[Susceptibility of Ornithodoros papillipes (Argasidae) ticks to the rickettsia Coxiella burnetii].

A definition of susceptibility of an arthropod to an infectious or invasional agent is given. The susceptibility means a capacity of the arthropod for infection in a result of which the agent either completes a part of its life cycle in the organism of its invertebrate host or reproduces repeatedly in the organs and tissues of its internal medium causing or not causing a noticeable damage. On the model of the tick O. papillipes and Q. fever agent there were shown differences in the resistance of arthropods to Rickettsiae depending on the developmental phase of the invertebrate host. Under otherwise equal conditions the susceptibility of nymphs of O. papillipes to C. burnetii was in nymph II 146 times, in nymph I 280 times and in larvae 50 000 times less than in nymph IV. It has been established that nymphs I, II, and IV engorge blood only 2.5, 14 and 50 times more respectively than larvae. It shows that the success of infection depends to a larger extent on the degree of susceptibility of an arthropod to a corresponding agent than on the quantity of engorged infected blood. The formula of estimation of agent concentration necessary for successful infection of 50 per cent of ticks is given.