Nymphal densities and infection rates of Ixodes scapularis nymphs collected from residential properties in Lyme disease-endemic areas of Connecticut, Maryland, and New York

Background: Lyme disease, transmitted by the blacklegged tick (Ixodes scapularis Say), is the most common vector-borne disease in the U.S.  In the northeastern U.S., risk of exposure to the blacklegged tick is considered highest peridomestically. Previous reports of regional tick densities and infection rates primarily focus upon ticks collected from public lands and forested research sites.  We present data collected from residential properties as part of a two year multi-site tickborne disease intervention study involving the Centers for Disease Control and Prevention (CDC) and the Emerging Infections Programs in Connecticut (CT), Maryland (MD) and New York (NY).

Methods: After providing informed consent, heads of households in freestanding homes in select areas in Fairfield County, CT; Baltimore, Carroll, Harford and Howard Counties, MD; and Dutchess County, NY were enrolled.  Tick habitat on randomly selected enrolled properties was flag- or drag-sampled for host-seeking nymphal blacklegged ticks during peak nymphal activity in late spring through early summer in 2011 and 2012.  Up to 40 30-second drags per property were conducted and ticks removed after each drag. Nymphal density was estimated as the number of I. scapularis nymphs collected per hour. I. scapularis nymphs were tested for Borrelia burgdorferi, Anaplasma phagocytophilum and Babesia microti using real-time multiplex polymerase chain reaction. 

Results: In total, 952 ticks were collected from 267 properties, of which 865 (91%) were I. scapularis nymphs.  The overall average nymphal density was 27.4 nymphs/hour (range 13.6-46.1) in 2011 and 7.6 nymphs/hour (range 5.1-10.2) in 2012.  The overall nymphal infection rate for B. burgdorferi was 18.5% (range 16.4%-23.2%) for 2011 and 15.3% (range 9.3%-23.1%) for 2012. A. phagocytophilum was detected from all study sites, with infection rates of 1.5%-4.8%.  The B. microti infection rate ranged from 5.9% in CT in 2011 to 15.4% in NY in 2012. B. microti was not detected in MD nymphs.  Coinfections with B. burgdorferi and B. microti were identified in 22 nymphs collected in CT and NY and one CT nymph was coinfected with B. burgdorferi and A. phagocytophilum. Coinfected nymphs were not detected in MD.

Conclusions:  In contrast to data collected from public lands and research sites, we report nymphal densities and infection rates determined from residential properties, reflective of what homeowners encounter in their backyards.  While the overall nymphal density for 2011 was much greater than that in 2012, the B. burgdorferi infection rates were similar.  Infection rates in MD are consistent with previous reports but infection rates in CT and NY are lower than previously reported.  The results of this study suggest peridomestic risk for Lyme disease, anaplasmosis, and babesiosis in endemic regions of CT, MD, and NY. The rates of pathogen coinfection from nymphs collected from CT and NY might indicate the need for provider education.  Additional study is warranted to determine if these pathogen rates correlate with risk of disease in humans.