Adaptation of Field Strains of Rhipicephalus Appendiculatus, Neumann to Host Resistance to Tick Infestation

Abstract

The possibility of adaptation to host resistance by field strains of R. appendiculatus was investigated by comparing the feeding and breeding performance of two field strains with a laboratory strain (Muguga) which has been bred and maintained for about 30 years on susceptible rabbits. Results have shown that the laboratory strain has smaller eggs and smaller unfed larvae, nymphs and adults than the field strains. When fed on susceptible rabbits the laboratory strain females laid eggs with a mean weight of 41 -+1 μg while those of the field strains were 47 -+1 μg and 46 -+μg respectively. The sizes of unfed larvae, nymphs and adults showed similar differences. .eggs and larvae of laboratory and field strains from females fed on cattle and rabbits were also compared. In both laboratory and field strains, eggs and larvae from ticks fed on cattle hosts were larger than those from rabbits. Since cattle are the main hosts of R. appendiculatus, it is possible that the use of rabbit hosts has exerted selection pressure for smaller size on the laboratory strain of ticks. When fed successively on the same hosts, field strain larvae and nymphs remained significantly larger than those of the laboratory strain. laboratory and field strain females fed to similar engorged weights on susceptible rabbits, but during the 2nd and 3rd infestations on the same hosts, the field strains yielded females twice as heavy as the laboratory strain females. The proportion of ticks surviving the feed decreased with subsequent infestations for both the laboratory and field strains. But although there was no significant difference in the proportions of nymphs and adults, the proportion of laboratory strain larvae that fed successfully decreased to a significantly lower level over the 4 feeds than that of the field strains. When proportions of larvae, nymphs and adults were combined, it was observed that a slightly higher proportion of the laboratory strain fed on susceptible rabbits than the field strains. But on previously exposed rabbits the situation was reversed. A comparison of reproduction efficiency showed that the laboratory strain females reproduced better on susceptible hosts, while the field strains reproduced significantly better on previously exposed hosts. When hosts previously exposed to ticks were challenged with laboratory and field strains, it was observed that cross-protection was low. Field strains, in particular, fed significantly better on hosts previously exposed to the laboratory strain. Cross-protection between the field strains, however, was found to be high. Observations made on cattle hosts showed that similar results to those reported above for rabbits could be expected on cattle. These results indicate that the laboratory strain has a higher reproductive ability on susceptible hosts than the field strains. The field strains, on the other hand, have a higher reproductive ability on previously exposed hosts. This reflects adaptation to the host environment that the strains have been exposed to. Caution is therefore needed when interpreting results on host resistance against ticks obtained with ticks bred in captivity for a long time. The absence of high cross-protection is another aspect to consider in tick control by host resistance.