Host parasite relationship, rabbit reactions to the brown ear tick
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The objective of these investigations was to study the rabbit reactions to the repeated infestation by each of the life cycle stages of Rhipicephalus appendicu1atus and to try and e1ucidatefue possible mechanisms on which the tick resistance phenomenon, a characteristic of this particular host-parasite system, is based. Eighteen mature intact male rabbits (Orycto10gus Cunicu1us) were divided into three groups of six rabbits each, each group being set aside for the infestation by one of the life cyc1e.stages according to methods of Bailey, (1960). Each group was itself subdivided into three subgroups of two rabbits.,each. One subgroup had three infestations, another, two infestations and the third subgroup had only one infestation. The infe~tations were done at two weeks interva1s. Rabbit reactions to the ticks were studied during each infestation. Two weeks after the last respective nymphal or larval infestation, adult feeding readiness and the associated rabbit reactions were examined. Similar investigations were made 'in the case of nymphs and for larvae feeding on the adult infested group of rabbits. To find out if histamine release during the infestations played a role in the resistance mechanism, one of the rabbits which had three previous infestations of its respective developmental stage ~f the tick was treated with antihista mine (Promethazine HeL) at a doss rate of Smg/kg for three consecutive days after applying the corresponding life cycle stage of the ticks. The feeding readiness of the ticks was assessed. To demonstrate the involvement of humoral factors in the resistance mechanism(s), sera from tick resistant rabbits, was transferred into six! rabbits with no previous tick exposure. Feeding readiness of each life cycle stage and the accompanying host reactions were examined. Six other rabbits divided into three groups of two rabbits each were subcutaneously inoculated with O.Sml of 1:40 unheated whole larval saline extract conjugated in 1:1 ratio with Freund's Complete Adjuvant. Twenty days after the last sensitizing infestation, each group of two rabbits was artificially infested with one of the developmental stages of the tick to assess the respective feeding readiness and the attendant .host reactions. feeding readiness, an index of resistance to the ticks, was assessed according to the criteria of length of engorgement period, weight and colour of engorged ticks and percentage of successful engorgement. The gross, haematological, immunological and histopathological reactions accompanying any level of resistance attained during any infestation were studied. It has been observed that the feeding readiness of each developmental stage of appendiculatus declined with successive infestations; prior infestation by one of the life cycle stages reduced the feeding readiness of another life cycle stage latter applied on the same rabbits; prior sensitization decreased the feeding readiness . of the larvae. Treatment of the resistant rabbits with an established antihistamine, improved the feeding readiness of larvae only. It has also been observed that tick infestation and the resultant tick resistance was accompanied by the rabbit physiopathological "reactions .of epidermal hyperplasia, lengthening of hair/follicles, epidermal tongue like projections into the dermis and inflammatory cell infiltration of the feeding lesion. Immunological reactions manifested as the appearance of both homocytotropic and precipitating antibodies in sera and the occurrence of blood and tissue eosinophilia also accompanied the subsequent infestations. It is concluded that the time relationship between the occurrenceof the various rabbit reactions and the development of resistance suggest that tick resistance in rabbits is acquired and its mechanisms are multicomponent in origin and based on mainly Arthus type of Immediate hypersensitivity which is superimposed on and which enhance the non-specific and probably innate physiopathological reactions of the rabbits. These tick resistance mechanisms are non absolute in effect but rather are geared to offsetting the high reproductive potential of the tick by rendering the attachment and feeding site unsuitable for the feeding of the tick.