Bovine babesiosis caused by Babesia bovis remains a significant constraint to beef
and milk cattle production throughout the world. Exoantigens released by the parasites
in culture supernatants are a potential source of antigen to induce protective immunity.
An attenuated strain of B. bovis from Brazil, catalogued as BbUFV1, was maintained
in vitro by the MASP method, and exoantigen-containing supernatant fluids were collected
daily to form a pool representing a 72-h culture cycle for preparation of the vaccine.
Exoantigen concentration was estimated using a two-site EIA. Three groups of susceptible
non-splenectomised male Bos taurus cattle, 14 months old, were used. Group A (vaccinated)
received two subcutaneous immunizations with a 21-day interval of B. bovis supernatant,
content 6500 EIA units of exoantigens plus 1.5 mg saponin, and Group B (adjuvant control)
received two injections of adjuvant alone. Four weeks after the second immunization,
Groups A, B and C (control) were challenged intravenously with 10(8) virulent parasites
of a heterologous B. bovis strain. The results showed that exoantigens present in
in vitro cultures can induce a high degree of protection against virulent heterologous
challenge exposure. In Group A only one animal showed discrete parasitaemia; all developed
a fever and slight decreases in PCV, with a rapid return to normal values. One animal
of Group B died; the survivors showed fever, anaemia and parasitaemia. All animals
of Group C died between 7 and 13 days after challenge. Vaccination elicited both humoral
and cell-mediated immune responses. In Group A, after the challenge, the maximum antibody
titer was 12,800. When vaccinated, cattle were tested at the moment of challenge for
B. bovis-specific cell-mediated immunity by the monocytemigration inhibition test.
A mean inhibition index of 60 +/- 0.33 was observed. Preliminary Western blot analysis
of the immunogen revealed at least four proteins of molecular weight ranging between
30 and 160 kDa.