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Leech as vectors

Mark Siddall mes at zoo.toronto.edu
Sat Feb 26 16:11:53 EST 1994

In article <9402260937.aa18297 at cc.dordt.edu> mahaffy at dordt.edu (James Mahaffy) writes:
>	Are there any significant human parasites that are transmitted by
>leeches.  I can't recall hearing of any, but the question came up in a
>general biology class and it appeared to me that a leech would be a good
>vector, but I haven't heard of any parasites that they transmit. 

Glad you asked!  In factm so far as we know, there are no parasites 
transmitted to humans via leeches.  
Leeches serve as vectors of blood parasites of fishes, turtles, and frogs
(e.g., trypanosomes, haemogregarines).  The fact that we are not succeptable
to blood parasites transmitted by leeches likely stems from our being
only incidental hosts for leeches.  
Here in North America there are only two leeches that will bother with 
human blood: Macrobdella decora (and perhaps related species like M. ditetra) 
and Placobdella ornata.
Macrobdella decora is an arhynchobdellid leech (feeds by making an incision
with the 3 jaws, as does Hirudo medicinalis, the European medicinal leech).
Placobdella ornata is a rhynchobdellid (feeds by inserting a proboscis).

With the possible exception of the African buffalo leech, arhynchobdellids do 
not transmit blood parasites at all.  Moreover, though M. decora (and H.
medicinalis) will feed on us, they are very catholic in taste, feeding on
the blood of fishes and frogs, but also feeding by predation on 
oligochates, fish and frog egg masses, gastropods etc.  
Placobdella ornata is usually a parasite of turtles, where it can reach
densities of 1000's of leeches on a single snapping turtle.

I have been fed on by P. ornata so many times now that I've developed
a mild hypersensitivity to their bite.  I have never come down with any of
the parasites they transmit (e.g., Haemogregarina balli and Trypanosoma
chresymidis).  Undoubtedly this is due to the fact that 37 C is not a 
friendly environment for blood parasites of turtles, as well as my 
erythrocytes being anucleate.

A final note: many arynchobdellids serve as hosts for larval stages of digenea.
This may be incidental in some cases by cercaria penetratin anything 
soft enough.  On the other hand, they are important in the life cycles of
some trematodes of birds that eat leeches.  As such, I wouldn't go out
and eat leeches raw.  
Cook them first.  They end up tasting a little too "organic" if you don't
cook them well.  Especially those that feed on detritus (e.g., Haemopis spp.
and even Macrobdella spp.).

Overall, I'd say garlic butter will enhance the taste.



Mark Siddall                                     mes at zoo.toronto.edu
Department of Zoology
University of Toronto
Toronto, Ontario, Canada



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