Susceptibility of schistosomula at various ages to praziquantel during invasion in mouse skin and observations on certain characteristics of schistosomula surface

Praziquantel possessed a noticeable effect in vivo against 3-h old schistosomula in the mouse skin, but less so or even not at all on 6-48-h (skin stage) and 72-h (lung stage) old schistosomula. The susceptibility of 3-72 h old schistosomula to praziquantel in vitro was the same in stimulation of worm activity and contraction induced by praziquantel. While in 3-h old schistosomula the drug-induced tegument damage was more severe than that in 24-h and 72-h old schistosomula. In 3-h old schistosomula 86.4% of the worms would have their surface antigen exposed evidence by indirect fluorescent antibody technique, while in 24-48-h old or 72-h old schistosomula 55.-58.7% or only 3.9% of the worms showed the exposure of surface antigen. When 3-h old schistosomula were inoculated into the peritoneal cavity of mouse containing neutrophils or macrophages, the worm surface was quickly attached by either kind of cells. No or only few neutrophils or macrophages adhered on the worm surface when 24- and 72-h old schistosomula were tested.