Parasitogenic alterations of vector behaviour.

In many parasite-vector systems, alterations of the behaviour of the blood-sucking arthropods result in an increase of the transmission rate, but the underlying mechanisms are elucidated in only some systems. The more sluggish movements of the Trypanosoma rangeli-infected triatomine Rhodnius prolixus might increase the rate of predation by insectivorous mammals but also the transmission rate between the triatomines via cannibalism. Alterations of the feeding behaviour by which the number of attacks on hosts by blood-sucking arthropods can be increased seem to derive from two possible mechanisms. A competition for metabolites in the ingested blood induces an earlier starvation effect than in non-infected specimens and thus a new attempt by the insect to ingest blood. This may be relevant in T. cruzi-infected triatomines. Perhaps this is also the reason for the increased activity of ticks infected with the tick-borne encephalitis virus, resulting in a higher infection rate of ticks collected on humans than from the vegetation. The second, better elucidated mechanism is interference with the ingestion process, which causes a higher number of probings and low ingestion rates and is connected with disturbances of the digestive tract. Cells of the salivary glands are destroyed by the penetration of the parasites in Plasmodium-infected mosquitoes, T. rangeli-infected Rhodnius, and tsetse flies infected with salivarian Trypanosoma species. Some of the latter species attach to mechanoreceptive sensilla, which act as fluid flow meters and/or reduce the diameter of the foregut by a heavy colonization. This colonization effect is even more evident in several Leishmania-sandfly systems and in Yersinia pestis infection of the rat flea.