Induction of host resistance to Rhipicephalus Appendiculatus Neumann in local Masai Sheep in Kenya.
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Red Masai lambs, reared. tick free from ewes, themselves reared and managed in an area of Kitui with high tick challenge, and having undergone no regular dipping programme, appeared to show a degree of innate resistance to the feeding of nymphs and adults of the ixodid tick Rhipicephalus appendiculatus Neumann as compared to control Rhipicephalus appendiculatus naive rabbits. As more ticks were applied/signs of acquired resistance were Observed in the sheep. Full resistance appeared to be attained after successive applications of ticks which totalled 350 male and female adults and 800 nymphs over a period of 181 days. It must be stressed that different Rhipicephalus appendiculatus naive rabbits were used as controls at each infestation so as to monitor· any variability in the tick culture. Resistance was indicated in a number of ways. Fewer nymphs(20± 4%)and adults (41 ± 6%) engorged on the experimental lambs at the last 50 nymphal challenge infestation compared to 93 ± 1% of nymphs and 93 ± 1% adults which engorged on the previously Rhipicephalus appendiculatus naive control rabbits; also these nymphs and adult ticks which fed successfully on the lambs weighed less than those that fed to engorgement on the previously Rhipicephalus appendiculatus naive rabbits presumably due to resistance factors in the lamb blood. A substantial percentage of nymphs (54 ± 4%)and adults (29'± 3%) died 0n the host sheep; 26.73 ± 2.04% of nymphs and 30 ± 2% adult females were squashed while feeding as compared to 2.00 ± 0.83% and 3.50 ± 0.71% of adults in the previously Rhipicephalus appendiculatus naive control rabbits. Resistance also appeared to continue to the next development stage. Nymphal development to adults and the fecundity of female adult ticks fed on t.he experimental lambs appeared to be affected in a number of ways. The percentage of nymphs that moulted and developed to the adult stage after a successful feed on the sheep was significantly lower (P<O.001) compared with the previously Rhipicephalus appendiculatus naive rabbit controls. Adult females, having fed on the sheep, delayed their egg laying activities were unable to lay as many egg batches and the weights of egg batches were lower compared to those fed on the previously Rhipicephalus appendiculatus naive control rabbits (P<O.OOI). Hence it was indeed possible to observe the development of acquired resistance to Rhipicephalus appendiculatus ticks in Red Masai Sheep