Assessment of Duration of Tick Feeding by the Scutal Index Reduces Need for Antibiotic Prophylaxis After Ixodes scapularis Tick Bites

Abstract

Doxycycline is recommended for persons with Ixodes scapularis tick bites in certain geographic areas, if the tick had fed for at least 36 hours. Based on the scutal index, over 40% of I. scapularis tick bites from patients seen at the Lyme Disease Diagnostic Center did not warrant antibiotic prophylaxis.

Lyme disease, tick bites, Ixodes scapularis, scutal index, chemoprophylaxis

(See the Editorial Commentary by Pritt on pages 617–8.)

Lyme disease is caused by Borrelia burgdorferi, a bacterium transmitted by certain Ixodes species ticks. In some geographic areas in the Northeast, Middle Atlantic, and North Central Regions of the United States, the average infection rate of Ixodes scapularis ticks may exceed 20% [1, 2]. In these areas, a single 200 mg dose of doxycycline is recommended by the Infectious Diseases Society of America to prevent Lyme disease in patients with I. scapularis tick bites who have no contraindications to taking this drug, can receive the drug within 72 hours of tick removal, and for whom the estimated duration of tick feeding is at least 36 hours [3]. The 36-hour duration of tick feeding criterion was based in part on modeling of data on the duration of tick feeding required for transmission of B. burgdorferi by infected field collected nymphal I. scapularis ticks that had fed on mice for defined periods of time [4]. For I. scapularis ticks that had fed for 24 hours, there was no transmission, for ticks that had fed for 48 hours the observed rate of transmission was 13% rising to 79% by 72 hours of feeding and further increasing to 94% by 96 hours of feeding. Although not directly measured, minimal to no transmission was predicted for feeding times of <36 hours [4]. In a separate study in which infected field collected female adult stage I. scapularis ticks were fed on rabbits, no transmission of B. burgdorferi occurred in animals on which ticks had fed for either 24 or 36 hours [5].

Examination of the tick microscopically and assessing the scutal index is a means of estimating the duration of tick feeding [6]. In this report, we provide data on the estimated duration of tick feeding based on the scutal index for I. scapularis ticks removed from humans and submitted to the Lyme Disease Diagnostic Center (LDDC), located in Westchester County, New York, where Lyme disease is endemic.

METHODS

The LDDC is a walk-in clinic open in the evenings during the summer months of June through August, when nymphal I. scapularis ticks are active, to provide services for patients with tick bites or with symptoms of a possible tick-borne illness. In some years, the LDDC has remained open through October as well, when adult I. scapularis ticks are active. The I. scapularis ticks submitted are assessed for duration of tick feeding based on the scutal index, as described elsewhere [6]. The scutal index is defined as the body length of the tick divided by the width of the scutum. Because during feeding body length increases while the width of the scutum remains constant, the scutal index provides a measurement of the degree of blood engorgement of the tick, which has been demonstrated to directly correlate with the duration of tick feeding [6].

Only nymphal and female adult I. scapularis ticks were assessed in this study. Lyme disease would be unexpected from exposure to larval, or male adult, I. scapularis ticks, because nearly all larval ticks are uninfected with B. burgdorferi, and male adult ticks rarely bite humans and only feed for a short duration of time [7, 8]. The Fisher exact test was used to assess statistical significance. This study was approved by the Institutional Review Board at New York Medical College.

RESULTS

Over the 17-year period from 2000 through 2016, 441 nymphal or female adult I. scapularis ticks, which had been found on patients, were assessed at the LDDC. Of the 441 ticks, 312 (70.7%) were nymphal stage ticks, and 129 (29.3%) were adult stage ticks. The scutal index could not be determined for 41 (31.8%) of the adult ticks due to physical damage to the tick compared with 34 (10.9%) of nymphal stage ticks (P < .0001).

Of the 278 nymphal stage ticks that could be assessed for duration of feeding, 131 (47.1%) had fed for <36 hours compared with 61 (69.3%) of the 88 evaluable adult stage ticks (P = .0004) (Table 1). Of the nymphal stage ticks, 54 (19.4%) had fed for ≥72 hours compared with 16 (18.2%) of the adult stage ticks (P = .88).

Table 1.

