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Between 1959 and 1996, research was performed to change a vaccine against babesiosis in Australia and to improve it as actual or threatened untoward field responses became apparent. The most significant change occurred in 1964 with the traditionally used carriers of Babesia being replaced as vaccine donors by acutely infected splenectomised calves. This ensured the infectivity of the vaccine and was fortuitously associated with a reduction in the virulence of Babesia bovis in vaccine. Since then, more than 27 million doses of highly infective vaccine have been supplied from the laboratory at Wacol near Brisbane. This vaccine reduced serious losses from babesiosis in vaccinated cattle in Australia to very low levels and has now gained acceptance worldwide. Research to ensure the continuing effectiveness of the vaccine has proved to be essential.

Passage of the avirulent vaccine (K) strain of Babesia bovis (KA) through either Boophilus microplus ticks, intact calves, or intact calves and then ticks, resulted in two distinct protein and protein antigen profiles as analyzed by two-dimensional gel electrophoresis of biosynthetically labeled proteins and immunoprecipitates. Different degrees of expression of two major acidic antigens of KA designated Ka1 (Mr 47,500) and Ka2 (Mr 43,000) were observed. Ka1 was apparently lost following passage of KA B. bovis through intact calves but was strongly represented in the parasite population following a single tick passage. In contrast, passage through ticks of the virulent KV B. bovis (from which KA was derived by passage in splenectomized calves) did not lead to strong representation of the Ka1 protein although there was increased representation of another major acidic protein antigen, designated KV (Mr 35,000). These data suggest that the previously recognized reversion to a strain-dependent basal antigenic type in the tick vector depends also on intrastrain characteristics such as virulence and strain heterogeneity. The data suggest that KA is a more heterogeneous population than KV although cloned isolates are required to establish this point. Comparable syringe passage of another strain of B. bovis, designated C strain, through splenectomized calves resulted in less marked differences between the putative CA and CV B. bovis. This may explain the less stable avirulence of CA compared to KA B. bovis. Various selection pressures must act, in either the tick or the vertebrate host, on subpopulations in heterogeneous isolates to produce the changes described in protein antigen profiles of B. bovis. The possible relevance of changes in representation of proteins to biological characteristics of B. bovis (such as virulence and tick transmissibility) is discussed.

Procedures used to prepare and test frozen vaccine against bovine tick fever are described. Blood from splenectomised calves infected separately with Babesia bovis, Babesia bigemina and Anaplasma centrale was diluted in the ratio of 3:1 with 8 M dimethyl sulphoxide in phosphate buffered saline and cooled at rates between 5 degrees C and 110 degrees C per minute to -196 degrees C. After varying periods of storage up to 369 days, blood was thawed by immersing containers in a 40 degrees C water bath and its infectivity tested in a total of 150 nonsplenectomised cattle. Subcutaneous inoculation of the cryopreserved blood, and 10-fold and 50-fold dilutions prepared from it, showed that high infectivity of all three parasites was retained during storage. The use of frozen vaccine exported from Australia to Trinidad and Tobago is briefly discussed.

A short account of the epidemiology and control of babesiosis in Australia is presented. Epidemiological topics discussed include differences in the transmission of Babesia bovis and B. bigemina by the cattle tick, Boophilus microplus and the relative prevalence, disease incidence and pathogenicity of B. bovis and B. bigemina. Circumstances under which babesiosis occurs in Australia are described. In the Section on control, only vaccination is discussed. Changes in the preparation of babesial vaccines, particularly those resulting in a highly infective vaccine containing relatively avirulent B. bovis are described. Fluctuations in demand, such as the increase from about 100,000 to over 1,000,000 doses in 4 years in the mid-1960s are shown. An unexpected increase in the use of A. centrale in 1973 is discussed, and the supply of B. bigemina for cattle exported from Australia reported.