REGITANO, Luciana Correia de Almeida.; OLIVEIRA, Márcia Cristina de Sena.; ALENCAR, Mauricio Mello de.; NÉO, Thalita Athiê.; MOREIRA, Ingritt Carolina.; BARONI JÚNIOR, Waldomiro.; CARVALHO, Minos Esperândio.; SILVA, Ana Mary da.
Resumo:
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.