Differential Bos taurus cattle response to Babesia bovis infection.

Bovine babesiosis is a tick-borne disease caused by Babesia spp. haemoprotozoans. The disease is of great importance at tick enzootic unstable areas and hampers cattle production in several developing countries. The available immunisation alternatives are pre-immunition and attenuated vaccines. Despite being efficient and protective, they are unsafe as they use cattle blood cells as inoculum and may potentially spread other diseases. Another alternative to help in babesiosis control would be the identification of genetically resistant cattle to Babesia bovis infection. The objective of this work was to phenotype cattle based on primary response against B. bovis infection. Two-hundred and forty half-sib Hereford and Aberdeen Angus heifers (120 animals from each breed), 12-18-month-old naïve cattle, originated from a tick-free area in Southern Brazil, were used in the experiment. Animals were monitored following an inoculation with 1x10(7)B. bovis parasitised erythrocytes. Results showed three different phenotypes: 1-'susceptible', animals with babesiosis clinical signs that received treatment to avoid death; 2-'intermediate', animals with clinical signs: parasitaemia, >or=21.5% reduction in packed cell volume (PCV) and increase in body temperature when compared to their pre-challenge physiological parameters, no specific treatment was needed as animals self recovered from the disease, and 3-'resistant', animals without clinical signs that showed B. bovis presence in blood smears, <21.5% PCV reductions, with little or no increase in body temperature and no need for babesiosis treatment. The frequencies of each phenotype were: 45.4, 26.7, and 27.9%, respectively, demonstrating the existence of phenotypic variation for B. bovis in Bos taurus cattle.