Tick Bites: To Treat or Not to Treat?
By Jonathan Edlow, MD
Tick-bite studies suggest that 24-48 hours of tick attachment is usually required for infection with Borrelia burgdorferi, the etiologic agent in Lyme disease (LD), to occur.1-3 The time delay is explained by the fact that in most unfed ticks, the spirochetes are localized to the midgut. After the tick attaches to the host for a blood meal, the organisms migrate to the salivary glands, a journey that requires hours to days.4-6
Although many authors state categorically that more than 24 hours of attachment is required for transmission, clinical and experimental data suggest that transmission is clearly possible within a shorter duration, although the frequency of this occurrence in North America is unknown. In one European study of 231 culture-confirmed cases of erythema migrans (EM), data were reported on those 34 patients who specifically recalled not only the tick bite but also the duration of attachment. In nine of the 34, the duration of attachment was less than six hours, and in an additional 16 of the 34, it was less than 24 hours.7 Moreover, clinical LD has also been documented after as little as six hours of attachment in North America.8 This probably occurs because some ticks are infected systemically, and these ticks could transmit the disease faster.
Endemic Areas. Several studies have examined the likelihood of developing clinical infection with B. burgdorferi in individuals who have had a tick bite in areas endemic for LD.9,10 Showing fairly consistent results, these studies demonstrate that 1-3% of patients develop the marker rash for EM after placebo treatment for a tick bite, even when the affected ticks had an infection rate of between 15-30%. Late symptoms or seroconversion were not seen in the placebo group. One possible methodological flaw in a larger study of over 300 patients was that serologic follow-up was only done at six weeks and three months.9,10 Only 75% of LD patients develop the marker rash of EM.11 While serologic conversion likely would have occurred within the three-month window, it is possible that some patients who never developed EM could have seroconverted after the three months and developed late disease after one year of clinical follow-up.
Treatment Considerations. Based on these investigations, the official recommendation is not to treat tick bites with prophylactic antibiotics. As a general rule, this represents a prudent, outcome-effective approach to clinical management. However, each case should be considered individually, and the clinical decision in the emergency-medicine setting should be based on a number of relevant risk factors, historical features, and physical findings.
Without question, duration of tick attachment is a critical parameter influencing the likelihood of acquiring clinical infection. Generally speaking, the longer the duration of attachment, the greater the likelihood of disease transmission. The species of biting tick is also important because, as a rule, only Ixodes ticks are capable of transmitting the disease. Characterizing the species of tick in the ED, however, may be difficult. The morphological stage of the tick should also be documented, since adult ticks are twice as likely to be infected as nymphs, and larvae are very rarely infected.12 Geography can play a decisive role in determining the need for emergency treatment. For example, in hyper-endemic areas, up to 50% of the Ixodes scapularis ticks can be infected, while in other areas, the rate is much lower.8,13
From a frontline, practical perspective, patient preference must enter into the management equation. Some patients, particularly those who live in endemic areas, are so anxious about acquiring LD that even after a thorough, reassuring explanation of the risks and options, a significant percentage will insist on antimicrobial prophylaxis and threaten go elsewhere if antibiotics are not prescribed. While there is no standard of "prophylactic" therapy for these situations, tick-bite studies that used a 10-day antibiotic course (amoxicillin in children, doxycycline in adults) have found no disease in the treatment groups.9,10
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Dr. Edlow is Associate Director, Department of Emergency Medicine, Mount Auburn Hospital, Cambridge, MA; Clinical Director, Division of Emergency Medicine, Beth Israel Deaconess Medical Center, Boston; Instructor in Medicine, Harvard Medical School, Cambridge, MA.