Scutal Index and Its Role in Guiding Prophylaxis for Lyme Disease Following Tick Bite

(See the Brief Report by Falco et al on pages 614–6.) In this edition of Clinical Infectious Diseases, Falco and colleagues describe their use of the tick “scutal index” to estimate duration of tick feeding and guide administration of doxycycline for Lyme disease prophylaxis after tick bite [1]. They calculated the scutal index for 278 suitably intact ticks that were brought to the Lyme Disease Diagnostic Center in Westchester County, New York, by patients over a 17-year period, and found that only 47.5% of the evaluable ticks had fed for a sufficient period of time (≥36 hours) to warrant offering doxycycline prophylaxis to the patient. Thus, the authors conclude that assessing the scutal index may be useful for guiding Lyme disease prophylaxis and avoiding unnecessary antibiotic usage [1]. The scutal index is an uncommonly used but easily obtained calculation determined by dividing the length of the tick body by the width of the scutum (Figure 1) [2]. As the tick feeds on the blood of its host, its body expands significantly whereas the rigid scutum remains constant in size; thus, the scutum can be used as a fixed point against which body size can be compared and degree of engorgement determined. This measurement of tick engorgement can then be used to estimate the duration of time that the tick was attached to its host by comparing the calculated index to previously obtained scutal indices from timed tick-feeding experiments [3]. The length of tick attachment is clinically important as it correlates positively with the risk for pathogen transmission; the longer the tick is attached, the greater the likelihood of transmission [3]. For Borrelia burgdorferi, the primary causative agent of Lyme disease in the United States, experimental data estimate that >36 hours of tick attachment is needed for transmission from tick to host [4, 5]. The Infectious Diseases Society of America (IDSA) uses this knowledge of transmission risk to guide its recommendations for administering doxycycline to patients for Lyme disease prophylaxis following tick bite [6]. Specifically, the IDSA recommends that a single dose of doxycycline be considered for adult patients and children aged ≥8 years when all of the following criteria are met: the tick is estimated to have been attached for ≥36 hours; the local rate of tick infection with B. burgdorferi is ≥20%; the tick can be accurately identified as a nymphal or adult Ixodes scapularis; prophylaxis can be given within 72 hours of tick removal; and doxycycline therapy is not contraindicated [6]. While the duration of tick attachment can sometimes be inferred from the patient’s history, this is dependent on the patient’s memory and may be challenging to estimate for patients with ongoing tick exposure (eg, those regularly exposed to the outdoors in a Lyme-endemic setting). Thus, scutal index assessment may provide a more accurate means for determining the duration of tick attachment. Anatomy of an Ixodes scapularis adult female delineating the measurement regions for tick body length (A) and scutum width (B ). The scutal index is calculated by dividing the tick body length by scutum width. How can healthcare providers incorporate scutal index determination into their own practice? Although the authors of the current manuscript do not provide details on how or if they routinely use the scutal index for guiding Lyme disease prophylaxis, one can consider the associated laboratory and clinical processes that would be needed for using this index in a given healthcare system, and then make a determination regarding the feasibility of this endeavor. A primary consideration is how tick identification and scutal index assessment would be performed. Tick identification should be performed to at least the genus (and preferably the species) level, and should be reported along with the life-cycle stage and gender. Keys are widely available for the human- biting ticks, including those most commonly found in North America [7]; also necessary for tick identification and scutal index calculation are a dissecting microscope, ocular micrometer, and experienced laboratory personnel. Only female and nymphal Ixodes require scutal index assessment, whereas males and non-Ixodes species do not. Another important consideration is that tick examination needs to be done in an expedient manner so that antibiotics, if indicated, can be provided to the patient within 72 hours of tick removal. The ideal approach would be to examine the tick at the point of care so that clinical decisions can be made immediately. In the future, there may be a mobile application that will allow the provider, or even the patient, to identify the tick and calculate the scutal index. However, at this time, it is likely that the specimen would need to be sent to a local laboratory where sufficient equipment and technical expertise are available, thus adding requirements for timely transportation and transmission of results. Of course, all providers who routinely see patients with tick bites would need to be made aware of the available tick testing options and how to interpret the results. There also needs to be a system for prescribing and delivering an antibiotic when indicated by the test result (ie, I. scapularis female or nymph with a scutal index indicating ≥36 hours of attachment). Consideration would need to be given for how the patient will be managed when the tick was damaged during removal and the scutal index is not able to be calculated (as was the case for 37% of the ticks in the current study) or when the tick is not available for examination. Finally, it is important to note that the scutal index has been most widely studied for I. scapularis ticks; the cutoffs for other Ixodes species that transmit Lyme disease–causing agents may differ. All of these aspects merit consideration when considering the approach to Lyme disease prophylaxis. By routinely incorporating tick identification and scutal index assessment into the evaluation of patients with tick bites, it may be possible to significantly reduce unnecessary antibiotic use. Potential conflicts of interest. The author: No reported conflicts of interest. The author has submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.