The Host Relationships of the Tick Amblyomma Variegatum in Rabbits and Cattle

Abstract

Repeated feeding of the different life stages of Amblyomma variegatum and Rhipicephalus appendiculatus on foμr groups of rabbits induced resistance in these animals. Resistance to the adults was manifested by reduced engorged weights, subsequent reduction in the egg-conversion factor and fewer ticks that were able to lay eggs. Similarly, resistant hosts allowed significantly less immatures to complete their engorgement and these also dropped with reduced engorged weights. The feeding of ticks was also greatly inhibited. Thus, some ticks were observed to die and shrivel while still on the hosts; others died immediately after detachment; while · a third group fed partially but were unable to complete their blood meal or detach as the-mouth-parts were masked by host scar tissue. It was also found that 72% of nymph3 fed on resistant animals dropped prematurely, with body sizes in feeding -categories ' N2 and N3, and only 28% were able to complete their blood meal and reach N4 (engorged) . Resistance in rabbits also seem to interfere with some vital physiological processes in the tick and possibly inhibited some factors or hormones which are responsible for the scutal pigmentation pattern. Thus, two additional spots were regularly observed on male A.variegatum which had fed as nymphs on resistant animals. High levels of intra-specific cross- resistance between the life stages of ticks have been demonstrated for the two genera. It was also demonstrated that ( rabbits sensitized to R. appendiculatus showed appreciable level of resistance to A.variegatum nymphs. However, R. appendiculatus appeared unaffected in its ability to engorge on A. variegatum resistant rabbits. Cattle also became resistant to R.appendiculatus after repeated· infestations. They also showed considerable resistance to A. variegatum nymphs. Although these results substantiate the findings from rabbits, data from control cattle susceptible to ticks would be required to confirm these observations fully. Antibodies have been demonstrated in the serum of all animals within the four groups of rabbits which had been exposed to natural tick feeding. The immune diffusion ' tests also demonstrated intra-specific cross-reactivity. Thus, s8ra from rabbits infested with adults nymphs, and to a lesser extent larvae, reacted to give intense precipitin lines against various adults organs (salivary glands , midgut, ovaries, whole internal organs) , and whole nymphal and larval extracts . However, the test did not reveal detectable inter generic cross-reactivity between these two species " of ticks. The histophathological studies on the attachment -sites of nymphs , 48 hours post attachment, have revealed differences between the three groups of rabbits which were related to the nature of their resistance. Thus, rabbits which were .made resistant to A. variegatum and R. appendiculatus by r epeated infestations showed epidermal vesiculation ....(continue from pdf attached)