Effect of ale, garlic, and soured cream on the appetite of leechesBMJ 1994; 309 doi: http://dx.doi.org/10.1136/bmj.309.6970.1689 (Published 24 December 1994) Cite this as: BMJ 1994;309:1689
- Correspondence to: Dr Baerheim.
The medicinal leech has regained some of its lost popularity by its present use in microsurgery. Sometimes, however, the leeches refuse to cooperate properly. To overcome this problem doctors in the 19th century used to immerse leeches in strong beer before applying them to the patient.1 In the 1920s a deaconess experienced that a little soured cream on the skin would encourage the leeches' feeding behaviour,2 and we recently found that they seem to be attracted by garlic.3 We designed a study to evaluate the effect of these remedies.
Materials, methods, and results
Six leeches were dipped briefly in one of two different types of beer (Guinness stout or Hansa bock) or in water (control) before being placed on the forearm of one of us (HS). We measured the time from when the leech touched the skin until HS felt it bite. Each leech was exposed three times to each liquid in random order. As serotonin is known to control the feeding behaviour of the leech,4 we also measured the serotonin content of both beers by high pressure liquid chromatography.
Six other leeches were then placed on the same author's forearms without being dipped in beer or water. The left forearm was either not prepared or smeared with soured cream. The right forearm was smeared with garlic. The leeches were placed on the differently prepared arms in random order. We planned to place each leech on each arm seven times, but because of adverse effects only two leeches were exposed to garlic. The four remaining leeches completed the study as planned with the other preparations.
We analysed the median time from application to biting. Some leeches did not bite within the predefined time limit of 300 s, but as the study was run according to the intention to bite principle, these incidences were registered with a time interval of 301 s in the statistical analysis (paired Wilcoxon test).
After exposure to beer some of the leeches changed behaviour, swaying their forebodies, losing grip, or falling on their backs. Leeches dipped in Guinness bit after 187 s, those dipped in Hansa after 136 s, and controls after 92 s (table). The serotonin content was low and similar for the two beers (0.1 μg/ml).
Two leeches placed on the forearm smeared with garlic started to wriggle and crawl without assuming the sucking position. They were placed in water, but their condition deteriorated. When placed on a bare arm they tried to initiate feeding but did not manage to coordinate the process. Both died 2 1/2 hours after the exposure to garlic. For ethical reasons the garlic arm was abandoned at this point.
Leeches exposed to soured cream sucked frantically on the wall of their container after they had been on the arm. While on the arm smeared with cream, however, they bit no sooner than the controls (table).
Exposure to beer tended to disrupt the leeches' normal behaviour and made them erratic. However, the most interesting effect was that of direct exposure to garlic. Ingested garlic has been reported to be lethal to some animals,5 but we believe this to be the first study showing garlic to be lethal by skin absorption. Garlic has a definite force of attraction on leeches,3 but further research into this fatal attraction will require in depth qualitative methods.
The alleged effect of soured cream may have been an extrapolation of the deaconess's own preferences, an example of the placebo effect. An inert substance in the hands of the believer may work well, even on leeches. This study provides a reminder of how medical beliefs can stand uncontradicted for decades. We should never forget the necessity of critical research on commonly accepted medical truths.
We thank Ole Helland, Hansa Brewery, Bergen, for supplying sufficient amounts of their precious liquid to satisfy the needs of all participants of the study. We are also greatly indebted to professor Ole Jacob Broch, Division for Pharmacology, University of Bergen, for analysing the ales for serotonin. The leeches are by all accounts grateful to Hogne Sandvik for supplying his own precious liquid, but in any event we wish to express our appreciation for their enthusiasm.