Epidemiologic aspects of bovine anaplasmosis in semiarid range conditions of south central Idaho.

The prevalance of Anaplasma marginale-infected cows, as determined by use of the modified rapid card agglutination (MRCA) test, was measured during a 4-year period (1980-1983). The prevalence of A marginale-infected cows, defined as positive reactors on the MRCA test, remained constant (31%-37%). The apparent incidence of A marginale transmission to susceptible cows was approximately 7% from 1980 to 1981, 8% from 1981 to 1982, and no transmission from 1982 to 1983. The occasional MRCA-positive cow became negative on the MRCA test, and 1 cow was determined to be free of A marginale infection by subinoculation of 100 ml of the cow's blood into a susceptible, splenectomized calf. Dermacentor andersoni, a known vector of A marginale, was often found on the cattle and in their environment. However, A marginale was not transmitted to susceptible, splenectomized calves, using collected ticks. Of 56 calves born to MRCA-positive cows, 82% were MRCA-positive within the first 3 months of life. These calves converted to MRCA-negative status and were determined to be free of A marginale infection by subinoculation of their blood into susceptible, splenectomized calves, indicating the passive transfer of colostral antibodies.