Lyme Disease occurs most commonly in areas where deer ticks are abundant and where the proportion of ticks that are infected with the bacteria that cause Lyme Disease is high (20% to 50%)-southern New England, southeastern New York, New Jersey, eastern Pennsylvania, eastern Maryland, Delaware, and parts of Minnesota and Wisconsin. The majority (greater than 75%) of the reported cases occurs in a small number of counties (less than 70) in these endemic areas.
Lyme Disease is uncommon in the Pacific states because, although the tick found there-Ixodes pacificus (the Western black-legged tick)-can transmit B. burgdorferi, few of these ticks are infected with the bacteria. People with increased occupational, recreational, or residential exposure to tick-infested woodlands and fields (the preferred habitat of ticks) in endemic areas are at an increased risk of developing Lyme Disease.
There are a number of factors associated with the risk of transmission of B. burgdorferi from ticks to humans. First, a tick has to be infected to be able to transmit the organism. The proportion of infected ticks varies greatly both by the geographic area and by the stage of the tick in its life cycle. Even in areas with the highest incidence of Lyme Disease, only about 20% to 30% of nymphal-stage ticks (the stage most likely to cause Lyme Disease) are infected.
Lyme Disease develops after an infected tick inoculates bacteria into the skin of its human victim. The risk of transmission of the bacteria from infected deer ticks is related to the duration of feeding. It takes hours for the mouth parts of ticks to implant fully, and much longer (days) for the tick to become fully engorged.
Experiments with animals have shown that nymphal-stage ticks must feed for 48 hours or longer, and adult ticks must feed for 72 hours or longer, before the risk of transmission of B. burgdorferi from infected ticks becomes substantial. The duration of time that a tick has fed can be estimated from measures of engorgement derived from experiments with animals.
Based on these studies, there is evidence that approximately 75% of persons who recognize that they have been bitten by a deer tick remove the tick less than 48 hours after it has begun to feed. Indeed, the majority of persons who develop Lyme Disease do not recall a tick bite. Unrecognized tick bites probably are associated with a greater risk, since unrecognized ticks may feed longer.
How do the bacteria cause disease?
The bacteria cause inflammation in the tissues that are infected. First, the inflammation usually causes a rash in the skin (known as erythema migrans) at the site of the tick bite. The bacteria may then enter the bloodstream and spread to other sites, which include other parts of the skin, the nervous system, and the heart. The bacteria may also infect the joints and cause inflammation that leads to arthritis. Rarely, but particularly in people who have a certain genetic predisposition, an autoimmune inflammatory process may develop in the joint so that inflammation continues even after the bacteria are killed by antibiotic treatment.
What are the common findings?
By far, the most common manifestation of Lyme Disease is the characteristic reddish rash, erythema migrans, which usually develops 7 to 14 days (and up to as many as 30 days) after the tick bite. This rash occurs in approximately 90% of children with Lyme Disease. It is a circular (or oblong) rash that increases in size over time, sometimes growing to as much as a foot or more in diameter. Although central clearing may occur (causing a characteristic "bull's eye" appearance), it is probably even more common for the rash to be uniformly red. Fungal infections of the skin ("ringworm"), insect bites, and circular or "nummular" eczema rashes commonly are confused with erythema migrans.
Erythema Migrans
Approximately one-quarter of the children with erythema migrans will have multiple erythema migrans, which is a sign that the bacteria have spread through the bloodstream and "seeded" other sites in the skin. The "secondary" rashes are usually smaller than the primary rash, and fever; pains in the muscles, joints, or neck; headache; and/or fatigue often accompany them. Multiple erythema migrans is a manifestation of "early disseminated" Lyme Disease.
Another finding in the "early disseminated" stage of the disease is paralysis of the facial nerve. Rarely, patients may have meningitis or arrhythmia of the heart.
The common manifestation of late Lyme Disease is arthritis. The knee is usually affected (greater than 90% of the cases), but any joint may be involved. Although the joint is swollen, it may be either very painful or it may be associated with relatively little pain, and the patient sometimes is able to pursue most normal activities with little impairment. Fever and other signs, such as headache or fatigue, may or may not be present.
It is important to realize that although patients with Lyme Disease often have non-specific symptoms, such as headache, fever, fatigue, or muscle pain, these symptoms are virtually always accompanied by specific signs of Lyme Disease, such as the characteristic rash, facial palsy, or a swollen knee. These non-specific symptoms, when present without more specific signs of Lyme Disease, are virtually never caused by Lyme Disease.
How is Lyme Disease diagnosed?
The most common sign of Lyme Disease-the erythema migrans rash-is usually diagnosed based on its typical appearance; the history of possible exposure to ticks; and the fact that it enlarges over time, if untreated.
Tests for antibodies are not appropriate in persons with a single erythema migrans rash because they usually are negative at this early stage of the illness. By contrast, if the rash is not present, but other signs, such as facial palsy or a swollen knee, are present (and the clinical and epidemiologic history is consistent with Lyme Disease), antibody tests usually are used to diagnose the illness.
It is of critical importance to realize that, because false-positive tests are common, tests for antibodies against B. burgdorferi should not be used as a "screening test" for persons with only non-specific symptoms, such as fatigue, muscle or joint pain, or a headache. Use of the antibody tests for Lyme Disease in such situations has resulted in a very high frequency of misdiagnosis of Lyme Disease, and it has helped to perpetuate the myth that Lyme Disease is difficult to diagnose and to treat.
How is Lyme Disease treated?
An antibiotic administered by mouth (usually doxycycline or amoxicillin) is very effective in treating Lyme Disease. Occasionally, an antibiotic (usually ceftriaxone or penicillin) is given intravenously to treat infection of the central nervous system, such as meningitis, or in the rare instances in which arthritis does not respond to one or two courses of oral treatment.
What are the complications?
Complications are very rare. The most common cause of failure to respond to treatment is misdiagnosis, that is, the patient either does not have (and never did have) Lyme Disease, or the patient had Lyme Disease in the past, and the current symptoms are unrelated to the previous Lyme Disease. Very rarely, facial palsy may persist (usually only to a very mild degree). Very rarely, an autoimmune arthritis that does not respond to antimicrobial treatment and that is not related to persistence of bacteria may develop in persons with a genetic susceptibility to this problem, especially when antibiotic treatment has been delayed.
Chronic inflammation of the central nervous system with memory loss has been reported as a rare complication in adults for whom treatment was delayed for a long time (often not until years after the presumed time of the infection). Congenital Lyme Disease (transmission from a pregnant woman to her child) has not been documented; likewise, transmission of Lyme Disease via breastfeeding has not been shown to occur.
Links to additional information
There are many Internet sites that provide information about Lyme Disease; however, many are not supported by scientifically sound data, including many sites controlled by "patient-advocate" groups or individuals. One worthwhile site is that of the American Lyme Disease Foundation at www.aldf.com. Information also is available through the Web site of the Centers for Disease Control at www.cdc.gov/ncidod/dvbid/lyme/.
About the Author
Dr. Shapiro trained at the UCSF School of Medicine, Children's Hospital of Pittsburgh, and the Yale School of Medicine, where he is currently a Professor of Pediatrics and of Epidemiology and Public Health.
He has conducted numerous research studies of Lyme Disease, the efficacy of vaccines, and bacteremia in children.
Copyright 2012 Eugene D. Shapiro, M.D., All Rights Reserved