Doxycycline offers effective prophylaxis after tick bite. (Single Dose Within 72 Hours).

NEW YORK -- A single dose of doxycycline given within 72 hours after a deer tick bite can prevent the development of Lyme disease, but not all patients require prophylaxis, Dr. Robert B. Nadelman said.

"Worrying after a tick bite is something we all do, but there are lots of reasons not to worry," he said at an international conference on tick-borne diseases sponsored by Imedex.

First, most tick bites don't result in illness, he noted. Disease is most likely to occur with ticks in the nymphal stage of growth; at that point, they are larger than the larvae and are more likely to be noticed and removed.

Also, the infectious agent, Borrelia burgdorferi, is transferred to the host via infected saliva only at a late stage of its blood meal, generally after the tick has been feeding for more than 72 hours.

When illness occurs, treatment is effective in most patients and the prognosis is excellent, said Dr. Nadelman of the division of infectious diseases, New York Medical College and the Lyme Disease Diagnostic Center, Westchester Medical Center, Valhalla, N.Y.

In contrast, there are a great many concerns regarding antibiotic prophylaxis, such as the occurrence of adverse events. "Antibiotics are often used as anxiolytic agents, and that's not the best role for them," Dr. Nadelman said.

Moreover, there have been questions about the efficacy of antibiotics and concerns that their use might alter the natural history or clinical manifestations of Lyme disease or contribute to the development of resistance in the community. "For those reasons the recommendation for some years now has been to observe the patient after an Ixodes scapularis tick bite and not to give prophylactic antibiotics," he said.