Monday, July 7, 2008

Myth Busters: Leech Anaesthetic

   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).



Anonymous said...

Great bust! When we were in Malaysia in 2000, my colleague Alison was acutely sensitive to leech bites as well. Four field season in tropical SE Asia will do that to you!

Frank Burbrink said...

I was just in Borneo, Mark, and those damned terrestrial leech bites do hurt. I especially felt the bites around my stomach. I also placed a leech on my hand to see how long it took for it attach (which took less than one minute) and clearly felt that bite.

Anonymous said...

Ya, leechbite DO hurt, especially when it bites on the hand. You absolutely can feel its intermittent biting. I think that maybe leeches have some other ways to suppress the feeling and the pain of host, such as unique microstructure of the jaw or simple chemicals that inhibit the reaction of nerves, but not the anaesthetic.

AND HERE I AM, Mark~:)

Ponder Stibbons said...

I found that when I first started backpacking in Malaysian jungles, I could not feel the bites at all. This seems to be the case for many novices. As I was bitten more and more, though, I developed a greater sensitivity, so that now it does hurt or itch enough for me to detect it.

Great blog, by the way.