Maggots in Wound Debridement - an Introduction
- S Thomas B Pharm., Ph D., Director SMTL.
- M Jones BN., RGN. Senior Research Nurse SMTL
- S Shutler BN., RGN. Research Nurse SMTL
- S Jones BSc PGCE. Laboratory Scientist SMTL
Flies, unlike most other insects, have only a single pair of functional wings. They are therefore classified as Diptera, (di + pteron) `two-winged' a word coined by Aristotle in the fourth century BC.
About 120 000 fly species have been described, a small number of which are vectors of diseases such as malaria, filariasis and trypanosmiasis. These diseases are transmitted by adult flies which have become adapted structurally and physiologically to a bloodsucking mode of life. Other species, such as the common housefly, Musca domestica represent a potential threat to health because the adult may contaminate food with pathogenic organisms. 
In some fly species the larval forms feed upon live or decaying animal tissue and in some instances the larvae can be highly invasive. For example the screw worm fly Cochliomyia hominivorax, literally `man-eater,' lays its eggs in the margins of wounds or mucous membranes of body openings such as the nose or vagina. The newly hatched larvae burrow downwards into the tissue causing massive tissue damage or even death. 
Other larval species are fortunately much less aggressive and limit their activity to dead or necrotic tissue.
Although a number of different species, including the common housefly, have been isolated from wounds and body orifices following accidental infestation,  the fly most commonly used for larval therapy is Lucilia sericata which is also responsible for the condition known as `blow-fly strike' in sheep. The adults are an attractive metallic coppery green, hence the common name `greenbottles'.
The key morpological features of medically important flies have been described by Crosskey and Lane. 
The beneficial effects of larvae (maggots) upon the healing of infected wounds have been recognised for hundreds of years. In the main, these larvae found their way into wounds by accident, particularly under battlefield conditions, but it was recorded that when this occurred the wounds tended to heal more quickly and with fewer complications than comparable wounds that had not become infested. Chernin,  in a review entitled `Surgical Maggots' quotes Joseph Jones, a ranking Confederate medical officer during the American Civil War as follows,
"I have frequently seen neglected wounds ... filled with maggots.. as far as my experience extends, these worms only destroy dead tissues, and do not injure specifically the well parts."
Chernin also suggests that the first therapeutic use of maggots may be credited to a second Confederate medical officer J.F. Zacharias, who reported that
"Maggots.. in a single day would clean a wound much better than any agents we had at our command ... I am sure I saved many lives by their use. "
Over fifty years later during the first world war Baer, then an orthopaedic consultant to American forces in France, treated two wounded soldiers who had remained overlooked on the battlefield for a week after the fighting. He found that although their compound fractures and abdominal wounds swarmed with maggots, the wounds had begun to granulate with no evidence of sepsis. Some ten years later, when Clinical Professor of Orthopaedic Surgery at the John Hopkins Medical School, he recalled his wartime experiences when trying to treat several cases of intractable osteomyelitis. He obtained a supply of maggots and placed them in the wounds which proceeded to heal within six weeks.  Following these experiences the use of larve in wound management became very common in the 1930s, particularly in the USA where larvae of the greenbottle, Lucilia sericata were produced commercially in large numbers for this purpose by the pharmaceutical company Lederle.  The early literature contains many references to the successful use of these larvae in chronic or infected wounds including osteomyelitis, abscesses, burns and sub-acute mastoiditis.        
It has also been reported that larvae may be of value in managing some forms of cancer. Weil et al  described how in two cases of inoperable cancer of the breast, the application of larvae removed the malignant tissue leaving a cavity which then formed healthy granulations.
Similarly, Bunkis et al.  described how accidental infestation of a facial squamous cell carcinoma produced a wound that was clean and free of any necrotic residues and Reames et al  reported how a similar wound that was clean and odour free following accidental contamination with maggots, deteriorated rapidly when the maggots were removed and the wound was managed with conventional treatments including hypochlorite solution. For additional information on the history of maggot therapy see Sherman and Pechter  and Morgan. 
