A Canadian bison isolate of Anaplasma marginale (Rickettsiales: Anaplasmataceae) is not transmissible by Dermacentor andersoni (Acari: Ixodidae), whereas ticks from two Canadian D. andersoni populations are competent vectors of a U.S. strain.

Anaplasma marginale Theiler is a tick-borne rickettsial pathogen of cattle with a global distribution in both temperate and tropical regions. The pathogen is endemic in regions within the United States, whereas the Canadian cattle population is considered to be free ofA. marginale. Farmed bison, Bison bison L., in central Saskatchewan have been found to be infected with A. marginale; however, there is no evidence of transmission from bison to cattle. We tested a Saskatchewan bison isolate of A. marginale (SB1) to determine whether it is transmissible by the Rocky Mountain wood tick, Dermacentor andersoni Stiles. Colonized D. andersoni from the United States and Canada failed to transmit SB1. A separate transmission trial using D. andersoni adults reared from ticks collected in Alberta and British Columbia showed that ticks from these populations could successfully transmit the St. Maries, Idaho, strain of A. marginale. Although the Saskatchewan bison isolate of A. marginale seems not to be transmissible by D. andersoni, in the event of the introduction of a tick-transmissible strain, Canadian D. andersoni are likely to be competent vectors.