MOLECULAR AND BIOLOGICAL CHARACTERIZATION OF A NEWLY ISOLATED ANAPLASMA-MARGINALE STRAIN

Anaplasma marginale, a rickettsial hemoparasite of cattle and other ru minants, results in significant economic losses worldwide. Distinct st rains of A. marginale have been identified based on differences in tic k transmissibility, molecular size of surface proteins and DNA restric tion fragments, and reactivity to a panel of monoclonal antibodies. Th ese different strains vary considerably in their virulence, antigenic composition, and ability to protect against heterologous challenge. In this paper, we report on the molecular characterization of a newly is olated strain of A. marginale, designated St. Maries, recovered from a n acutely infected cow in northern Idaho. Dermacentor andersoni ticks taken from the infected animal were tested for infection by RNA probe analysis. The infection rate of male ticks (as determined by midgut in fection) was 100%, and the infection rate of female ticks was 83%. Inf ected male ticks were able to transmit the St. Maries strain to a susc eptible calf. The high infection rate in male ticks may be particularl y relevant, given that male ticks are believed to be epidemiologically important in transmission of A. marginale because of their intermitte nt feeding behavior, which promotes interhost transfer. The newly isol ated strain differs from other US strains, including strains previousl y isolated in Idaho and Washington, based on reactivity to a panel of monoclonal antibodies and restriction fragment length polymorphisms. T hese results imply that antigenically distinct strains of A. marginale may arise within the same region. However, because epitopes previousl y demonstrated to be conserved in other A. marginale strains were also present in the St. Maries strain, it is possible to develop diagnosti c tests against conserved epitopes that could be used for early identi fication of infected cows or ticks.