Acquisition of different isolates of Anaplasma phagocytophilum by Ixodes scapularis from a model animal.

The prevalence of etiologic agents in ticks reflects the intensity of their transmission in natural cycles and is an important measure of their potential to cause human disease. The distribution of Anaplasma phagocytophilum within the range of its primary vectors is patchy. Even nearby sites differ dramatically in the prevalence of Anaplasma in questing ticks. We hypothesized that this irregular distribution may be due in part to variations in acquisition rates of different isolates of A. phagocytophilum by I. scapularis ticks from infected animals. BALB/c mice were infected with seven isolates of A. phagocytophilum from different geographical regions: four isolates from the Northeastern United States (Bridgeport, Dawson, Gaillard, NY-8), two from the Midwest (Webster, Sp-Is), and one from California (MRK). Larval ticks were placed on infected mice for 16 consecutive weeks and allowed to feed to repletion. The prevalence of infection in the freshly molted nymphs was then determined by PCR. The proportion of ticks that became infected with either isolate fluctuated over the duration of infection. Mice harboring the isolate Sp-Is were most infectious for ticks at 3 weeks postinfection. Mice infected with the other six isolates exhibited several peaks of infectivity. Timing and relative heights of these peaks differed between isolates. Geographical proximity of the studied isolates did not predetermine their similarity, and isolates originating from the same region differed more in their ability to infect ticks than isolates from different regions. However, it appears unlikely that described differences in agent acquisition by ticks alone are sufficient to account for the irregular distribution of A. phagocytophilum in vector populations.