19 March 2007

Leeches and frogs

The medidicinal leech (Hirudo medicinalis) feeds on vertebrate, mainly mammalian, blood. A short paper from 2002 by Merilä & Sterner1, provides a compilation of published reports of medicinal leeches feeding on the blood of amphibians (frogs and newts).

The authors also present their own observations from the Swedish island of Gotland where they found many moor frogs (Rana arvalis) parasitized or killed by medicinal leeches. They note that frogs can't seem to be able to remove the leeches attached to their bodies and the parasitized frogs invariably die from blood loss.

Hirudo medicinalis is native to Europe, but its populations have been declining. Many amphibian species are also threatened with extinction.

The discussions in the paper outline what seems to be a complex network of several interconnected phenomena operating simultaneously in Europe and possibly affecting the longterm well-being of both the leech and the frog populations. According to the authors, the decline in the numbers of free-ranging cattle in Europe may have caused leeches to rely more on frogs. But frog blood, having an energetic value lower than that of mammalian blood, may have decreased the growth and fecundity of leeches. At the same time, increased predation by leeches may have been affecting the frog populations negatively.

If the leeches indeed switched from preying on mammals to frogs recently, I suppose the historical cause was first the decline of the numbers of large native mammals in the wild, followed by the decline of the numbers of large domesticated mammals.

1Merilä, J. & Sterner, M. 2002: Medicinal leeches (Hirudo medicinalis) attacking and killing adult amphibians. Ann. Zool. Fennici 39: 343–346. pdf


ümit said...

It is very strange. Looks same to me like "Lions shifted to humans". Is it possible that amphibians seemingly avoided by the leech (not leeches) for millions of years can be alternative hosts?
All parasites have special 'systems' enabling host specific behavior, and many are so. I think Dr. Neubert, ideas of whom i would also like to know also, once worked on these animals taxonomically, maybe he's got some more to say.

Clare said...

I remember reading once that there was a shortage of leeches in the nineteenth century too. Leeches were just too popular. As soon as they had finished their meal they were put into salt water to be sick and with their appetite restored they could be used again.

I'm not surprised they kill off the frogs. A sated leech must be almost the same size as a small frog.