Abstract
Boophilus microplus infestation is one of the most serious limitations to cattle industry in tropical regions, even though bovines show natural resistance to ticks. This resistance was evaluated in Cross-bred Carora Bulls (CCB) a tropicalized dairy breed from Venezuela. Seven CCB were experimentally infested with B. microplus larvae, "Mozo" strain, they were considered tick-naive because they had never been infested with ticks. The mean inoculum size applied on each bull was 6,477 larvae. After life cycle was completed adult female body weight (BW), egg mass weight (EW), egg hatching rate (%EH), and reproductive index (RI) were recorded. Results revealed a high variability in the levels of resistance to B. microplus. Thus, one animal showed greater resistance (Dunnett, p < 0.05) for the analyzed parameters in contrast with three non-resistant bulls. The others had moderate resistance. The trait "resistance" should be included together with other traits often used in genetic selection of cattle.
Highlights
REVISTA DE BIOLOGlA TROPICAL control must be used if we want to fight suc cessfully against these ectoparasites, and simul taneously to keep an acceptable level of milk andJor meat production in the tropics
Tick parameters: Boophilus microplus larvae completed their lifeeycle in aH infested buIls after 21 ± 2 days PI and no clínica! signsof tíek bome diseases wete seen in these bovinesdyripg the evaluation>period
This study has shown that cross-bred Carora bulls (CCB) experimen tally infested with B. microplus showed great variability in their natural resistance to tbis tick, one bull out of seven was statistical1y detected as tick-resistant after experimentally infested, three were non-resistant, and three showed moderate tick-resistant
Summary
REVISTA DE BIOLOGlA TROPICAL control must be used if we want to fight suc cessfully against these ectoparasites, and simul taneously to keep an acceptable level of milk andJor meat production in the tropics. Host natural or acquíred resistance to ticks by a cattle breed is deterrnined after assessing several tíck's feed ing and fertility parameters, i.e., reduced engorgment weight, length of feeding period, reduction of tick egg production, reduced per centage of egg hatching, and egg and larvae mortality (Wikel 1996) The application of this methodology and later the genetic selection of those tick resistant cattle led in several coun tries, i.e. Australia, Brazil and sorne regions in Africa, to develop tick-resistant cattle breeds after cross-breeding B. taurus x B. indicus. This breed is the prod uct of empirical genetic crosses among Brown Swiss cattle, indigenous Creole bovines, and Bos indicus in a minor degree (Herrera 1960, Meléndez & ForJano 1'996). As a con tinuatíon of this previous assay this work was undertaken wíth the maín objective of assessing the Jevels of natural resistance to B. microplus in seven (7) tick-naíve cross-bred Carora bulls (CCB) which were experimentally ínfected with larvae of this tick
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