So you’ve pulled off a deer tick

What’s the next step? Perhaps you know the recommendation for a single dose of 200mg of doxycycline for prevention of Lyme disease, but is it based on solid evidence?

Well, the study at hand enrolled 506 patients, which they then pared down to 482 that had the ticks positively identified by an entomologist. They had this many because their power calculations determined they would need a sample size of 129, but once their data indicated that nymphal ticks were highly correlated with erythema migrans, they extended the study to the point it lasted almost 10 years. Due to the length of the study, 6 patients were enrolled twice due to being bitten in separate years.

Even with the large sample size only 9 patients developed EM, with 8 of these occurring in the placebo group. This allowed statistical significance even with relatively low prevalence. This gives them an efficacy rate of 87%, but unfortunately the 95% CI is 25-98%.

It seems like it works, and the theoretical risk of a single dose of antibiotics is fairly low. However, 30% of the doxy group had side effects from that single dose, usually nausea and vomiting.

Based on this study, giving 200mg of doxycycline as a single dose within 72 hours of tick removal appears sensible. You diminish the risks of developing erythema migrans, while also preventing a prolonged course of antibiotics. My only warning would be to tell patients to continue to watch for symptoms of Lyme, as prophylaxis is not 100%.

Nadelman RB et al. Prophylaxis with single-dose doxycycline for the prevention of Lyme disease after an Ixodes scapularis tick bite. N Engl J Med. 2001 Jul 12;345(2):79-84. [PMID 11450675]