Medicinal use of terrestrial molluscs (slugs and snails) with particular reference to their role in the treatment of wounds and other skin lesions
Abstract
The slime produced by terrestrial molluscs (slugs and snails) has interesting properties which have been utilized for centuries for the treatment of minor wounds and other skin disorders such as warts. This paper provides an introduction to the properties of slug slime and considers its potential value in modern wound management. It also reports the results of a small study in which this material was successfully used to treat a longstanding wart. This is believed to be the first illustrated and fully documented account of the use of slug slime for this indication.
Introduction
Slugs and snails are terrestrial molluscs which have similar morphology except that slugs, unlike snails, have no obvious shell, although some species possess a partial or internal vestigial shell. Widely distributed around the world, the largest species of slug in the UK, the Ashy-Grey slug (Limax cinereoniger), can exceed 25 cm in length.
Both slugs and snails secrete visco-elastic slime or mucus which acts both as an adhesive and lubricant and enables the creatures to adhere to, and glide over, all types of surfaces including rough or potentially hostile terrain. Mucus also helps to prevent the creatures from drying out, renders them fairly unattractive as food for predators,[1] and is also thought to help prevent infection and facilitate healing.
For centuries snails, and to a lesser extent slugs, have been used both as a food and as a treatment for a variety of medical conditions. Quave et al.[2] described how, in southern Italy, the common garden slug, Arion hortensis, is sometimes swallowed whole as a treatment for gastritis or stomach ulcers. In America slugs are not thought to be swallowed live in this way, but a recipe for ‘Slug Syrup’ is recorded on the website of the University of Saskatchewan. This instructs that a jar be filled with alternating layers of slugs and sugar. After about a day, when the sugar has ‘dissolved’ the slugs, the resulting mixture is run through a sieve, after which 1/3 grain alcohol is added by volume. The site quotes the original authors who recommend the resulting syrup be used for the treatment of ulcers, bronchitis, asthma, claiming that it is able to ‘heal these conditions when nothing else will.’[3]
Snail and slug slime have been used sporadically as skin treatments since the time of the Ancient Greeks; Hippocrates reportedly recommended the use of crushed snails to relieve inflamed skin and some 20 years ago, the potential of snail slime was noted by Chilean snail farmers who found that skin lesions healed quickly, with no scars, when they handled snails for the French food market. This observation resulted in the production of ‘Elicina’ a Chilean snail slime-based product. In 2010 ‘Missha’ then launched Super ‘Aqua Cell Renew Snail Cream’, claiming that its 70% snail extract ‘soothes regenerates and heals skin’. Snail slime based products are claimed to be the new miracle face-fixer in the U.S where they are used to treat acne, reduce pigmentation and scarring, and combat wrinkles.[4]
In the publication cited previously, Quave et al.[5] also described how slugs are used in Italy to treat dermatological conditions. Mucus collected from a slug is rubbed onto the skin to treat dermatitis, inflammations, calluses, and acne, and to promote wound healing. In addition, in a special ritual slugs themselves are used for the treatment of warts. Mucus from a live slug is first rubbed onto the wart, and then the slug is hung out in the sunshine to dry out and die. It is believed that once the slug has dried up, the wart should as well.[2] The use of slugs for the treatment of warts is not, however, confined to Italy. Records exist of the use of slug slime in the US and UK some of which recommended that the slime be collected at certain phases of the moon to ensure maximum effect.
