to the editor: I was happy to see the article on the diagnosis of Lyme disease¹ in the July 15, 2005, issue of American Family Physician. Although the article¹ correctly makes reference to coinfection with human granulocytic ehrlichiosis, it incorrectly identifies the causative organism as Babesia microti (an organism that also causes coinfection with Lyme disease). This distinction is a critical one because the treatments for these infections are different. Although ehrlichiosis (caused by Ehrlichia phagocytophila), like Lyme disease, can be treated with doxycycline (Vibramycin), babesiosis is an intracellular parasite treated with antiprotozoal medications in addition to traditional antibiotics (atovaquone [Mepron], 750 mg every 12 hours for 10 days combined with azithromycin [Zithromax], 1,000 mg once a day for three days followed by 500 mg once a day for seven days).

Further, the article states that “empiric antibiotic prophylaxis is not recommended for patients who seek care after a tick bite but who are asymptomatic.”¹ A recent randomized controlled trial² demonstrates that a single 200-mg dose of doxycycline, if administered within the first 72 hours after a likely exposure, will prevent approximately 90 percent of the cases of Lyme disease. For physicians, such as myself, who practice in endemic areas, this is a very good option. Because the clinical symptoms of Lyme disease do not appear until well after this three-day window, it is imperative that patients and physicians be aware of this valuable prevention technique. I use this option in my own practice and have found that it decreases the incidence of Lyme disease significantly and gives my patients a much-needed sense of security.

in reply: I would like to thank Dr. Marshall for his comments on Babesia microti and Ehrlichia phagocytophila. Although these coinfections are far from the main thrust of our article on the diagnosis of Lyme disease,¹ the information is a valuable correction and a service to readers.

I would like to focus on the recommendations for prophylactic treatment of tick bites. The article² Dr. Marshall refers to makes a decent case for treating certain tick bites prophylactically, but it has several limitations and should not be overapplied in the decision-making process. There are several reasons why the Centers for Disease Control and Prevention has not adopted the same conclusion as this study.²

The study² points to an 87 percent effectiveness rate in preventing primary Lyme disease infection in patients with a bite from a deer tick (Ixodes scapularis). This may be true, but the question is whether this treatment is worth the risk. The overall infection rate after the definite bite of a deer tick (as identified by an entomologist) was only 3.1 percent. In the family physician’s office, without the benefit of an entomologist, many bites from dog ticks would end up being treated. So, of the total number of bites for which physician attention is sought, what percentage would actually result in erythema migrans, let alone more clinically significant Lyme disease? The actual percentage is probably much lower than 3.1 percent, even in highly endemic areas. Therefore, most courses of antibiotics given prophylactically for tick bites would be given in vain.

Also, the characteristic rash of Lyme disease, erythema migrans, is easily identified in most endemic areas and generally results in prompt treatment without further complications. Would the overall benefit of treating all exposures really be significant? The overuse of antibiotics in situations with low risk of infections is a serious problem in all developed countries and is one of the main reasons for antibiotic resistance. Antibiotics are not without risk; this should be considered if most of the patients treated did not need the antibiotic.

Finally, there is the theoretical risk that only erythema migrans is being prevented and that the disease may present insidiously at a later stage, having bypassed the easily treatable first stage, as can happen with inadequate treatment of early syphilis. This issue is addressed in the study² and was deemed unlikely by the authors for several plausible reasons; however, I do not feel that this disturbing possibility has been definitively ruled out.

As with all treatment decisions, the risk-to-benefit ratio of treating tick bites prophylactically has to be weighed carefully. A reasonable case can be made either way, but the evidence to solidly recommend this course of action for all tick bites does not appear to be present yet.