Studies on failure of T strain live Babesia bovis vaccine.

Field investigations of the protection afforded by the Australian live Babesia bovis vaccine used in the early 1990s (T strain) revealed inadequate vaccine-induced protection in certain herds. Vaccination/challenge trials using 207 experimental cattle were conducted to evaluate the protection afforded by T strain B bovis against field isolates from these herds. The trials investigated whether isolates that could 'break-through' T strain immunity were present in the field, the ability or inability of specific cattle to develop protective immunity after vaccination with T strain and the effect of attenuation and maintenance procedures on the immunogenicity of T strain. The results showed that B bovis parasites present early in the process of attenuation of T strain were more protective than those remaining late in the process. They also showed that cattle from properties experiencing vaccine failures were less protected by T strain vaccination than Bos taurus cattle randomly selected from the general population if vaccinated with highly attenuated T strain. A hypothesis is offered to explain these findings.