Maggot Debridement Therapy (MDT) is the medical use of live maggots (fly larvae) for cleaning non-healing wounds. Medicinal maggots have three actions: 1) they debride (clean) wounds by dissolving the dead (necrotic), infected tissue; 2) they disinfect the wound, by killing bacteria; and 3) they stimulate wound healing.
Maggot debridement therapy Larvae used for MDT need to be sterile to prevent contamination, and therefore must be bred in a controlled sterile, moist environment. Newborn larvae should be used within 8 hours or stored in a refrigerator at 8º to 10ºC, so as to slow their metabolism. To maximise debridement, it is important to ensure an optimal body temperature, adequate oxygen supply and moisture, though too much moisture may kill the larvae. The use of occlusive dressings should be avoided, as larvae require oxygen to survive. Propylene glycol from hydrogel dressings can limit the growth and viability of larvae, while systemic antibiotics do not affect larval development.
Mechanisms of maggot debridement Therapy
Debridement
Scientists first postulated that the debriding action of maggots was due to their mechanical wriggling. Maggots use a pair of mandibles/hooks for movement and attachment, and it was believed that the probing from the hooks may facilitate wound debridement. Recently, three proteolytic enzyme classes have been identified in the maggot excretions/ secretions (ES). These enzymes effectively degrade extracellular matrix components, including laminin and fibronectin. The ES could thus assist in the digestion of the wound matrix, leading to effective debridement.
Disinfection
The presence of antibacterial substances has long been identified in maggot ES. Thus, ES has an inhibitory effect on Gram-positive and Gram-negative bacteria, including methicillin-sensitive Staphylococcus aureus, methicillin-resistant S aureus (MRSA), Escherichia coli and Pseudomonas aeruginosa. This activity of the ES was thermally stable and protease-resistant. Using ultrafiltration, the latter study identified two fractions with inhibitory effects on S aureus and MRSA. It was also believed that ammonia excreted by maggots increases wound pH, thereby creating an unfavourable environment for bacterial growth. According to another hypothesis, maggots ingest bacteria and kill them within their alimentary tract. Mumcuoglu et al investigated the fate of E coli after ingestion by L sericata. It transpired that the proximal alimentary canal was more heavily infested, with 67% of the throat harbouring live bacteria. The figures dropped dramatically as the bacteria moved distally along their alimentary canal. Only 18% of the posterior hindgut was found to contain live bacteria.
Enhancement of wound healing
It was believed that the enhancement in tissue growth was due to an increase in fibroblast proliferation brought about by the ES. Horobin et al demonstrated that the ES altered fibroblast adhesions to collagen and fibronectin, and it was subsequently shown that it increased the migration (but not proliferation) of fibroblasts. This was attributed mainly to the action of serine and metallo proteinases. These investigators then developed a three-dimensional model to better simulate a human wound.Their results were consistent with the previous studies and supported by later investigators. An upregulation oftyrosine phosphorylation was also detected, which possibly enhanced the motility of the fibroblasts. Others have postulated that maggots secreted cytokines, which help wound healing. High levels of gamma-interferon and interleukin-10 (IL-10) were found in the ES, but as to whether these cytokines are responsible for increasing granulation requires further investigation.
Indications for maggot debridement therapy
Maggot debridement therapy is mainly used for the cleaning and disinfection of chronic wounds that are sloughy, necrotic, and infected. Various clinical studies have demonstrated the efficacy of MDT in treating wounds that fail to heal following alternative forms of treatment. Larvae were effective in removing necrotic tissue and exudation without damaging adjacent healthy tissue. This action stimulated tissue granulation and reduced offensive odours brought about by infections. Compared to conventional hydrogel therapy, MDT was more effective for chronic venous ulcers, diabetic ulcers, and pressure ulcers
Contra-indications and side-effects
Dry wounds are a relative contra-indication as maggots require a moist environment. The use of maggots should also be avoided in open wounds of body cavities or wounds in close proximity to large blood vessels so as to facilitate the removal of the larvae. Also, maggots should not be used in patients who are allergic to eggs, soybeans, or fly larvae. Maggot debridement therapy has not been associated with major adverse effects or complications, but has been reported to cause mild discomfort, malodour at the first change of dressing, and escape of larvae. Excessive pressure applied on to parts of the wound may also kill maggots in that area, leading to uneven wound debridement. Despite
these shortcomings, even patients experiencing pain during treatment tend to report improved appearance.
Advantages and disadvantages of maggot debridement therapy (“MDT")
- Efficacy, as demonstrated in several small but significant controlled clinical studies.
- Takes about 15-30 minutes to apply a secure dressing to keep the maggots in place.
- Maggots are highly perishable and should be used within 24 hours of arrival.
- Excellent safety record.
- Simple enough that non-surgeons can use it to provide thorough debridement when surgery is not available or is not the optimal choice. This means that it is also possible to provide surgical quality debridement as an outpatient or in the home.
- Low cost of treatment.
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