It is believed that larvae combat wound infection by ingesting microorganisms which are then destroyed in their gut but there is published evidence to suggest that they also secrete chemicals such as allantoin and other agents with with pronounced broad-spectrum antibacterial activity.    Studies on the screwworm, however, appear to indicate that this antimicrobial activity may be due to phenylacetic acid and phenylacetaldehyde produced by the bacteria Proteus mirabilis which is a commensal of the larval gut. 
The clinical importance of these antimicrobial agents has never been fully investigated.
With the advent of the widespread use of antibiotics, the practice of larval therapy declined in the 1940s, but since that time isolated reports of the benefits resulting from the accidental contamination of wounds with fly larvae (termed myiasis) have appeared in the literature. As a result of these observations and the serious clinical problems associated with the emergence of antibiotic resistant strains of bacteria such as methicillin resistant Staphylococcus aureus (MRSA), some clinicians have begun to reconsider the use of larvae, and one centre in America has been using this technique in the treatment of pressure sores and other wounds for some time with very good results.  
In the United Kingdom, initial clinical experiences with this once popular form of therapy, now termed `Biosurgery', using sterile larvae reared in the Surgical Materials Testing Laboratory (SMTL) proved very encouraging. The Bridgend and District NHS Trust therefore provided facilities to establish a dedicated Biosurgical Research Unit, believed to be the only one of its kind in the world.
The Biosurgical Research Unit
The Unit consists of a self contained suite of rooms comprising a fly room, where the adult insects are kept, a clean room facility with a lamina flow cabinet where the eggs are processed and sterilised, a room with air extraction for the breeding stock of larvae and a treatment room where patients can receive larval therapy on an outpatient basis.
Life history of the greenbottle
In the wild, the adult fly will lay large numbers of eggs in clusters or `rafts' on organic matter such as suppurating wounds or carrion. The larvae, which hatch within a day or so, produce a powerful mixture of proteolytic enzymes  including collagenase  which breaks down the dead tissue to a semi-liquid form which is then reabsorbed and digested. In order to maximise the potential of this extra-corporeal digestive process, the larvae tend to congregate into groups and feed in the head-down position, concentrating initially on small defects or holes in the tissue.
The larvae a have a pair of mandibles or hooks, which they use to assist in locomotion and provide attachment to the tissue. Under the microscope they have also been observed apparently using these hooks to scrape at the surface of food possibly to disrupt tissue membranes, although this has not been confirmed.
The larvae increase in size very rapidly. They moult twice and when they are fully grown - usually after about 5 days - depending upon the temperature, they cease feeding, leave the food source and seek a place to pupate. The puparium, is formed from the hardened last larval skin on which structures such as the posterior spiracles, holes through which the larvae breath, are still visible.
The adult fly develops within this puparium and emerges by rupturing the skin with the ptilinum, a bladder-like organ that is temporarily extruded from the head. The rate of development of the adult fly within the puparium is temperature dependent. Under adverse conditions, development may be arrested for weeks or even months.
The newly emerged adult bears little resemblance to the familiar greenbottle but within a short while the wings expand and the body takes on its familar green hue. The life cycle is thus complete.
Preparation of sterile larvae
The external surface of the eggs are normally very heavily contaminated with bacteria. These must be removed or killed before the eggs hatch if the emerging larvae are to remain sterile.     
Eggs are collected on raw liver. Using a novel technique devised within the Unit, the eggs are cleansed and sterilised (Figure 7) under aseptic conditions using equipment more commonly used for the production of sterile pharmaceuticals.
The sterilised eggs are transferred aseptically into sterile flasks containing an appropriate substrate on which they will hatch. This substrate is formulated to maintain the viability of the larvae without allowing them to grow too rapidly. Provided they receive adequate oxygen, these newly emerged larvae may therefore be stored in a cool place for extended periods until they are ready for use or despatch.
Each batch of sterilised eggs is tested for sterility prior to use by incubating samples in Tryptone Soya Broth to detect the presence of aerobic organisms and Thioglycollate Broth to facilitate the growth of anaerobes.
Clinical use of larvae
Where possible, a tracing of the wound is prepared on a sterile plastic sheet. This is then cut out and transferred onto a suitably sized hydrocolloid dressing. The shape of the wound is cut from the hydrocolloid and discarded and the hydrocolloid sheet with the wound-sized hole is applied to the patient.
