Overview

The maggots' ability to prevent infections and promote wound healing has been known since the 19th century. Increasing problems with treatment resistant wounds and antibiotic resistant bacteria has re-aroused interest in this therapy. Maggot therapy is the medical use of disinfected fly larvae (usually Lucilia sericata) to treat wounds. The maggots work by debriding wounds by dissolving necrotic tissue, clean wounds by killing bacteria and promote wound healing. They have broad antibacterial action against gram-negative and gram-positive bacteria, including MRSA. It is used to debride complicated skin and soft tissue wounds like pressure ulcers, venous stasis ulcers, traumatic wounds and burns and is used to treat osteomyelitis (47).

 

They are usually applied for 3 days but more applications may be required. Introduction of LarvE biobags has meant that the larvae are contained and cause less distress to the patient. They also don't have to be ‘counted’ in and out. Example: LarvE. Indications: Necrotic, sloughy, malodorous and infected tissues, chronic ulcers. Pros: Rapid healing. Cons: Low patient acceptability. Notes: inappropriate for wounds where the blood supply is insufficient to permit healing to take place and malignant fungating wounds (48). Maggot therapy has not been associated with serious side effects (47).

 

Description of System

Larval therapy has been used to treat a wide spectrum of wounds including necrotising fasciitis, traumatic necrotic leg wounds, primary burns, pressure sores and digital amputations in diabetic feet. The wounds are successfully debrided of devitalised tissue and granulation tissue develops rapidly. Anecdotal evidence has consistently suggested that larval therapy results in a reduction in wound pain and odour, and promotes the healing process with relatively few side effects. The most commonly mentioned disadvantage of larval therapy is the negative perception from patients and practitioners (48).

 

Although the so-called ‘yuk factor’ of its clinical appearance is often reported in case studies, there is little evidence that patients refuse larval therapy when it is offered. Pain has occasionally been reported by patients suffering from ischaemic wounds, perhaps due to the sharp mouth hooks and spicules with which larvae anchor themselves onto tissue. Contraction of necrotic tissue or pH changes within a wound may affect pain receptors in healthy tissues nearby. Occasionally, inflammation of adjacent tissue may pose problems for adherence of dressings, and treatment should be delayed until inflammation has subsided. Several authors have proposed that skin surrounding the wound should be protected with hydrocolloids or zinc paste to prevent possible damage from potent proteolytic enzymes in larval secretions. No occurrences of allergic reaction have been recorded, but blood infections (with Providencia stuartii and Candida albicans) have been reported where larvae of Protophormia terraenovae were used. Alteration of the disinfection process appeared to eliminate this problem, with no further cases of sepsis occurring (48).

 

Mechanisms Involved

Wound debridement:  Larvae feed on necrotic tissue, cellular debris and exudate within the wound, thus debriding it of devitalised tissue. In many randomised controlled clinical trials, researchers noted that significantly more wounds healed with frequent debridement, regardless of the topical preparations used. The digestive juices secreted by larvae during feeding, contain proteolytic enzymes, including trypsin-like and chymotrypsin- like enzymes and collagenase. The enzymes selectively debride necrotic tissue, leaving viable tissue unharmed (48).

 

Wound disinfection: Chronic wounds are often colonised and infected with wound flora, including Staphylococcus, P. aeruginosa and E. coli. Increased bacterial burden may impair healing, particularly if a wound becomes infected with antimicrobial resistant bacteria. Disinfection is therefore critical for wound healing. The natural habitats of larvae include corpses and wounds, which typically contain an array of pathogenic microorganisms. In response to these conditions larvae are believed to have evolved several effective mechanisms for removing bacteria. During feeding, larvae ingest bacteria within devitalised tissue thus physically removing microorganisms. Research has suggested that any bacteria which is not destroyed in the acidic alimentary canal are held in a tubular structure called the peritrophic membrane, preventing recontamination.

 

Movement of larvae may stimulate the production of serous exudate by the wound, thus increasing irrigation and removing bacteria, or wounds may be physically irrigated by actual larval secretions.

Early research has shown that larval secretions contain a variety of alkaline components, including ammonium bicarbonate, calcium and urea that inhibit bacterial growth. The subsequent increase in pH provides an optimum environment for enzymatic activity, and also renders the wound bed uninhabitable to many bacteria (48).

 

It has been proposed that larvae release antimicrobial substances as part of an innate response to high levels of bacteria. In vitro research isolated a protease resistant, thermally stable compound from larval secretions, which exhibited strong antibacterial activity. Some antibacterial compounds isolated, e.g. phenylacetic acid are thought to be released by Proteus mirabilis, a commensal species of bacteria found within the larval alimentary canal. The symbiotic relationship between larvae and particular bacterial species appears to facilitate wound disinfection, but further research is required into exactly how.

 

Following treatment with larvae, remittance is greatly reduced because of an increased quantity of granulation tissue within the wound bed. Therefore, larval secretions have a positive effect on wound healing because of the development of granulation tissue and increased tissue oxygenation. The properties of substances within larval secretions, described as a healing ‘active principle ‘have been the subject of subsequent research and suggested that the secretion of allantoin, ammonium bicarbonate and urea provide an optimal growth environment for cells involved in wound healing by acting as growth factors. The alkaline nature of these substances has been reported to have a role in the promotion of healing by altering the pH of the wound (48).

 

Costs and Evidence

It is expensive compared with conventional wound dressings. A study investigated the efficacy and cost-effectiveness of larval therapy vs. hydrogel, and reported that all wounds treated with larval therapy were successfully debrided following one application at a median cost of £78.64. Treatment with hydrogel was proven to be less efficient where it was noted that, following 1 month of treatment, one-third of wounds still continued to require treatment. The use of larval therapy often resulted in quicker healing, and a subsequent reduction of nursing time and materials. A further advantage of larval therapy is that, as larvae are typically applied for 3 days, wounds are disturbed less frequently than conventional dressings that require changing every 1–2 days.

 

Treatment can usually be carried out in outpatient and community settings. A study at an outpatient wound clinic on chronic wounds of varying aetiologies reported that using larval therapy resulted in a 62% decrease in the need for amputation. In a comparative study of chronic wounds of multiple aetiologies, it was reported that all wounds healed following 4 weeks of treatment with larval therapy, whereas necrotic tissue was still present on the surface of conventionally treated wounds following 5 weeks (48).