risk of getting Lyme disease
Two Yale researchers examine the risk of contracting Lyme disease and the benefits of treating it with the antibiotic doxycycline in the July 12 issue of the New England Journal of Medicine (NEJM).
Durland Fish, associate professor of epidemiology, was a coauthor on an article titled "Prophylaxis with Single-Dose Doxycycline for the Prevention of Lyme Disease after an Ixodes Scapularis Tick Bite." The study, conducted by researchers at New York Medical College, 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 doxycycline, administered within 72 hours to people bitten by a deer tick, was 87% effective in preventing Lyme disease.
Fish points out, however, that the study was based only on patients who had seen and removed a deer tick.
"Most patients contracting Lyme disease never see the tick that caused infection," he says. "The nymphal stage of the deer tick is about the size of a poppy seed, and even though it takes three to four days for it to feed before removing itself, most people will not notice it. This is because the tick also secretes an anesthetic while it is feeding, as well as infectious bacteria."
One published study showed that only about 20% of patients with Lyme disease recalled a tick bite. "This is an amazing, but true, fact of tick biology," says Fish, associate professor in the Department of Epidemiology and Public Health at the School of Medicine. "The key difference in this issue is between tick bites that are recognized by the patients and those which are not. Most are not."
Unrecognized tick bites in humans cannot be studied, but animal studies done by Fish show that more than 80% of infected ticks will cause Lyme disease in mice after 72 hours of feeding, the maximum feeding time for nymphs. This rate, coupled with a 25% natural rate of infection of deer ticks in the Northeast, yields a 20% chance of infection from a nymphal deer tick.
"People who live in Lyme disease areas should take the risk of tick bites seriously and do all they can to prevent them ...," Fish says. "Bites from deer ticks are extremely common in the Northeast."
One Centers for Disease Control study coauthored by Fish estimates that one in five Westchester Country residents are bitten by deer ticks each year, an annual average of 179,000 tick bites.
The NEJM study also reported that over 18% of the tick-bite patients received a second tick bite within the 90-day study period and over 6% of the patients had multiple ticks attached when they were initially observed. "With such a high frequency of contact between ticks and people, prevention methods other than treating tick bites with antibiotics are badly needed," Fish says
In an editorial in NEJM about the doxycycline study, Yale researcher Dr. Eugene D. Shapiro contends 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 says 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," says Shapiro, who is professor in the Departments of Pediatrics and of Epidemiology and Public Health at the 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."
Antibiotics like doxycycline would not be indicated for most tick bites because of the low risk of Lyme disease, says Shapiro, noting, "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."
He adds, "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."
-- By Karen Peart
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