EFFICIENCY OF TRANSOVARIAL TRANSMISSION OF THE LYME-DISEASE SPIROCHETE, BORRELIA-BURGDORFERI, IN THE WESTERN BLACKLEGGED TICK, IXODES-PACIFICUS (ACARI, IXODIDAE)

The efficiency of transovarial transmission of Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner was evaluated in Ixodes pacificus Cooley & Kohls collected from two areas of northern California where Lyme disease is endemic. In total, 132 (8.8%) of 1,499 replete females examined by direct immunofluorescence were demonstrated to be infected with B. burgdorferi. Larvae or eggs from 119 of these females were examined for the presence of spirochetes by direct immunofluorescence, placing them in culture, or both; none was found to contain B. burgdorferi. The fecundity of 20 midgut-infected (mean = 874.2) and 20 uninfected (mean = 1,048.3) I. pacificus females did not differ statistically. Likewise, the fertility of infected (mean = 87.0%) and uninfected (mean = 89.9%) females and the mean engorged weights of both groups (infected, 120.8 mg versus uninfected, 132.7 mg), were comparable. The fecundity, fertility, and mean weights of six replete females having ovarian infections, six females having midgut-restricted infections, and six uninfected females were also similar. We conclude that transovarial transmission is not efficient for maintaining B. burgdorferi in populations of I. pacificus, a known vector of that pathogen. Infection with the spirochete does not appear to affect either feeding or reproductive success adversely in females of this tick.