Re-assessment of the medicinal leech Hirudo medicinalis Linnaeus, 1758, in Ireland

The apparent absence of the medicinal leech Hirudo medicinalis Linnaeus, 1758, in Ireland has been noted for over 150 years. Furthermore, not a single native Irish specimen has been preserved to prove its existence. The status of the Irish medicinal leech has been subject to several retrospective interpretations which need to be put into perspective in view of new thinking and research. The predominant view that this leech was never indigenous to Ireland is based predominantly on the assumption that Ireland was once totally glaciated, and this leech did not naturally colonise the island in the post-glacial period. Recent genetic evidence that the common frog survived in an Irish glacial refugium invites re-examination of some of this argument. Another widely held view is that within historic times, leeches imported into Ireland for medicine established itself in the wild but disappeared in the 19th century. In fact, Hirudo medicinalis was notoriously difficult to transplant (hence its threatened status). In spite of the millions of leeches imported into the British Isles and North America in the 19th century not a single example of an escaped population has been demonstrated. A third interpretation that the medicinal leech was indeed indigenous to Ireland is, in the author’s view, most compatible with a re-assessment of the historical and more recent evidence detailed in this paper. The author leaves open as a real biological possibility that the medicinal leech may be rediscovered in Ireland, but points out the historical precedent in mainland Britain that premature proclamation of extinction may delay potential rediscovery by decades.