Percutaneous infection by Schistosoma mansoni "tailless" cercariae.

Cercariae of Schistosoma mansoni are actively motile in water. A glycocalyx coat on the outer surface of their tegument provides protection from adverse changes in permeability (McLaren, 1980, Schistosoma mansoni: The parasite surface in relation to host immunity. Research Studies Press, New York, p. 36). Although the natural transformation of cercariae into schistosomula is effected during skin penetration (Stirewalt, 1963, Exp. Parasitol. 13: 395-406; Clegg and Smithers, 1968, Parasitology 58: 111-128), several mechanical (Colley and Wikel, 1974, Exp. Parasitol. 35: 360-372) and chemical (Gilbert et al., 1972, Parasitology 64: 333-339; Eveland and Morse, 1975, Parasitology 71: 327-335) treatments have effected the transformation in the absence of the process of penetration. These artificially-induced schistosomula were sensitive to water, gradually discharged their acetabular gland contents, and lost their glycocalyx with the tegument ultrastructurally changing from trilaminate to heptalaminate (see review of McLaren, 1980, loc. cit.). Percutaneous infectivity of these in vitro-transformed schistosomula was lost although they developed to adult worms after parenteral inoculation (Holanda et al., 1976, Rev. Inst. Med. Trop. Sao Paulo 18:410-414; James and Taylor, 1976, J. Helminthol. 50: 223-233). In all of these studies, the cercariae treated by the various procedures were immediately resuspended in media isotonic with mammalian tissue. It appeared that the loss of cercarial "tails" signaled the onset of the transformation process to schistosomula. We were interested in the ability of "tailless" cercariae (cercarial bodies) to percutaneously infect mice for several hours after mechanical decaudation. Schistosoma mansoni (Puerto Rican strain) cercariae were shed from Biomphalaria glabrata snails in a volume of spring water (Arbor Springs, Ann Arbor, Michigan) to provide approximately 1,500 to 2,000 cercariae/ml. After a 3-hr exposure under a fluorescent lamp, the cercariae were decanted and one-half was rapidly and repeatedly passed through a syringe fitted with a 21-gauge needle (Colley and Wikel, 1974, loc. cit.) over a 2to 3-min span. The entire volume was then examined with a dissecting microscope to ensure the complete absence of intact cercariae. The concentration of cercarial bodies was then determined. Viability was assessed by methylene blue dye exclusion (Ellner and Mahmoud, 1979, J. Immunol. 123: 949-951).