The young larvae, which are about 2 mm long, are then introduced into the wound. A sterile piece of a fine nylon mesh, a little larger than the wound but smaller than the hydrocolloid dressing, is then stuck to the back of the hydrocolloid using a suitable adhesive tape. A simple absorbent pad is applied to the outer surface of the net to contain any exudate or liquified necrotic tissue.
This dressing fulfills two important functions. It provides a sound base for the second component of the dressing system, and also protects the intact skin from the potent proteolytic enzymes produced by the larvae.
For practical reasons it is usually best to introduce the larvae into the wound once the hydrocolloid is in place and before the net is completely sealed down.
For difficult areas such as the toes, or very extensive wounds around the ankle, an alternative dressing technique has been developed. A piece of net is heat-sealed on two or three sides to form a sleeve or bag which is slipped over the appropriate part and stuck to a piece or pieces of hydrocolloid dressing placed around the wound to protect any areas of vulnerable skin. As before the bag or sleeve is covered with an absorbent dressing held in place with tape or a bandage as appropriate.
The outer absorbent dressing can be changed as often as required. Because the net is partially transparent, the activity of the larvae can be determined without removing the primary dressing.
The number of larvae used depends upon a number of factors. Robinson  suggested that an injured finger tip may require as few as 5-6 maggots, while a deep femur wound may require 500-600. As a rule of thumb, however, it has been recommended that no more than 10 larvae per cmÂ² should be introduced into a wound, fewer if the wound contains only a limited amount of necrotic tissue.
The larvae are generally removed from the wound after 3 days, but they may be left longer in some instances. Although the literature contains isolated references to the use of agents such as chloroform  to facilitate the removal of larvae, in our experience this is generally a straightforward process. Once the net is removed the majority of larvae will fall out of the wound or leave it of their own volition. These can be caught in a suitable receptacle or retrieved with a forceps or a gloved hand. Any individuals that remain can be gently removed manually or irrigated out of the wound with a jet of saline.
The larvae can either be retained for examination or placed with the dressing residues doubled wrapped in sealed plastic bags to await destruction by incineration.
In many instances, a single application of larvae is sufficient to effect complete debridement, but for particularly extensive wounds, further applications may be required.
Problems associated with Biosurgery
A review of the literature has revealed no significant risks or adverse events causally linked with the clinical use of sterile larvae of Lucilia sericata in manner described.
Contamination of a wound with pathogenic organisms as a result of using non-sterile larvae is the most serious potential problem but this risk may be eliminated by the introduction of the measures described previously.
There is also a theoretical possibility that a patient could develop an allergic reaction to the foreign protein of the larvae but such an effect has never been reported.
The larvae will not burrow under the skin or attack healthy tissue and there is no danger that they will stay within the wound and `breed'. A mature larva must leave the wound to pupate (the stage before it becomes an adult insect) or else it will die. Once a larvae is fully grown, therefore, it will come to the surface of the wound from where it is easily removed.
According to the literature, the principal disadvantage of larval therapy appears to be a tickling sensation. Even this should be eliminated, however, if the larvae are prevented from leaving the wound and migrating onto the surrounding skin by the use of an appropriate dressing system.
In one or two instances, it has been noted that the presence of large numbers of larvae in a relatively clean wound has caused bleeding, possibly as a result of proteolytic enzyme activity upon highly vascularised granulation tissue.
The use of fly larvae in wound management has a sound basis in the literature. It appears to be free of any serious or significant side effects, and may have major advantages over conventional treatments for certain types of wounds.
It is recognised that some patients, and indeed clinicians, may find the presence of `maggots' in a wound to be unacceptable. However, the results of a survey conducted in the USA published in the American Journal of Surgery in 1935 involving 605 surgeons and 5750 patient treatments, revealed that a favourable opinion on larval therapy was expressed by 552 individuals (91.2%) of those who took part. 
Experience gained locally indicates that although some patients and nursing staff may find the use of larvae unacceptable, the technique is much more readily accepted than might have been anticipated.
Provided that a specific patient has no objection to the use of larvae, therefore, there appear to be no ethical barriers to their widespread use.
