The use of short antibiotic regimens in treating postexposure to spirochetal infections such as syphilis and leptospirosis has been effective; however, small studies of antibiotic treatment after tick bites did not demonstrate consistent efficacy in the prevention of Lyme disease. Nadelman and colleagues studied the efficacy and safety of a single 200-mg dose of doxycycline in preventing Lyme disease after a tick bite.
Patients were enrolled in Westchester County, New York, where Lyme disease is hyperendemic. Study subjects were accepted if they presented within 72 hours of a tick bite and if the tick was available for verification by an entomologist as being from the Ixodes scapularis group that is the vector for Lyme disease. Participants were randomly assigned to receive either two 100-mg capsules of doxycycline or two identical placebo pills. Skin culture for Borrelia burgdorferi (through biopsy of the bite site) was performed, and serologic testing was performed at study entry, and again at three weeks and six weeks post-bite.
Entomologists confirmed I. scapularis ticks in 482 of the 506 patients (95 percent) who presented with a tick bite. Multiple tick bites were seen at presentation in 28 patients and 59 patients reported additional tick bites after enrollment but during the six-week study period. Erythema migrans (a dilating red ring of rash surrounding the tick bite site) developed in 3 percent of the patients who were receiving placebo and 0.4 percent of patients who were receiving single-dose doxy-cycline, which represented a statistically significant level of protection. Serologic conversion or a positive skin culture were found in seven of nine subjects (78 percent) who developed a rash. There were no seroconversions or positive cultures in any patients without rash. Entomologists examined each tick for its life stage (larva, nymph, adult). Only nymphal ticks that were at least partly engorged because of blood feeding were associated with transmission of Lyme disease, but the authors did not describe how to distinguish this stage of the tick.
Side effects were more common in the doxycycline group than in the placebo group. Nausea and vomiting (reported in 15 percent and 6 percent of treated patients, respectively) were the most common adverse events.
The authors concluded that a single 200-mg dose of doxycycline given within 72 hours of a bite from an I. scapularis tick was 87 percent effective in preventing the development of early Lyme disease. Three percent of patients in the placebo group developed Lyme disease.
An accompanying editorial by Shapiro notes that most tick bites in the United States are not caused by I. scapularis ticks. Depending on the statistical model used, the efficacy of doxycycline might be as low as 25 percent rather than the 87 percent quoted by Nadelman and associates. Shapiro suggests that prompt removal of all ticks before they begin to feed (within 48 hours) might be the most effective remedy for the prevention of Lyme disease and other tick-borne illnesses.