Host Identification by Schistosoma japonicum Cercariae

Abstract

Attachment, the first phase of host identification by Schistosoma japonicum cercariae, can occur in 2 different ways. Cercariae clinging to the water surface simply swing around and transfer to the host skin. Free-swimming cercariae behave like S. mansoni: upon touching a substrate, they switch from tailward to forward movement, swim in an arc, and attach to it with the penetration organ. Neither type of attachment is influenced by chemical, thermal, or specific mechanical stimuli from the host. The second phase, remaining on the host, requires a solid hydrophobic surface and seems to depend only on the cercaria's ability to cling to it. This phase is not influenced by chemical or thermal stimuli. The third phase, creeping across the host surface, is independent of chemical and mechanical stimuli. Cercariae migrate in thermal gradients to a preferred temperature of 37 +/- 3 C and then attempt to penetrate. Penetration, the fourth phase, was evoked by human skin surface lipids. The free fatty acid (FA) fraction was identified as exclusively stimulating components. Saturated FA's were effective at chain lengths between 10 and 14 carbon atoms (pH 7.0), and unsaturated FA's were effective at longer chains and their activity increased with increasing number of double bonds. Dog skin surface components did not stimulate cercarial penetration, which can be attributed to the lack of free FA's. A temperature of 32-40 C also stimulated penetration responses, which might be the main stimulus in animal hosts, whose skin surfaces contain no or only a few free FA's. FA's and heat evoked a transformation of cercarial tegument simultaneous with penetration behavior, making the organisms osmotically sensitive. The host identification of S. japonicum cercariae is very nonspecific compared with the differentiated host recognition of S. mansoni.

Infective larvae of the human hookworms Necator americanus and Ancylostoma duodenale differ in their orientation behaviour when crawling on surfaces

“…Obviously, the sensitive thermotaxis seems to be sufficient for the human hookworms to reach the skin. Other skin invaders, such as cercariae of S. japonicum and S. spindale, are also successful without such a chemo-orientation (Haas et al 1987(Haas et al , 1990. The two human hookworm species differ in many life history and epidemiological parameters (Hoagland and Schad 1978;Schad 1991).…”

“…The orientation of crawling hookworm larvae in thermal gradients was studied with methods modified after Haas et al (1987) and Granzer and Haas (1991). Briefly, the gradients were created on a silver plate (30·40·1 mm), which was connected to a heated and cooled copper block.…”

“…Human skin surface extract was obtained with methods modified after Haas et al (1987Haas et al ( , 1994. Briefly, the skin was rubbed with distilled water, the formed emulsion removed with a blade, homogenized by sonication, the sediment removed after centrifugation, and then freeze-dried.…”

Behavioural strategies used by the hookworms Necator americanus and Ancylostoma duodenale to find, recognize and invade the human host

“…Human skin surface lipids were obtained by washing the chest, back and arms of volunteers with ethanol. These were separated into lipid classes by thin-layer chromatography (TLC) using the methods described by Haas et al (1987. The lipids were arbitrarily divided into three classes: free fatty acids, polar lipids (including free sterols, diacylglycerols, phospholipids, ceramides, glycosylceramides), and nonpolar lipids (including triacylglycerols, wax esters, sterol esters, hydrocarbons).…”

“…Human skin surface extract was obtained using methods modified after Haas et al (1987Haas et al ( , 1994. Briefly, the skin was rubbed with distilled water, the formed emulsion removed with a blade, homogenized by sonication, the sediment removed after centrifugation, and then freezedried.…”