Evidence of this practice may be found amongst the specimens contained in the Pitt Rivers Museum in Oxford where a glass specimen jar filled with alcohol contains a slug impaled on a thorn. Purchased by the Museum in July 1898 from Thomas James Carter of Oxford, it provides hard evidence of a cure used in several parts of the UK. The label on the jar instructs ‘Go out alone and find a large black slug. Secretly rub the underside on the warts and impale the slug on the thorn. As the slug dies the warts will go’. In other parts of the UK such as Berwickshire, Northumberland and Lancashire, the museum suggests that the slug is replaced by a snail: ‘Take a black snail, rub the warts with it, and then suspend it upon a thorn; as the snail melts away, so will the warts. This must be done nine nights successively, at the end of which time the wart will completely disappear.’[6]
Snail products may even have a role in orthopaedics. Researchers at Herriot-Watt University found that the slime of Giant African land snails contains unusual crystals of calcite. Under adverse conditions the snail will retract into its shell and produce significant quantities of this slime which dries and quickly hardens to form the animal’s epiphragm - a protective covering formed across the opening of the shell when the snails go into periods of deep rest. The authors postulated that in the long term their observations could point the way to the development of bone cement based on a natural process involving inorganic crystals in an organic matrix; a biologically compatible material which might contribute to mechanisms of bone healing. [7]
The principal benefits associated with the use of slugs and snails as topical treatments are therefore associated with the chemical and/or physical properties of the slime or mucus that they produce in abundance, particularly when threatened or irritated.
Production and properties of slug mucus relevant to wound healing
The mucus producing cells are located in the epithelium of the skin, both on the foot and upper surface of the body. Slugs produce at least two types of mucus; pedal mucus, which is relatively thin and contains about 96-97% water, and a second form which is produced over the entire body. This tends to be more thick and sticky. Both types are hygroscopic.[1]
The precursor of slime is initially produced by the slug or snail in the form of highly hygroscopic grains which are stored within the cells in the form of granules coated with a protective water resistant membrane which keeps them dry. These packets only break open after they have been released from the cell, a process which is thought to be mediated by contact with extracellular ATP.[8] At this point the granules very rapidly absorb up to 100 times their initial volume of water to form the familiar mucus or slime trail.
Slime is a complex material with non-Newtonian properties. In simple terms, the slime acts like a solid glue at rest, but liquefies when an adequate stress (or force) is applied to it - rather like non-drip paint or ketchup. When the applied stress is removed, the slime quickly re-solidifies. This may have important implications for its use as skin or wound treatment but slugs and snails use this property to create ‘pedal waves’ in a process known as adhesive locomotion. By exploiting this ‘yield-heal’ property, the creature can keep one part of its foot stuck to a surface whilst the remainder moves forward.[9][10]
Composition of slug/snail slime
The composition of slime is thought to vary according to species, and it is believed that it is possible that each may also be able to vary its formulation.[9]
Mucus consists of a complex mix of proteoglycans, glycosaminoglycans, glycoprotein enzymes, hyaluronic acid, copper peptides, antimicrobial peptides, and metal ions.[11] Atomic absorption spectrometry showed that glue from the slug Arion subfuscus contains substantial quantities of zinc, iron, copper and manganese. Experimentally it was shown that the addition of iron or copper to dissolved slug glue causes the proteins to precipitate rapidly but the addition of zinc had no effect, suggesting that some metal ions play an important role in gel formation.[12]
The presence within the slime of these complex polymers, may have particular relevance for wound healing as the literature contains many references to the importance of these materials in the healing cascade.
Studies have also shown that mucus contains peptides such as mucin which possess antibacterial activity against both Gram positive and Gram negative bacteria. These antimicrobial peptides not only act as natural antibiotics, but also stimulate many elements of the immune system, including barrier repair and inflammatory cell recruitment. The antibacterial factor from the body surface of the Giant East African Snail, Achatina fulica, for example, exhibited highly positive antibacterial activity both for the Gram-positive bacteria, Bacillus subtilis and Staphylococcus aureus and for the Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa, but this activity was lost when the material was heated at 75º C for 5 min. The antibacterial factor of the snail mucus was shown to be a glycoprotein with a molecular weight of about 160,000.[13]
Slug slime is also said to contain a local anesthetic and for this reason there are anecdotal accounts of live slugs being used to treat toothache.
These local anaesthetic properties (if confirmed) coupled with the antimicrobial properties and hygroscopic nature of the slime might offer significant benefits in the treatment of minor but painful wounds such as superficial burns in humans.
In the United States a patent has been filed (US2009026349) which describes the possible use of slug slime as a carrier for therapeutic agents in the treatment of burns and skin conditions and also as a protective covering for these and other wounds. Within the