Reducing Transmission of Lyme Disease Spirochetes in a Suburban Setting

Abstract

Recently, a new method has been developed that distributes acaricide to white-footed mice (Peromyscus leucopus), the principal reservoir of the Lyme disease spirochete (LDS) Borrelia burgdorferi, in order to kill immature Ixodes dammini infesting these hosts.'This method relies on the nesting and foraging behavior of these mice.Standard cotton balls, impregnated with permethrin (7.4% w/w), are dispensed from weatherresistant tubes placed at 10-meter intervals in suitable mouse habitats.Mice collect the cotton as they forage for fibrous material to build their nests, thus incorporating the acaricide into their burrows.This method, using such permethrin-treated cotton (PTC), was empioyed in a suburban community where ticks ( I .dammini) are abundant and Lyme disease is endemic, in an attempt to reduce the risk of human exposure to LDS-infected ticks.The study area, located in Ipswich, Massachusetts, is bordered on its perimeter by salt marsh and is transected by a road.An 18-acre tract containing seven residences located on the south side of the road was selected for treatment with PTC.A total of 850 tubes, each packed with 4.5 grams of PTC, were distributed in grid fashion in all potential mouse habitats.The treated cotton was distributed in early May 1986, 15 August 1986,lO May 1987, and 4 August 1987.Thus, PTC was distributed twice each summer during seasons corresponding to the feeding activity pattern of the immature stages of the tick.*To evaluate the effectiveness of PTC in reducing tick infestations on P. leucopus, mice were live-trapped on the perimeter of yards located in the centers of the treated and nontreated sites.A total of 33 of 34 mice captured around five nontreated yards on 10 August 1986 were infested with an average of 20 larval and nymphal I .dammini each.In contrast, 39 of 40 mice captured on the same date around five yards in the treated site were found to be tick-free.The only tick-infested mouse captured in a treated yard had three larval ticks.Evaluations conducted prior to and following treatment during 1987 produced similar results.While ticks were found infesting 43 of 45 mice captured in nontreated yards, none of the 52 mice captured in treated yards were tick infested.Distribution of PTC appeared to be nearly completely effective in eliminating infestations of ticks on P. leucopus.To determine whether such reduced tick infestations on the mouse reservoir indeed resulted in lowered transmission risk, we measured the relative abundance of hostseeking nymphal I. dammini in both treated and nontreated yards, as well as the LDS infection rate in samples of these ticks.Host-seeking nymphal I .dammini were collected on the perimeter of yards by dragging a 1-meter square flannel flag for a measured amount of time.Samples of nymphs collected in this manner were examined for LDS using a direct fluorescent antibody procedure.' Prior to treatment (in 1985) and following initial treatment (in 1986), the abundance of host-seeking nymphs varied from yard to yard but was statistically equivalent in yards throughout the study