Yorkshire’s giant leeches

Leech found by Graham Sheperd
Leech found by Graham Sheperd

Graham Shepherd was in touch with a tale of giant leeches. ‘You may be interested in the attached pictures of what is clearly a leech, found in a Stocksbridge garden.

Leeches in my own pond do not come on land, are much smaller and semi-transparent. This one is capable of changing length between one and six inches as shown but bigger ones have been seen at the same place. They anchor down at the thick end and extend the pointed end out. The strange thing is that they are on land where there are no ponds in close proximity, just a small land drain nearby. Do we have any terrestrial leeches in our area? Are they harmless?’

This is undoubtedly the horse leech, haemopis sanguisuga, which, despite the name, does not attack horses.

Indeed it cannot bite through mammalian skin at all, but as a very large species, reaching up to 15 centimetres or about six inches, they catch the eye.

Their greenish colour and size help to identify them and they are very common around still water. These leeches are often found under stones near to water, and can be seen all year round.

The good news is that they feed on smaller animals such as midge larvae and snails, occasionally moving on to drier land to feed on earthworms.

Some of the larger terrestrial leeches, found in the tropics, detect their victims through sensitivity to body heat, but the horse leech is able to sense chemicals in the water and this is how they find their prey.

This animal is probably under-recorded in Britain, but is very widespread.

The true biting leech, which we have, is the medicinal leech, hirudo medicinalis, which achieves a body length of 20 centimetres (about eight inches).

This species has strong teeth that make a Y-shaped cut in mammalian skin, and it secretes anticoagulant via its digestive juices to stop the clotting of the victim’s blood. Consuming two to five times its own body weight in blood, a full meal can take it about 200 days to digest; it can manage without feeding for 18 months.

It was formerly widespread but bridges replaced fords, drinking troughs replaced ponds, and with the great fenlands drained, it decreased dramatically. This was a combination of habitat destruction and the loss of easy access to its victims.

Today, medicinal leeches raised commercially provide ‘hirudin’, an anti-coagulant used medicinally to prevent blood clots, and for the live animals which are used for delicate surgical operations such as on the eyes.

In times past, the stout women-folk of Yorkshire collected leeches for sale by standing in pools, ponds, and other wet areas. The leeches attached themselves to the ladies’ legs and could then be removed and collected.

- Sightings: The movement of wading birds through the region is clearly underway with, for example, Blackburn Meadows Nature Reserve turning up green sandpiper and greenshank. Nearby Orgreave Lakes had whimbrel, common sandpiper, little ringed plover, ringed plover and more – some birds having bred locally, others on the move. A green woodpecker was yaffling at Fairholmes in the Upper Derwent Valley and lapwings are building up into small flocks. Interesting gulls and terns are turning up as well, and swallows, martins and swifts are around in good numbers; some are still breeding, others are in post-breeding flocks.

- Professor Ian D. Rotherham is a researcher, writer and broadcaster on wildlife and environmental issues.