Isolate of Anaplasma marginale not transmitted by ticks

The tick-borne transmissibility of 2 isolates of Anaplasma marginale was compared. Dermacentor variabilis were exposed to A marginale as nymphs by feeding on 1 of 4 splenectomized calves during periods of ascending parasitemia (maximum 49% to 81% parasitized erythrocytes) induced by injection of a stabilate. Tick-borne transmission was attempted, using 26 to 224 adult ticks within 30 to 220 days after molting. Adult D variabilis did not transmit an Illinois isolate of A marginale in 7 tick-borne transmission experiments (P = 0.0047), including 2 experiments in which calves were inoculated IV with homogenates of adult ticks. In contrast, a Virginia isolate of A marginale was readily transmitted by the same tick colony. Thus, previously reported morphologic and immunologic differences among A marginale isolates may extend to tick-borne transmissibility. The Virginia and Illinois A marginale isolates had an inclusion appendage that was not a marker for tick transmissibility.