Lyme disease, the most common vectorborne illness in North America, is caused by a spirochete, Borrelia burgdorferi. This multi-system infectious disease is transmitted by the bite of an infected Ixodes tick (most commonly belonging to the species scapularis). Initially, a localized rash (erythema migrans) is often noted at the site of inoculation. Spirochetes hematogenously spread to other organ systems, causing multisystem symptoms. Prompt recognition and early antibiotic treatment markedly reduce the likelihood of later symptoms. Unrecognized and untreated Lyme disease can cause late arthritic and neurologic syndromes that become difficult to treat, possibly because of permanent tissue damage.
Edlow reviewed the current knowledge of tick-borne illness. The attack rate for Lyme disease is highest in children younger than 15 years of age and in adults older than 29 years of age. Establishing the diagnosis involves assessing: (1) seasonality of initial symptoms (“tick season” is May through September), (2) patient activity (persons who spend much time outdoors in endemic areas are at the greatest risk) and (3) geography (more than 80 percent of cases originate in eight states: New York, New Jersey, Connecticut, Rhode Island, Maryland, Massachusetts, Pennsylvania and Wisconsin).
The vector for infection is the tick from the genus Ixodes. The life cycle is two years, with three motile stages (larvae, nymph and adult), each requiring a blood meal from a vertebrate host to move on to the next stage. Nymphs and adults can infect humans, although the preferred hosts are small rodents and white-tailed deer, respectively. Nymphs are the most common vector to humans because their smaller size makes them less physically detectable. The likelihood of infection increases with longer duration of tick attachment. Fewer infections occurred when the tick was attached for 24 hours or less, whereas infection rates increased in tick attachments of 72 hours or more. Once transmission occurs, the spirochete may be killed by host defenses, establish local infection or disseminate to distant sites. When local infection is established, erythema migrans is the reaction. Dissemination occurs within one week and is often identified using polymerase chain reaction (PCR) to detect spirochetal DNA in blood. PCR testing for spirochetal DNA is an unreliable indicator of infection.
Treatment depends on many factors, including identification of tick species and stage, and duration of tick attachment. With attachment of more than 72 hours, there is a 20 percent chance of spirochete transmission. Estimates of tick engorgement by visual examination aid in determining the length of attachment. Early tick removal can be accomplished using steady tension with a fine forceps. When risk of infection is high, antibiotic therapy is appropriate.
Erythema migrans usually occurs seven to 10 days after the bite and may be accompanied by mild fatigue, myalgias, arthralgias, headache, fever and chills. True erythema migrans is flat, will spread over a period of days to weeks (not hours) and is rarely found on the distal extremities. The physical examination may reveal fever, neck stiffness and local adenopathy. In approximately 20 percent of patients, secondary lesions may illustrate hematogenous spread. The diagnosis of Lyme disease is clinical. Early infection is often accompanied by false-negative serologic tests, although this can occur late in the disease. The positive predictive value of serologic testing is low in patients with vague symptoms unaccompanied by any objective signs.
Patients with a high-risk asymptomatic tick bite can be treated with antibiotics for 10 days, while treatment of patients with Lyme disease who present with erythema migrans should last two to three weeks. First-line drugs are amoxicillin or doxycycline; alternatives include cefuroxime axetil, tetracycline, azithromycin or erythromycin. Patients with more severe disease should be treated with parenteral antibiotics (ceftriaxone, cefotaxime or penicillin G) for two to four weeks.
Prevention includes avoidance of ticks, which is accomplished by tucking long pant legs into socks, staying in the center of trails, applying diethyltoluamide (deet) to the skin and inspecting the skin every day. In two large trials, the Lyme disease vaccine appeared to be 76 to 92 percent effective after the full series of three injections.
Other tick diseases that may present with an undifferentiated febrile illness include babesiosis and ehrlichiosis. Coinfections of more than one of these tick-borne infections have been recognized, and these patients have a longer duration of illness. Babesiosis generally does not require treatment, while doxycycline is the drug of choice for the treatment of ehrlichiosis. Rocky Mountain spotted fever is another tick-borne illness with an untreated fatality rate of up to 25 percent. Patients are usually treated with tetracycline or chloramphenicol. In one retrospective study, doxycycline was associated with higher survival rates.