A serological survey for Babesia in cattle in Scotland. III. The rates of acquisition and loss of antibody to Babesia and their effects on observed levels of antibody incidence.

Data for age and the incidence of antibody to Babesia divergens in cattle from areas of high and low endemicity have been analysed using three simple epidemiological models to estimate the rates of gain and loss of antibody. The models adequately described the observations but did not allow for an increased recovery rate attributable to acquired immunity in older animals. The rate of loss of babesial antibody in the absence of challenge was estimated directly from data for age and the incidence of antibody in Irish bred cattle located in nonendemic areas of Scotland. These data suggested an antibody loss rate of 0.25 per annum but did not provide firm evidence for the form of the antibody loss curve. Measurements of the titre of babesial antibody supported the conclusions drawn from studies of antibody incidence. In Irish bred cattle from non-endemic areas titres declined gradually over eight to ten years, whereas in cattle from an area of high endemicity, titres did not change appreciably with age. Cattle tested within one to three months of infection exhibited high titres.