The effect of natural exposure to Anaplasma and Babesia infections on native calves in an endemic area of Colombia.

The age at which native calves first became infected with Anaplasma marginale and Babesia bigemina and the effect of the infections as characterised by anaemia was determined by the biweekly examination of newly born calves on four ranches located in an endemic area of anaplasmosis and babesiosis on the north coast of Colombia. All of the calves examined became infected with A. marginale at 4-24 weeks of age and with B. bigemina at 2-34 weeks of age. The mean age at first infection with both haemoparasites was 11 weeks. A significant decrease occurred in the mean packed cell volumes of the calves following infection. However, the packed cell volumes returned to preinfection levels with 4 weeks following infection and most calves recovered from their first natural infections without apparent serious consequences. The mild clinical response and rapid recovery indicated that the protection of native calves through immunisation or other procedures prior to natural exposure would be of questionable value.