Avaliação da resistência de bovinos de diferentes grupos genéticos ao carrapato e à babesiose.

Abstract

Ticks (Boophilus microplus) and tick-born diseases are some of the major causes of economic losses in worldwide tropical and subtropical cattle production. Losses may be attributed to spoliation and to disease transmission, as well as to economic and environmental costs of chemical control. Genetic variability for tick resistance has been described in several bovine genetic groups, indicating the potentiality for genetic improvement of this characteristic in cattle. In this project, eight molecular markers, located close to genes of the immune system, were analyzed in 708 females from four genetic groups: Nelore, % Aberdeen Angus + % Nelore, % Canchim + % Nelore and % Simmental + % Nelore. Tick resistance was evaluated under natural infestation, from July 2003 to December 2004. A subset of the animais were also evaluated under artificial infestation in four occasions. Influence of genetic group in rates of Babesia bigemina infection was investigated in animais and in ticks. When ali four tick artificial infestations were analyzed together, animais from the % Aberdeen Angus + % Nelore and % Simmental + % Nelore genetic groups had the highest recuperation percentage, compared to Nelore. The % Canchim + % Nelore group had intermediate recuperation percentage. From these results one can postulate that Nelore cattle have the highest resistance, while RC have intermediate resistance and TA and TS are more susceptible to ticks. Although heritability estimates for number of ticks were low (0,15), these estimates indicate that there is at least some genetic variation that would allow for genetic progress under selection. Amplification of B. bigemina DNA by the combination of PCR and N-PCR was positive in 100% of the animais (calves and cows), in ali the genetic groups analyzed, suggesting that the greater resistance of Zebu animais to babesiosis can not be interpreted as an ability to keep themselves free from or with low infection levels. Presence of parasites in bloodstream was particularly affected by cattle age. Molecular markers close to the genes for interleukins 2 and 4 were associated to measures of tick resistance under natural infestations. Associations between molecular markers and B. bigemina infection could not be investigated, since no variation for infection rate was detected.