Observations on anaplasmosis following field exposure to heavy and light infestations with Boophilus microplus.

Two groups of Normandy calves were exposed to heavy (mean half body count = greater than 300) or light (mean half body count = 9) natural infestations with Boophilus microplus. All of the calves became infected with Anaplasma marginale. Despite the difference in tick challenge level, there was no significant difference in the incubation period, increase in body temperature, level or duration of Anaplasma parasitemia, decrease in packed cell volume, or complement-fixing antibody response, between the heavily and lightly infected calves. Neither the incubation period nor the clinical severity of anaplasmosis was significantly influenced by the number of infested tick vectors.