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Sex Life of the Schistosome

Al Shostak al.shostak at ualberta.ca
Wed Oct 6 09:46:02 EST 1999

Assuming that you do have swimmer's itch, I wonder what you were actually 
seeing with your microscope.  The cercariae of schistosomes do have separate 
sexes, but they do not differentiate morphologically until after they have 
migrated to the liver. Also, each welt typically results from the entrance of a 
single cercaria.  They do not group up prior to penetrating.  I think it is 
highly unlikely that you would have extruded from the pustule, let alone been 
able to identify, a group of females around a male.  When the cercariae leave 
the snail the males do not yet even have a gynaecophoric canal. What you MAY 
have been interpreting as a male surrounbded by females, is some cellular 
debris around a cercaria that resulted from your body's immune response, 
which killed the cercaria that penetrated your skin at that site.

In a subsequent article you mention that you wish to make a presentation to 
your local parks board, to convince them to warn swimmers of the risk.  Before 
you do so, I suggest that you educate yourself about the parasite more fully.  
Any general textbook on parasitology will give you information about 
schistosomes.  Trichobilharzia is just one genus, but the general aspects of 
the life cycles are similar.  To address a few of the inaccuracies from other 
of your postings: (1) The cercariae do not have "pincers"; the forked structure 
is actually their tail, and it is generally left behind when the cercaria 
penetrates skin.  (2) Towelling off is not necessarily a simple preventive 
measure.  It may remove some cercariae on the surface of the skin, but 
cercariae penetrate many individuals while they are still in the water.

Swimmer's itch is a world-wide problem, with no easy solution.  If you have 
waterfowl and appropriate snails in a lake or pond, there is a good chance that 
some snails will be releasing the cercariae that cause the itch.  Preventing 
infected waterfowl from visiting the lake, or removal of snails, would be 
effective, but is difficult to accomplish in practice.  Any claims that  
application of certain creams or lotions will prevent the penetration of 
cercariae are inconclusive.  Recent studies indicate that swimming in the 
morning may increase the chances of exposure because the snails tend to release 
the short-lived cercariae at first light.


In article <7tbqup$4mv$1 at nnrp1.deja.com>, pspangle at worldnet.att.net says...
>In the last few weeks I have become intimately acquainted with
>Trichobiharzia ocellata - hundreds of them (i.e., I have Swimmer's
>Itch).  When I examine extrusions from the pustules on my legs and arms
>under a microscope, I notice that each male T.o. is accompanied by 4-6
>Question: Does the male schistosome carry his whole harem in his
>gynecophoral canal during his trip from snail to warm-blooded host, or
>do they swim separately and joing up after he has penetrated the host?

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