The absence of clinical disease in cattle in communal grazing areas where farmers are changing from an intensive dipping programme to one of endemic stability to tick-borne diseases.

A two-year field study was conducted in four communal grazing areas in South Africa. Sera were collected from young cattle (6-18 months old) in these areas during the winters of 1991 to 1993. The sera were tested for antibodies to Babesia bovis, Babesia bigemina, Anaplasma marginale and Cowdria ruminantium. In two of the four areas, treatment with acaricide was erratic and dependent on the discretion of individual owners. In these areas the drought of 1992 had a major impact on tick burdens and there were changes in the seroprevalence to tick-borne diseases. In the other two areas there was a reduction in the intensity of acaricide application and this was associated with an increase in seropositivity to the tick-borne diseases. Increases in the prevalence of seropositivity and the presence of endemic instability, as calculated from inoculation rates, were not accompanied by outbreaks of clinical disease. Possible reasons for this are discussed.