Studies on the cellular and humoral immune responses of the tropical bont tick Amblyomma varibgatum and the brown ear tick rhipicephalus appendiculatus against pathogens and parasitoids

Abstract

Amblyomma variegatum and Rhipicephalus appendiculatus are pests of domestic and to a certain extend wild animals. While feeding they transmit important diseases, among which are cowdriosis and East coast fever. It is generally known that pathogens after being ingested with the blood by the tick, move through the gut and reach the salivary gland where they are injected in a host with the saliva of the tick during feeding. Hardly anything is known on tick immune system. The goal of this work was to study the immune system of both A. variegatum and R. appendiculatus. Five types of haemocytes were found in both tick species. They were plasmatocytes, granulocytes, spherular cells, prohaemocytes and oenocytoids. There was no significant difference between total haemocyte count in adults and nymphs of A. variegatum and adults of R. appendiculatus. However total haemocyte count in infected ticks were significantly (P<0.05 ) higher than non-infected ones. Differential haemocyte count in non-infected showed a significant predominance of spherules and granular cells. The same trend was observed in ticks infected with bacteria. Tick haemocytes reacted to the presence of foreign bodies like bacteria, fungi and parasitoids by forming nodules or granulomma. All the ticks injected with Metarhizium anisopliae and Beauveria bassiana died within 5 to 10 days. None of the infected engorged females layed eggs. None of the engorged nymphs molted. Encapsulation was demonstrated using araldite as implant. Lysozyme activity was induced by injecting bacteria. Bacillus thuringiensis was observed to have induced more lysozyme activity than other bacteria. The tick parasitoid Ixodiphagus hookeri laid eggs in all the stages of both A. variegatum and R. appendiculatus but completed its development in the nymph of A. variegatum despite the cellular and humoral reactions of the host. The parasitoid goes through 3 larval stages, pupates, molt and emerges generally through one hole after 4 to 6 weeks post-infestation, leaving a dried empty cuticle.