The blacklegged tick, Ixodes scapularis, is the primary vector of Ehrlichia phagocytophila (the etiologic agent of human granulocytic ehrlichiosis) in the eastern United States. Several arthropod-borne agents are known to be harmful to their vectors, but the effects of E. phagocytophila on its tick-vectors have not been studied. We assessed effects of sympatric and allopatric strains of E. phagocytophila on the survival of I. scapularis larvae that were derived from ticks collected in the northeastern US. Six strains of E. phagocytophila, originating from different geographical regions of the United States, were tested: three strains from the northeast (NY-8, Bridgeport, and Dawson), two from the Midwest (Webster, Spooner), and one from California (MRK). BALB/C mice were infected with each of the six Ehrlichia strains via exposure to infected ticks. Both infected and control-uninfected mice were infested weekly with uninfected larvae. The molting success of infected and uninfected ticks was calculated as the percentage of larvae successfully molting into nymphal stage, and prevalence of infection in molted nymphs was determined by PCR. In ticks that became infected with the Bridgeport or Spooner strains, the molting success decreased slightly with an increase in the prevalence of infection. Ticks that fed upon mice infected with the MRK or Spooner strains showed significantly lower levels of survival than those fed upon control mice or mice infected with the other four strains regardless of the prevalence of infection. Three strains (NY-8, Dawson, Webster) had no apparent effect on the tick survival through the molt. Thus, some but not all of the E. phagocytophila strains have adverse effects on ticks. Ticks exposed to "harmful" strains may experience higher levels of bacterial metabolism or reduced quality of their blood-meal thereby reducing their survival. Noted differences between strains may be due to the origin of a particular strain and/or the degree of co-adaptation between the pathogen and its vector on the population level.
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA