Reduction of Lyme disease exposure by recognition and avoidance of high-risk areas

Abstract

Lyme disease is caused by the spirochaete Borrelia burgdorferi transmitted via a tick bite from Ixodes scapularis. White-footed mice, which are maintenance and reservoir hosts for the subadult tick, are an important mammal involved in the transmission of Lyme disease. To reduce tick exposure and the risk of contracting Lyme disease, practical preventive measures are needed. Current publichealth recommendations to wear long sleeves and tuck one’s trousers into socks are rarely practised. We postulated that ticks were most likely to be found in areas supportive of mice. Consequently, if people recognised and avoided these areas they could reduce their risk of exposure. Mice have three principal requirements to inhabit an area: variety of food, nearby water, and ground cover, which is extremely important for protection, whereas open space is dangerous. We tested our hypothesis in endemic wooded areas in northeastern USA. Ixodes nymph ticks were collected in 10 10 foot grids (1 foot=0·3048 m), and divided into 1 square foot sectors with magnifying glasses and tweezers. Since food and water were available, the areas were independently classified as putative high, intermediate, or low suitability/risk, based upon ground coverage. Coverage was assessed visually from standing height and then measured from ground level. High suitability, where bare ground was not readily visible, corresponded to cover greater than 3 inches high (1 inch=2·54 cm). Low suitability was defined by ground coverage of less than 1 inch with bare ground visible. Included were contiguous sites of high and low cover. Accidental introduction of ticks was avoided by tick checks on individuals who were collecting and by collection from the perimeter inward. Deer trails were avoided as a confounding variable. Habitation by mice was confirmed by signs of bedding areas, hairs, track imprints, chew marks of incisure areas, feeding areas, scat, and urine discolouration of areas. As shown in the table, the number of ticks averaged 108·8 in the high suitability niches compared with 4·6 in the low suitability niches (p<0·01). The average 23-fold increase also included a 51-fold increase of ticks in one sampling site. With greater than 100 ticks per grid as a conservative number for high-risk areas and less than 10 per grid for low-risk areas the positive-predictive value of visually classifying the terrain attained 100%. This study and our other observations show that density was the key to favourable ground cover, which is often provided by a mixture of low-level, thick, and broadreaching plants that can grow up to 1 foot in height. When one could see the bare ground from standing height the area was not likely to be favourable for mice and we did not find signs of mice inhabitation or tick abundance. The opposite was found when the surface appeared covered, at least above the ankles, and food was available. With a minimum of training, areas can be distinguished. Recognition and avoidance of the high-risk areas could reduce the risk of contracting Lyme and, possibly, other tick-borne diseases.