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Transmission of Babesia bovis by the tick Boophilus microplus was studied in 4 breeding herds of European and Zebu x European cattle under different levels of tick infestation. The observations consisted of weekly counts of female ticks on the cattle, monthly serological tests for antibodies to B. bovis, examination of tick larvae from the pasture to determine B. bovis infection rates and comparison of the suitability of paddocks for tick reproduction. The rate of transmission (inoculation rate) was estimated in terms of the daily probability of infection and consisted of the product of the mean daily tick infestation head-1 and the babesial infection rate in tick larvae. The theoretical value of the minimum inoculation rate required to produce stability of babesiosis is 0.01. This value was exceeded only in a herd of European cattle with minimal tick control and grazing on a pasture favourable for tick reproduction. Instability of babesiosis occurred in the other herds of European cattle after tick numbers had been reduced by pasture spelling and strategic dipping and after reduction in the babesial infection rate in ticks apparently caused by unfavourable environmental conditions. Over a period of years, the tick infection rates also declined as a consequence of the reduction in numbers of ticks. The Zebu x European cattle failed to generate inoculation rates greater than the minimum level, even though no tick control measures were applied. This was attributed to lower babesial infection rates in the ticks than those observed in a comparable herd of European cattle and to the high tick resistance of the Zebu crosses which maintained the tick populations at low levels. Both factors combined to produce a low inoculation rate.

A Babesia bovis low-molecular-weight antigen was purified from crude material by using affinity adsorption techniques first with a mouse monoclonal antibody and then by adsorption with normal bovine sera. The antigen was then further purified by gradient gel electrophoresis. Analysis by Western transfer revealed only one antigen band, with an apparent molecular mass of 29 kilodaltons. A band (12 by 0.3 by 0.6 cm) corresponding to this antigen was excised from the acrylamide gel and injected twice 4 weeks apart, together with 2.5 ml of Freund complete adjuvant, into nine nonsplenectomized adult cattle. The vaccinated cattle and five susceptible control animals were challenged with a virulent homologous strain 4 weeks after the second vaccination. None of the vaccinated animals was clinically affected, whereas three of five controls were severely affected. Control animals had significantly greater declines in packed cell volume and greater rises in temperature and parasitaemias than vaccinated animals.