Which medicinal leech?

31st August 2008, Sunday

Cliff Dean’s recent post on the medicinal leech Hirudo medicinalis reminded me that I have been meaning to catch this animal on the Marsh all through the summer. Romney Marsh has the largest and most extensive populations of this animal in the UK. In our country it has a curiously disjointed distribution. The closest population to ours is in the New Forest, and animals occur in scattered isolated populations all the way to the north coast of Scotland

The reason for my interest is that recent studies have revealed that there is more than one species of medicinal leech in Europe and that we do not know which one occurs in the UK. This discovery has a number of implications. First of all leeches are known to produce a number of pharmaceutically interesting chemicals. They are likely to differ in the two species, so there may be medicinal benefits from this. Interestingly most of the captive stocks appear to be of the newly recognised species, Hirudo verbana, so a lot of the research on this animal may be wrong!

The second implication is that Hirudo medicinalis is a protected species. The new species, Hirudo verbana is not.  So, is our conservation legislation appropriately applied?

One paper on the internet decribes Hirudo medicinalis (see here) as having numerous dark pigmented spots on the underside. The new species has few dark spots on the underside and a pair of black marginal stripes.

The upper surface of these leeches can also be used (see photographs on this link). Hirudo medicinalis has two narrow orange-red lines passing down either side of the centre of the back. In Hirudo verbana these lines form a series of connected blotches. To further confuse matters this second publication refers to DNA analyis that suggests that Hirudo verbana may need to be split into two species, although it is not mentioned if the animals actually look different.

Cliff’s photographs look more like Hirudo medicinalis, but it would be useful to check a few more specimens of this animal.

PS – another interesting observation on the internet. Turkey exports 6000kg of leeches annually!