The pathogenicity and immunologic relationship of a virulent and a tissue-culture-adapted Babesia bovis.

Babesia bovis grown in tissue culture was used to inoculate 12, 2-year-old Holstein steers. All 12 developed serological evidence of infection but only six had a febrile response of greater than or equal to 40 degrees C, and only one had a demonstrable B. bovis parasitemia. An average modest drop of 19% was observed in packed cell volume (PCV) during the period of reaction. All 12 steers were subsequently challenged with virulent B. bovis: seven on day 78 post inoculation (p.i.), two on day 106 p.i., and three on day 251 p.i. No demonstrable clinical response was observed in any of the 12 steers previously exposed to the tissue-culture organism, whereas severe signs of babesiosis occurred in seven 2-year-old, non-vaccinated control steers given a comparably virulent B. bovis challenge. All seven controls showed a febrile response, B. bovis parasitemias, with an average drop of 55% in PCV and a 28% mortality.