Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2004
Publication Date: 1/3/2005
Citation: Scoles, G.A., Ueti, M.W., Palmer, G.H. 2005. Variation among geographically separated populations of dermacentor andersoni (acari: ixodidae) in midgut susceptibility to anaplasma marginale (rickettsiales: anaplasmataceae). Journal of Medical Entomology. 42(2):153-162.
Interpretive Summary: Many cattle in the United States are infected with Anaplasma marginale, a pathogen which causes a disease called Anaplasmosis. Cattle that are not killed by the infection remain infected for the rest of their lives and can be a source of infection for other cattle (i.e. they become carriers). In the western U.S. the Rocky Mountain Wood Tick transmits the infection between cattle, but in order to transmit the infection the tick first has to become infected itself. Not all ticks can become infected with Anaplasma when they feed on an infected carrier. In this research we show that ticks collected from populations in different areas have different levels of susceptibility to infection with Anaplasma. Only 12.5% of ticks collected from a site near Hamilton, MT become infected when fed on a carrier, compared to 62.5% of ticks collected from a site near Riley, OR. The test was repeated in two different years and the results were the same. We also showed that ticks are not naturally infected when they are collected from the field, this supports the theory that intrastadial transmission by adult male D. andersoni is the most important mode of transmission. We used DNA sequencing to prove that all the ticks we compared in our study were Rocky Mountain Wood Ticks, confirming that the differences in susceptibility to infection represent intraspecific variation, not because the ticks in different areas are different species. In summary, this study shows that ticks from some populations are not as good at transmitting this important cattle pathogen as ticks from some other areas are. This may help to explain why there is a higher Anaplasma infection prevalence among cattle in some areas than in others.
Technical Abstract: Anaplasma marginale is a tick-borne rickettsial pathogen of cattle that is endemic throughout large areas of the United States. Cattle that survive acute infection become life-long persistently infected carriers. In the intermountain west the Rocky Mountain wood tick, Dermacentor andersoni Stiles, is the most common vector of A. marginale. Male D. andersoni acquire A. marginale when feeding on persistently infected cattle, and biologically transmit it when they transfer from infected to susceptible hosts. Host seeking adult D. andersoni were collected from four widely separated natural populations and tested for susceptibility to midgut colonization with A. marginale. Male ticks were fed on calves persistently infected with a strain of A. marginale naturally transmitted by D. andersoni. Gut infection rates ranged from 12.5% of ticks collected from a mountain site near Hamilton MT to 62.5% of ticks from a rangeland site near Riley, OR. Sites near Miles City, MT and Kamloops, BC had intermediate levels of susceptibility. The infection rates differed significantly among populations, and the same populations sampled in two consecutive years were not significantly different from one year to the next. Although there was variation among the populations in the size of ticks, size was unrelated to acquisition of gut infection. Quantitative PCR demonstrated that there was no significant difference between populations in the mean number of genome copies in the guts of infected ticks. A. marginale from infected ticks was genotyped to confirm that they were all infected with the laboratory strain, and a sample of 682 field collected D. andersoni was surveyed for A. marginale by nested PCR; none were found to be naturally infected. Infection of the gut is an essential constituent of vector competence for A. marginale; in this study we have demonstrated significant variation among populations in this key component of vector competence.