Feeding Time on Humans of Ixodes scapularis Ticks Determined by the Scutal Index

Feeding Time	All Evaluable Ticks (n = 366)	Adult Stage Ticks (n = 88)	Nymphal Stage Ticks (n = 278)	P-value for Comparison of Adult vs Nymphal Stage Ticks
<36 hours	192 (52.5%)	61 (69.3%)	131 (47.1%)	.0004
36–71 hours	104 (28.4%)	11 (12.5%)	93 (33.5%)	.0001
≥72 hours	70 (19.1%)	16 (18.2%)	54 (19.4%)	.88

DISCUSSION

Of the ticks submitted for which duration of feeding could be assessed, 47.1% of the nymphal stage I. scapularis ticks and 69.3% of the female adult stage I. scapularis ticks were regarded as posing a very low risk for transmission of B. burgdorferi, because they had fed for <36 hours [3–5]. Chemoprophylaxis with doxycycline would have been warranted only for 52.9% of the patients with evaluable nymphal stage ticks and for 30.7% of the patients with evaluable adult stage ticks. A study published in 1995 that measured the scutal index for over 500 I. scapularis ticks removed from individuals in Rhode Island or Pennsylvania found similar results with 59% of nymphal stage ticks and 36% of adult stage ticks estimated to have fed for >36 hours [9]. In prior studies, the mean duration of feeding time for nymphal stage I. scapularis ticks removed from humans also exceeded that of female adult stage ticks, and this observation was assumed to account for the infrequency with which patients bitten by adult stage ticks developed Lyme disease [7]. Thus, in our experience antibiotic prophylaxis is unnecessary for a substantial proportion of patients with I. scapularis tick bites, because 192 (52.5%) of the 366 evaluable ticks in our study had fed for <36 hours based on the scutal index. In highly endemic areas such as ours, however, by the time ticks have fed for ≥72 hours the risk of transmitting B. burgdorferi infection resulting in symptomatic Lyme disease is at least 20%, if antibiotic prophylaxis is not prescribed [7, 10]. I. scapularis ticks that have fed for <36 hours may still pose a risk for transmission of certain pathogens found in this tick species, such as Anaplasma phagocytophilum [4, 11], the deer tick virus subtype of Powassan virus [12], Borrelia miyamotoi [13], and, rarely, even B. burgdorferi [10]. Doxycycline is not recommended for chemoprophylaxis of infections caused by A. phagocytophilum or B. miyamotoi due to lack of data on efficacy. Furthermore, doxycycline chemoprophylaxis is not 100% effective for prevention of Lyme disease [7], and in addition, patients with a known tick bite may have had additional tick bites that were not recognized. Therefore, irrespective of whether antibiotic prophylaxis is prescribed, all patients with I. scapularis tick bites should be advised to seek medical attention if skin lesions develop at the bite site or at other skin locations, or if fever or other unexplained symptoms should develop within 30 days of tick removal [3].

Of note, the scutal index could not be calculated for 10.9% of nymphal stage I. scapularis ticks and for 31.8% of the female adult stage ticks. This difference was highly statistically significant (P < .0001), suggesting that removal of adult stage ticks is more likely to physically damage the ticks. For persons with damaged ticks, assessment of feeding duration depends on estimating the time between when the tick was likely to have been encountered and when it was removed. The accuracy of such estimates needs to be evaluated. Even if it were assumed that all of the damaged ticks had fed long enough to justify doxycycline prophylaxis, the overall proportion of tick bites that would still not have needed to be prophylaxed was 43.5%.

In conclusion, assessing the scutal index of an I. scapularis tick may help to prevent unnecessary use of antibiotic prophylaxis in a substantial proportion of patients with a tick bite and will identify those patients at highest risk for developing Lyme disease. Further evaluation of this testing is warranted to determine whether clinical microbiology laboratories should consider adding the scutal index to their diagnostic panels, along with tick identification.

Notes

Acknowledgments. The authors thank Julia Singer, Artemio Zavala, Shana Warner, John Kokas, and Lisa Giarratano for their assistance.

Potential conflicts of interest. G. P. W. reports receiving research grants from Immunetics, Inc., Institute for Systems Biology, Rarecyte, Inc., and Quidel Corporation. He owns equity in Abbott/AbbVie, has been an expert witness in malpractice cases involving Lyme disease, and is an unpaid board member of the American Lyme Disease Foundation. All other authors report no potential conflicts of interest. All authors have 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.