Submitted to: Journal of Medical Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/20/2004
Publication Date: 3/3/2005
Citation: Scoles, G.A., Ueti, M., Palmer, G. 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 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 infected when they are collected from the field. This is important because if some field collected ticks are infected our results about the number of ticks in each population that become infected could be incorrect. Also, we used DNA sequencing to prove that all the ticks we compared in our study were Rocky Mountain Wood Ticks, the differences are not because the ticks in different areas are a different kind. This study shows that ticks from some areas are not as good at transmitting this important cattle disease as ticks from some other areas are. This may help to explain why more cattle become infected with Anaplasma in some places 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 with A. marginale become life-long persistently infected carriers. In the intermountain west Dermacentor andersoni Stiles, the Rocky Mountain wood tick, is the most common vector of A. marginale. D. andersoni acquires the pathogen when feeding on persistently infected cattle. Following invasion and development in the tick midgut epithelium, ticks biologically transmit A. marginale in the saliva when infected male ticks transfer from one host to another. Questing adult D. andersoni were collected from four widely separated natural populations and tested for susceptibility to colonization of the midgut with A. marginale. Male ticks were fed on calves persistently infected with the St. Maries strain of A. marginale, a strain 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. 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. To rule out the possibility that these field collected ticks were naturally infected prior to their use in this study, A. marginale from infected ticks was genotyped to confirm that they were infected with the St. Maries strain. In addition, a sample of 679 adult D. andersoni ticks collected from three of these sites, and 14 additional sites, were surveyed for A. marginale by nested PCR; none were found to be infected. Infection of the gut is a key component of tick vector competence for A. marginale. This study demonstrates that there is significant population level variation in a key component of tick vector competence for this important tick-borne pathogen of cattle.