Delayed-type hypersensitivity to Phlebotomus papatasi sand fly bite: An adaptive response induced by the fly?

The saliva of bloodsucking arthropods contains a large array of pharmacolog ically active compounds that assist hematophagy, Arthropod saliva is also r esponsible for causing uncomfortable allergic responses in its vertebrate h osts. In this article, we investigate whether the sand fly Phlebotomus papa tasi, known to produce a strong delayed-type hypersensitivity (DTH) in huma ns. could benefit from, and possibly adaptively induce, this response in th eir vertebrate hosts. In this study, we show that flies fed on humans to co mpletion nearly twice as fast in DTH sites as compared with normal skin sit es. DTH sites had significantly larger blood flow as measured by the laser Doppler method. Sand flies feeding at sites in mouse ears that had a DTH re sponse also fed faster than at normal sites. We conclude that in the case o f P, papatasi, and possibly other arthropods such as fleas and bed bugs, th e strong saliva-induced DTH response may reflect an adaptation of the fly t o manipulate host immunity for the insect's own advantage.