Enhancement of Schistosoma mansoni infectivity by intradermal injections of larval extracts: a putative role for larval proteases.

Extracts of Schistosoma mansoni cercariae caused increased vascular permeability and edema if administered to CBA/Ca mice by intradermal injection. Percutaneous infection with cercariae over the skin site at which cercarial homogenate (CH) had been injected intradermally resulted in a significant increase in the infectivity of S. mansoni compared with that shown by worm recovery from control animals (P < .05). This effect was abrogated by inhibition of protease activity prior to injection. Injection of inflammatory mediators (bradykinin or zymosan-activated plasma) with or without prostaglandin E2 produced a similar amount of edema as did CH. Injection of these mediators did not, however, enhance infectivity of cercariae. Pancreatic elastase was found to induce edema and enhancement of infectivity comparable to those induced by CH. The protease(s) introduced into the site of infection may have facilitated larval migration directly by hydrolyzing host tissue or indirectly by inducing an inflammatory response (or both).