Risk of Lyme Disease Very Low for Most People Bitten by a Tick

Yale School of Medicine
Tuesday, 12 June 2001

Yale researcher Eugene D. Shapiro, M.D., writing in the July 12 issue of The New England Journal of Medicine (NEJM), says that although antibiotic treatment is effective in preventing Lyme disease, the overall risk of Lyme disease in any individual is a very low 3.2%, even in the most highly endemic areas, and is lower in most other places.

Shapiro's comments are part of an editorial he wrote for the NEJM about a study by researchers at New York Medical College titled, "Doxycycline for Tick Bites-Not For Everyone." The study found that in New York's Westchester County-where the risk of Lyme disease is very high-a single dose of 200 mg of the antibiotic doxyclycline, administered within 72 hours to people bitten by a deer tick, was 87% effective in preventing Lyme disease.

Shapiro said the only people who developed Lyme disease had been bitten by a nymphal-stage deer tick that was at least partially engorged with blood-meaning it had fed for up to 72 hours or longer.

"Ideally, antibiotics would be reserved for this small group that is at high risk of Lyme disease," said Shapiro, who is professor of pediatrics and in the Department of Epidemiology and Public Health at Yale School of Medicine. "Unfortunately, most people who are bitten will not have the expertise available to identify the species, stage and degree of engorgement of the tick."

Shapiro said antibiotics like doxycycline would not be indicated for most tick bites because of the low risk of Lyme disease. "The majority of people in the United States will not be bitten by a nymphal-stage deer tick, most people do not live in areas with a high incidence of Lyme disease, and most ticks will not have fed for 48 to 72 hours or longer," Shapiro said.

He added, "One factor rarely discussed is anxiety about Lyme disease, which often drives decision-making about antimicrobial prophylaxis for tick bites. In the overwhelming majority of people who do develop Lyme disease, treatment is highly effective and the long-term outcomes of persons who develop the disease are excellent."

For more information, or to contact Yale School of Medicine, see their website at: info.med.yale.edu/ysm/

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