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Schistosomes are parasitic flatworms that have a snail intermediate host and a human definitive host.

After developing in the snail, they (cercariae stage) escape into the water and can attach to the skin of a human. Then, it will penetrate the skin and migrate into areas of the human's body.

I have been unable to find any information on the mechanisms related to attachment to the human host's skin. More specifically, my questions are:

  1. Do they attach to whatever they can find, and can only develop into the next stage if they (by chance) attach to the right host?
  2. If they selectively attach to human host skin, how do they detect the skin? What makes the skin distinguishable to them?

Here is a great picture of the life cycle of these parasites, for reference. enter image description here

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Schistosoma recognize human skin by mechanical, thermal and chemical stimuli from host. Attachment of parasite to skin is stimulated by L-arginine. Schistosomes specifically bind to L -argine by chemotaxis. L-arginine guides this parasite towards site of penetration. The lipids on the human skin surface also serve as signal for host invasion.

It is also reported that parasite may use L-arginine to synthesize nitric oxide for blood vessel dilation so that it can easily penetrate into the host. Schistosoma may attach to L-arginine of surrounding tissues to escape the cellular immune response from host.

(Via: https://www.ncbi.nlm.nih.gov/pubmed/11860033)

These parasites enter the host through skin by producing several proteolytic enzymes to degarde the host tissue proteins including elastin, condromucoprotein, keratin, fibronectin, laminin and collagens.

Two distant serine proteases are responsible for digesting host tissue. One is Chymotrypsin-like protease which degrades protein with large hydrophobic side chains. And the second one is Trypsin-like protease which prefers proteins with positively charged side chains.

(Via: http://www.jbc.org/content/275/49/38667.full#ref-2)

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