Future developments in Biosurgery
Within the Unit, on a pilot basis, sterilised eggs have successfuly been reared through to bacteria-free (axenic) adult flies which in turn have been used to produce eggs free from microbiological contamination. If this process can be scaled up, it will eventually eliminate the need for a time consumming sterilisation procedure.
Laboratory studies are also underway to isolate and identify the enzyme systems and antimicrobial agents produced by larvae from different species of flies.
If Biosurgery is to become established as an accepted part of modern wound management, there is a need for hard scientific data to prove its efficacy. A randomised clinical trial designed to compare the effectiveness of larvae with conventional treatments in the management of necrotic wounds has just begun in South Wales.
Our experience with larval therapy in a range of wound types supports that of other workers in this area. Specifically we have found that compared with our personal experience with the use of hydrogel dressings and those of published studies, this technique appears to;
- Promote rapid cleansing of necrotic and sloughy wounds of all kinds,
- Control production of offensive wound odour produced by proteolytic bacteria,
- Prevent or control infection.
In addition it is our strong impression, yet to be confirmed in a controlled clinical trial, that larvae appear to stimulate the production of granulation tissue - an effect that has also been reported previously in the literature. Possible mechanisms for this effect are currently being sought.
Preliminary studies also suggest that larvae are able to eradicate antibiotic-resistant bacteria such as MRSA from infected wounds and this is also under investigation.
In many instances, all these benefits may be achieved whilst treating patients in their own homes or on an out-patient basis thus reducing or eliminating the costs associated with hospitalisation. Despite the understandable reluctance to adopt this technique that some patients and nursing staff exhibit, we believe that it has many advantages which make it worthy of serious consideration for the management of problem wounds which do not respond to more conventional treatments.
It should always be remembered, however, that larval therapy is a potent therapeutic tool and as such it must be used with caution by staff who have been properly trained in its use. Larvae produce very powerful enzymes which although primarily directed at dead or necrotic tissue, have the potential to cause irritation of healthy tissue if the larvae are applied in excessive numbers or left in place for too long after debridement has been completed. The use of large numbers of larvae in the immediate vicinity of exposed or damaged blood vessels is also probably best avoided.
This article is based upon a paper that was first published in the Journal of Wound Care and is reproduced here with the kind permission of the editor.
: "Introduction to the Diptera", R.W. Crosskey, In - Medical Insects and Arachnids. Edited by Richard P Lane and Roger W. Crosskey, Chapman & Hall, London., 1995
: "Diptera causing myiasis in man.", M.J.W. Hall, K.G.V. Smith, In - Medical Insects and Arachnids. Edited by Richard P Lane and Roger W. Crosskey, Chapman & Hall, London., 1995 , 429-469
: "Human myiasis in Australia", D.J. Lee , Med. J. Aust., 1968, 170-172, 1
: "House flies, Blow flies and other allies (Calyptrate Diptera)", R.W. Crosskey , Lane, In - Medical Insects and Arachnids. Edited by Richard P Lane and Roger W. Crosskey, Chapman & Hall, London., 1995, 403-428
: "Surgical Maggots.", E. Chernin , Southern Medical Journal, 1986, 1143-1145, 79(9)
: "The Treatment of Chronic Osteomyelitis with the Maggot (larva of", W.S. Baer , J. Bone and Joint Surg., 1931, 438-475, 13
: "Surgical Maggots-Lederle. Council on Pharmacy and Chemistry. New and nonofficial remedies.", JAMA., 1932, 401, 98
: "Maggots in the treatment of chronic osteomyelitis, infected wounds, and compound fractures", S.K. Livingston , Surg. Gynec. Obst., 1932, 702-706, 54
