Lyme Disease in California: Interrelationship of Ixodid Ticks (Acari), Rodents, and Borrelia burgdorferi

Abstract

The association of immature ixodid ticks, several species of rodents, and the Lyme disease spirochete, Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner, was studied in two habitats in northern California in spring and summer 1985 and year-round in 1986. A total of 428 rodents were collected from ecotonal chaparral and a woodland-grass-rock outcrop; the former habitat yielded six species, the latter three species. The deer mouse, Peromyscus maniculatus (Wagner), and the piñon mouse, P. truei (Shufeldt), were the dominant species year-round and collectively comprised 78% of rodents captured within chaparral and 87% from the rock outcrop in 1986. In both habitats, rodents were trapped most frequently in winter and spring, and least often in summer and fall. A total of 306 rodent blood films from all six species were assayed for spirochetes by direct immunofluorescence; of these, only one film prepared from P. truei (n = 123 films from 53 individual mice) was found to contain spirochetes. Immature western black-legged ticks, Ixodes pacificus Cooley & Kohls, and Pacific Coast ticks, Dermacentor occidentalis Marx, were collected from each species of rodent. Larvae of I. pacificus infested P. maniculatus and P. truei in low numbers year-round, but nymphs of this tick rarely parasitized these rodents. D. occidentalis larvae infested P. maniculatus and P. truei in spring and particularly in summer; nymphal ticks infested these mice primarily in summer. The efficiency of visual inspection for collecting immatures of these ticks from P. maniculatus ranged from 45 to 69% in spring and summer, whereas the efficiency of a drop-off technique appeared to be 100%. Spirochetes were detected in <1% of D. occidentalis larvae (n = 310) and nymphs (n = 120), and in approximately 4% of I. pacificus larvae (n = 75) derived from these hosts. The potential significance od these findings in the enzootiology of B. burgdorferi is discussed.