VOL: 98, ISSUE: 36, PAGE NO: 45
Steve Thomas, BPharm, PhD, FRPharmS, is director, Biosurgical Research Unit, Princess of Wales Hospital, Bridgend, Wales
For hundreds of years it has been recognised that the opportunistic infestation by maggots of wounds sustained in battle can prevent the development of infection or septicaemia (Sherman et al, 2000). The wound cleansing properties of maggots was first utilised in western medicine by surgeons during the American Civil War. However, the technique did not become widely accepted in civilian medicine until after the First World War, during which the life-saving effects of maggot infestations were once again reported by military surgeons.
One fly species in particular, the common greenbottle Lucilia sericata, is attracted to human wounds of all types. When these creatures lay eggs on an exudate-soaked dressing the resulting hatchlings find their way into the wound and feed voraciously on any necrotic tissue present, cleansing the area and combating or preventing infection.
Larval therapy (also known as maggot therapy) using this species of fly was widely prevalent for about 20 years but fell into disuse in the early 1940s with the introduction of antibiotics. It was revived in the USA in the mid-1980s and in the UK in 1995 when the Biosurgical Research Unit (BRU) was established in South Wales. In the past seven years the popularity of the treatment has grown rapidly. Over 20,000 treatments have been supplied to over 1,000 centres, and over 50 publications now refer to the use of sterile maggots supplied by the BRU. Numerous reviews on larval therapy have also been published that describe the history of the technique and the mechanism of action (Sherman et al, 2000; Goldstein, 1932; Thomas et al, 1996a).
The effects of maggots and their secretions on the wound bed are complex and not fully understood. The principal action is that of debridement, which is due to the maggots’ production of collagenases and trypsin-like and chymotrypsin-like enzymes (Hobson, 1931; Ziffren et al, 1953; Pendola and Greenberg, 1975; Vistnes et al, 1981; Casu et al, 1994), which break down the necrotic tissue into a semi-liquid form the creatures can ingest. It has also been reported that maggot secretions appear able to destroy unhealthy or abnormal tissue, leaving healthy tissue in its place (Weil et al, 1933).
The second important action of maggots is their ability to kill or remove bacteria from wounds.
The early literature contains many references to the successful treatment of chronic or acutely infected soft tissue injuries, including abscesses (Weil et al, 1933), carbuncles (Fine and Alexander, 1934), leg ulcers (Ferguson and McLaughlin, 1935), pressure ulcers, mastoiditis (Horn et al, 1976), and compound fractures (Fine and Alexander, 1934). They were also used extensively for the treatment of osteomyelitis (Baer, 1931).
It is believed that maggots’ ability to combat wound infection is at least partly due to the antimicrobial nature of their secretions (Thomas et al, 1999a), but a further and possibly even more important mechanism is the ability of the actively feeding larvae to ingest bacteria and destroy them as they pass through their gut (Robinson and Norwood, 1934).
The third benefit associated with larval therapy is its reported ability to stimulate wound healing. First described by Larrey in 1832, this property has since been noted by other authors. Baer (1931) and Fine and Alexander (1934) were so convinced of the value of maggots in this regard that they continued to apply maggots even when debridement was complete in order to keep the wound clean and to promote healing.
Support for the stimulatory activity of maggots on granulation tissue may be drawn from the work of Prete (1997), who showed that maggot secretions stimulate fibroblast cells in culture.
Clinical effectiveness of larvae
Because larval therapy is frequently employed when other treatments have proved ineffective, controlled studies involving larval therapy are difficult to perform. Nevertheless, three studies have been published in which some element of control has been attempted. Two of these involve groups of patients (Sherman et al, 1995; Wayman et al, 2000), and in the third a patient with bilateral ulcers acted as his own control (Stoddard et al, 1995). Wayman et al (2000) also showed convincing evidence for the cost-effectiveness of sterile maggots compared with the use of hydrogel dressings.
Numerous other papers have described the use of maggots in a variety of wound types, including pressure ulcers (Sherman et al, 1995), leg ulcers (Thomas et al, 1996a), diabetic foot wounds (Rayman et al, 1998; Murray and Benbow, 1999), traumatic wounds (Thomas et al, 1996b), surgical wounds (Jones and Thomas, 2000), burns (Namias et al, 2000), an infected insect bite (Chaffrey, 1997) and necrotising fasciitis of the neck (Dunn et al, 2002).
Detailed clinical outcomes for 21 ambulant patients treated with maggots by 12 therapists were described by Sherman et al (2000), who reported that in 20 cases (95%) the therapists expressed satisfaction with treatment outcomes. No patient discontinued therapy because of discomfort or ineffectiveness.
Method of application
The standard technique recommended for relatively small wounds involves the initial application of a hydrocolloid sheet with a hole cut to the size and shape of the wound. This protects the intact skin from attack by the maggots’ enzymes. Once in place, the maggots are introduced into the wound and covered with a piece of sterile nylon net that is supplied with each container of larvae.
The net is fixed to the hydrocolloid using waterproof adhesive tape to form a ‘cage’ that retains the maggots in the wound. The pores in the net allow the maggots to breathe and facilitate drainage of liquefied necrotic tissue and serous exudate. The process is extremely simple, taking only a few moments to perform.
A layer of moist gauze is placed over the net to prevent the maggots from drying out in the early stages of their development. The dressing is then covered with a simple absorbent pad, held in place with adhesive tape or a bandage.
For areas that are more difficult to dress, such as the toes or feet, the nylon mesh is supplied heat-sealed into a bag or sleeve that is taped to strips of hydrocolloid dressing placed above and/or below the wound. For patients whose skin is too fragile to permit the use of adhesive dressings, or those who are allergic to hydrocolloid dressings, zinc oxide paste can be used to protect the skin and form a seal with the nylon net. Thomas et al (1996b) published an illustrated guide to the application process.
It is important to use the correct number of maggots, so a simple calculator has been devised that provides general guidance on the number of pots required, taking account of the size of the wound and amount of slough present (Thomas et al, 2001). Copies of this calculator are available on request from the BRU.
Disadvantages of larval therapy
Some patients and clinicians still find the idea of introducing live creatures into their wound unacceptable. It has been suggested that presenting them in some form of ‘tea-bag’ would overcome these aesthetic problems while facilitating application and removal of the maggots.
Recent studies have shown, however, that even on relatively flat or open wounds the feeding mechanisms and therefore the growth rate of maggots applied in this way are greatly impaired, compared with the ‘free-range’ variety, with important implications for their wound debriding activity (Thomas et al, 2002).
For cavity wounds, sinuses or wounds with a significant degree of undermining experience suggests that maggots in bags will do little to facilitate debridement or combat infection. The use of maggots in bags should therefore be restricted to situations where the application of free-range maggots is contraindicated, either by the nature or position of the wound. It should also be remembered that the wealth of clinical evidence for the value of maggots all relates to the use of free-range maggots and cannot be used to support maggots applied in net or foam bags.