A strain of Babesia bovis, which has been routinely used in the locally produced babesiosis vaccine for two decades and maintained by needle passage, was of low virulence and therefore safe, but it induced poor protection to challenge with two field isolates. Animals infected with this attenuated strain and subsequently challenged heterologously with a field strain were solidly immune when challenged later with a second field strain. The two field strains, though more virulent, conferred a high degree of immunity to heterologous challenge.
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