Perhaps the most common misconception about leech bites, and one that I keep hearing, is that leeches have an anaesthetic in their salivary secretions. The idea seems to have started with Sawyer's
Leech Biology and Behaviour, and has even been repeated in normally
reputable journals and
academic websites frequented by the public for information on various animals.
The theory goes that leeches, being stealthy, inject an anaesthetic so as to avoid detection. Most notable was an
NPR Science Friday story from August 26, 2005 in which a homeopath (Woodson Merrell, of Beth Israel Hospital) and a neurobiologist (George Stefano, Director of the Neuroscience Research Institute, SUNY at Old Westbury) promogulating the story that leeches inject a morphine-like substance to "numb" host tissue.
Of course, the real story is that leech 'morphine-like' substances are in the neural tissues not in salivary tissue as had been known five years earlier (
Laurent et al., 2000. Morphine-like substance in leech ganglia. Evidence and immune modulation. Eur J Biochem. 267:2354-61).
It should come as little surprise that the myth of a salivary anaesthetic would be further repeated by
BioPharm,
Leeches USA,
Niagara Medical Leeches, and
Ricarimpex, (all purveyors of medicinal leeches). After all, patients might resist leech therapy if it is going to hurt.
In my experience, the bites do hurt. Usually just a little, sometimes rather acutely (especially if your skin is not numbed from having been in cool water for half an hour), and in at least one case,
intensely enough to cause one of us to nearly kick the dashboard off a rental car.
More importantly, there is
not a single refereed article in the scientific literature that in any way points to an anaesthetic in leech salivary gland secretions. Twenty years ago, Meir Rigbi and colleagues showed rather convincingly that it does not exist (
Rigbi et al., 1987. The saliva of the medicinal leech Hirudo medicinalis - II. Inhibition of platelet aggregation and of leukocyte activity and examination of reputed anaesthetic effects. Comp. Biochem. Physiol. 83C, 95).
BUSTED
McCallum and colleagues, in an article just published in the Southeastern Naturalist, convincingly demonstrate that the rather striking colors of Macrobdella species, the North American medicinal leeches, the dorsal polka-dots, ventral orange with black specks, represent a case of Batesian or Müllerian mimicry.
The Wi'-gi-e
I will not be outdone. Herewith, a pair of Chtonobdella (c.f.) bilineata in-copulo! (Credit to Ralph Davis of Sydney, Australia). 
Be sure to watch NOVA Science Now on your local public television station,Wednesday, July 23 at 9pm in which one of the segments is all about leeches!
The best student paper presentation at the 87th Annual American Society of Parasitologists went to Anna J. Phillips for her present entitled "Hirudinidae3: Towards a Revision of the World's Medicinal Leeches". Here's the abstract...
cytochrome c oxidase subunit I, were examined separately, and in combination, in a parsimony analysis. This analysis of representative species of the Hirudiniformes indicates that the world’s medicinal leeches comprise multiple independent groups. Clade membership is only partially indicated by continental origin. The African Hirudinidae are split between two clades, with the unexpected results of the North African and Eastern European taxa being sister to select genera of Central and South America, one of which is a new genus with a unique morphology among leeches. These results suggest that, what was previously considered the family Hirudinidae, is actually 3 families: the Macrobdellidae Richardson, constituted by most New World taxa; the “true” Hirudinidae, with the type species (Hirudo medicinalis); and a new family, including the subset of African species grouped with Limnatis nilotica and associated New World taxa. 
Leech therapy made it into Atlantic monthly's The 11 1/2 Biggest Ideas of the Year in this month's issue (Volume 301 No. 6 : July/August 2008).
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