: "The Baer maggot treatment of osteomyelitis - Preliminary report of 26 cases", E.H. Wilson , C.A. Doan , D.F. Miller , J. Am. Med. Assn., 1932, 1149-1152, 98
: "The therapeutic active principle of maggots with a description of its clinical application in 567 cases", S.K. Livingstone , J. Bone Joint Surg., 1936, 751-756, 18
: "Maggots and their use in the treatment of chronic osteomyelitis", J Buchman , J.E. Blair , Surg. Gynec. Obstet., 1932, 177-190, 55
: "The rationale of the treatment of chronic osteomyelitis with special reference to maggot therapy", J. Buchman , Ann. Surg., 1934, 251-259, 99
: "Maggot Therapy - A rapid method of removing necrotic tissues", L.K. Ferguson , C.W. McLaughlin , Am. J. Surg., 1935, 72-84, 29
: "Maggot therapy for subacute mastoiditis.", K.L. Horn , Jr. Cobb A.H. , G.A. Gates , Arch Otolaryngol, 1976, 377-379, 102
: "A Biological, Bacteriological and Clinical Study of Larval or Maggot Therapy in the Treatment of Acute and Chronic Pyogenic Infections", G.C. Weil , R.J. Simon , W.R. Sweadner , Am. J. Surg., 1933, 36-48, 19
: "Maggot Therapy Revisited.", M.D. Bunkis , S. Gherini , R. Walton , West J Med, , 1985, 554-556, 142
: "The use of maggots in wound debridement", M.K. Reames, C. Christensen, E.A. Luce, Annals of Plastic Surgery, 1988, 388-391, 21
: "Maggot therapy: a review of the therapeutic applications of fly", R.S. Sherman, , E.A. Pechter, Medical and Veterinary Entomology, 1988, 225-230, 2
: "Myiasis: The Rise and Fall of Maggot Therapy", D. Morgan, Journal of Tissue Viability , 1995, 43-51, 5(2)
: "Stimulation of healing in non-healing wounds by allantoin occurring in maggot secretions and of wide biological distribution", W. Robinson , J. Bone Joint Surg., 1935, 267-271, 17
: "The healing properties of allantoin and urea discovered through the use of maggots in human wounds.", W. Robinson , Ann Rep Smithsonian institution, Washington, DC, US Government Printing Office., 1938, 451-460
: "An antibiotic from maggots", E.R. Pavillard , E.A. Wright , Nature, 1957, 916-917, 180
: "Isolation and identification of two antibacterial agents produced by a strain of proteus mirabilis isolated from larvae of the screwwork (Cochliomyia Hominivorax) (Diptera: Calliphoridae)", G.R. Erdmann, S.K.W. Khalil, J Med Entomol, 1986, 208-211, 23(2)
: "Maggot Debridement Therapy - An Alternative Treatment for Nonhealing Ulcers", S.R. Stoddard, , R.M. Sherman, , B.E. Mason, , D.J. Pelsang, Journal of the American Podiatric Medical Association, 1995, 218-221, 85(4)
: "Maggot Therapy for Treating Pressure Ulcers in Spinal Cord Injury Patients ", R.A. Sherman, , F. Wyle, , M. Vulpe., J Spinal Cord Med., 1995, 71-74, 18(2)
: "Proteolytic activity of blowfly larvae secretions in experimental burns.", L.M. Vistnes , R. Lee , G.A. Ksander , Surgery, 1981, 825-841, 90
: "The secretion of collagenase by maggots and its implication", S.E. Ziffren , H.E. Heist , N.A. May S.C. Womack , Ann. Surg., 1953, 932-934, 138
: "The cultivation and sterilization of the fly larva or maggot", G.C. Weil , J.P. Henry , S. Nettrour , R. Sweadner , West Virginia Med. J., 1931, 458-461, 27
: "A Method of Producing Sterile Blowfly Larvae for Surgical Use", F.F. Murdoch , T.L. Smart , U.S. Naval Med. Bull., 1931, 406-417, 29
: "Sterilization of blowfly eggs in the culture of surgical maggots for use in the treatment of pyogenic infections", S.W. Simmons , Am. J. Surg.,, 1934, 140-147, 25
: "Maggot Therapy - Technique and Clinical Application", A. Fine , H. Alexander , J. Bone Joint Surg., 1934, 572-582, 16
: "Sterilising procedures and Agents, Antibiotics and Inhibitors in Mass Rearing of Insects", B. Greenberg , Bulletin of the Entomological Society of America, 1970, 31-36, 16
: "Suggestions to facilitate the use of surgical maggots in suppurative infections", W. Robinson, Am. J. Surg.,, 1934, 525, 25
: "Progress of maggot therapy in the United States and Canada in the treatment of suppurative diseases", W. Robinson , Am J. Surg., 1935, 67-71, 29