Warm weather inevitably heralds the return of diseases transmitted by insects, arthropods, and other creatures. Lyme disease is the most common vector-borne illness in this country, with about a third of the U.S. population living in endemic areas.
Lyme disease has established itself as a serious perennial problem over the past 30 years. Emergency physicians are often the first to encounter patients with initial signs of the infection, and are frequently called upon by patients with tick bites whose only apparent manifestation of Lyme disease is anxiety.
Development of the Epidemic
Erythema migrans, the classic rash of Lyme disease, was first described a century ago, and since the 1940s has been associated with tick bites. The present epidemic of Lyme disease dates to the 1970s when clusters of children with arthritis were noted around the town of Lyme, CT.
In 1982, a newly-recognized spirochete, Borrelia burgdorferi, was isolated from the blood, skin lesions, and spinal fluid of some patients with Lyme disease, and was noted to be identical to the organism recovered from local deer ticks (Ixodes). Antibodies to the spirochete were noted in most patients with Lyme disease, while control patients had no such antibodies. Borrelia burgdorferi then was established as the causative agent of Lyme disease, and the Ixodes tick was established as the vector of transmission (New Engl J Med 1983;308:733).
The incidence of Lyme disease is only 1–3% even in highly endemic areas
Lyme disease is considered endemic in 15 states as well as in central Europe and east Asia. Three foci of disease exist in this country: the Northeast from Maine to Maryland, the north central states of Wisconsin and Minnesota, and a minor focus in northern California and Oregon (New Engl J Med 2001;345:115). In 2000, the most recent year for which data are available, there were almost 18,000 new cases of Lyme disease reported to the Centers for Disease Control and Prevention, greater than in any previous year and almost certainly an underestimate of the true number of cases (MMWR 2002;51:29).
Ixodes scapularis transmits Lyme disease in the northeastern and north central states. Mice are the hosts of immature ticks, while deer are the hosts of mature ticks. The proliferation of the deer population in densely populated New England states is believed to be responsible for the current epidemic. In western states, Ixodes pacificus transmits the disease, and the risk of infection from this species is much lower.
Deer ticks are only 1 to 2 mm in size and much smaller than common dog ticks, which are often up to a centimeter in length. An attached tick frequently resembles a freckle and often goes unnoticed when latched onto hairy parts of the body.
Emergency physicians usually encounter Lyme disease in Stage 1, when patients present with erythema migrans. The lesion is usually described as a “bulls-eye,” with concentric red rings migrating centrifugally from the site of the tick bite. Erythema migrans begins between three days and one month after the bite, starting as a red macule and reaching a final diameter of about 15 cm.
The rash is noted in the majority of patients who develop Lyme disease, and is often accompanied by influenza-like symptoms, which may reflect bacteremia (New Engl J Med 2001;345:115). Because most patients don't actually recall the tick bite, diagnosis of early Lyme disease depends upon recognition of the characteristic rash and obtaining a history of exposure to ticks in an endemic area (Ann Intern Med 1983;99:76).
Lyme disease progresses to Stage 2 in a minority of untreated patients, starting a few days or weeks after the rash appears. Secondary lesions may develop, similar to but smaller than the initial rash and located distant from the site of the tick bite. Patients have malaise, fevers, and adenopathy; musculoskeletal pain is common.
About 15 percent of untreated patients develop neurologic deficits, such as frank meningitis, mild encephalitis with cognitive difficulties, cranial nerve palsies (particularly facial nerve palsies, mimicking Bell's palsy), mononeuritis multiplex, cerebellar ataxia, and myelitis. About five percent of untreated patients develop acute cardiac abnormalities, usually variable A-V block and occasionally mild ventricular dysfunction. The cardiac abnormalities are transient and rarely symptomatic or life-threatening (Med Clin N Amer 2002;86:285).
Stage 3 of Lyme disease, typified by joint involvement, is seen in more than half of patients with untreated illness, and it starts months after the tick bite. Patients have recurrent episodes of arthritis, particularly in the knees. Subacute encephalopathy is another finding in late Lyme disease, seen in about five percent of untreated patients, and is manifested by persistent memory, mood, or sleep disturbances.
Treatment of Lyme Disease
Patients with erythema migrans do not manifest antibody responses to B. burdorferi, and there are no serologic tests that reliably diagnose Lyme disease during Stage 1. Erythema migrans should, therefore, be empirically treated based on the appearance of the rash and a history of exposure to ticks in an area endemic for disease.
Prophylactic antimicrobial treatment, which is 87% effective in preventing illness, is recommended for those bitten by deer ticks
Doxycycline for 14 to 21 days is recommended for patients over age 8 (except pregnant and nursing women). Amoxicillin is the preferred alternative. Cefuroxime axetil is a third-line agent, and the macrolides are reserved for patients unable to take any of the other regimens (Ann Intern Med 1998;128:37).
More than 90 percent of patients treated for early Lyme disease have resolution of symptoms with no disease progression. IV therapy has no benefit over oral therapy in early disease, provided the patient has no neurologic manifestations other than facial palsy (New Engl J Med 2001; 345:85; J Infect Dis 2001;183:453).
Diagnosis and treatment are more complex for patients with Stage 2 or Stage 3 Lyme disease. While disease manifestations are protean, antibody tests are usually positive by one month after acute infection, and these greatly assist diagnosis. Emergency physicians are rarely called upon to order or interpret Lyme disease titers or manage subacute or chronic infections. A suspicion of the disease and appropriate referral for further testing is usually the extent of our involvement, although patients with meningitis from Lyme disease usually require hospitalization.
While I've seen maybe a single case of erythema migrans in the past decade (as far as I know), I've had dozens of encounters with patients who have had a tick bite and are terrified of contracting Lyme disease. Some bring the tick, some display the site of the alleged attack, and others come bearing only a description of their assailant. All want reassurance; many want antibiotics.
What should an emergency physician offer the asymptomatic patient, assuming he provides a reasonable description of a deer tick bite? Clearly, patients should be reassured that the likelihood of contracting Lyme disease is very low, probably only about one to three percent, even in highly endemic areas (New Engl J Med 1992;327:769). Based on these data and meta-analyses of relevant studies, many authorities have advised withholding prophylactic antibiotics (Clin Infect Dis 2000;31:S1).
A recent large and well-publicized study seems to have tipped the scales in favor of prophylactic antimicrobial treatment for patients sustaining deer tick bites — good news for those who seek to please their patients' requests for antibiotics. Patients living in an area highly endemic for Lyme disease who had recently removed Ixodes ticks (as identified by an entomologist) received either doxycycline as a single 200 mg dose or placebo. Three percent of the control patients developed erythema migrans, compared with 0.4 percent of the doxycycline-treated patients. This difference was statistically significant.
It is important to note that none of the 482 patients in this study developed later manifestations of Lyme disease. Doxycycline was associated with a high rate of nausea. Because most patients can't identify an Ixodes tick with the accuracy of an entomologist, the benefit to antibiotic therapy in real-world scenarios (and in areas where Lyme disease is less endemic) is unlikely to be as great as seen in this study. These limitations aside, I suspect that few of us would want to face the potential consequences of a patient contracting Lyme disease after we chose to withhold a simple therapy reported to be 87 percent effective in preventing the illness.