REVIEW OF RELATED LITERATURE

Fly maggots have been known for centuries to debride and heal wounds. Maggot

debridement therapy, also known as MDT, is a type of biotherapy involving the

intentional introduction of freshly emerged, sterile fly larvae into the non-healing skin

and soft tissue wounds of humans or animals for the purpose of selectively cleaning out

only the necrotic tissue within a wound in order to promote healing. It is an important

adjunct to conventional medicine in its application for the treatment of chronic wounds,

such as leg ulcers, pressure sores, diabetic and necrotic ulcers, as well as infected surgical

wounds, burns and trauma injuries1. Maggot therapy is gaining recognition around the

world among medical practitioners and patients since it is a simple, efficient, safe and

cost-effective tool for the treatment of wounds and ulcers unresponsive to conventional

treatment and surgical intervention.

Wound healing is the body's natural mechanism of regenerating dermal and

epidermal tissue. It is an interactive process involving soluble mediators, extracellular

matrix components, resident cells (keratinocytes, fibroblasts, endothelial cells, nerve

cells), and infiltrating leukocyte subtypes, which participate differentially in the three

phases of wound healing: inflammation, tissue formation, and tissue remodeling2---that

overlap in time.

Tissue injury results in the disruption of blood vessels and extravasation of blood

constituents. In the inflammatory phase, blood clotting takes place which reestablishes

hemostasis, or stop blood loss, and various factors. The clot also provides a provisional

extracellular matrix for cell migration. Further, platelets secrete several mediators of

wound healing (i.e. platelet-derived growth factor) that attract and activate macrophages

and fibrolasts that phagocytise debris, bacteria, and damaged tissue and successively

release factors that initiate the proliferative phase of wound healing3. The proliferative

phase or tissue formation begins when fibroblasts begin to enter the wound site,

approximately two to three days after the wound occurs. Neovascularization, fibroplasia

and granulation tissue formation, collagen deposition, reepithelialization and wound

contraction are all part of this phase. The formation of granulation tissue in an open

wound sets the pace for reepithelialization. Basal keratinocytes from the wound edges

and dermal appendages advance in a sheet across the wound site and proliferate from the

wound edges towards the middle. Keratinocytes migrate without necessarily proliferating

at first. However, epithelial cells require viable tissue for them to migrate across. Repair

of skin is thought to be influenced by growth factors such as TGF-β1, and by adhesion

molecules such as integrins12. Finally, tissue remodeling is said to have begun when the

levels of collagen production and degradation equalize, that is, type III collagen is

gradually degraded while type I collagen is laid down in place of type III. As the phase

progresses, the tensile strength of the wound increases, ultimately becoming as much as

80% as strong as normal tissue.

Debridement is the process of removing non-living (necrotic) tissue from pressure

ulcers, burns, and other wounds thus allowing the wound to heal faster. Necrotic tissue is

a good medium for bacterial colonization leading to inflammation and hampering the

body's ability to fight infection. It may also hide abscesses which can lead to spread of

infection that may lead to amputation or death. Example of such opportunistic bacteria is

the Pseudomonas aeruginosa, an opportunistic pathogen, which exploits any break in the

host defenses to initiate an infection.  The common predisposing factors are breakdown

of the integument due to burns, trauma or dermatitis and high moisture conditions such as

those found in the ear of swimmers. It causes urinary tract infections, respiratory system

infections, dermatitis, localized and diffused skin infections, soft tissue infections,

bacteremia, bone and joint infections, gastrointestinal infections and a variety of systemic

infections, particularly in patients with severe burns and in cancer and AIDS patients who

are immunosuppressed.5 Adding to its pathogenicity, this bacterium has minimal

nutritional requirements and can tolerate a wide variety of physical conditions4.

Pseudomonas aeruginosa is primarily a nosocomial pathogen. According to the CDC, the

overall incidence of P. aeruginosa infections in US hospitals averages about 0.4 percent

(4 per 1000 discharges), and the bacterium is the fourth most commonly-isolated

nosocomial pathogen accounting for 10.1 percent of all hospital-acquired infections.5

Pseudomonas aeruginosa is frequently resistant to commonly used antibiotics. 5

Only a few are effective against Pseudomonas and these include fluoroquinolones,

gentamicin and imipenem. The fluoroquinolones are divided into 2 groups, based on

antimicrobial spectrum and pharmacology: the older group includes ciprofloxacin,

norfloxacin, and ofloxacin, and the newer group, gatifloxacin, gemifloxacin,

levofloxacin, moxifloxacin, and trovafloxacin. Ciprofloxacin is particularly effective

against Pseudomonas aeruginosa.28

In MDT, maggots basically have three fundamental actions involved in wound

healing, namely: debridement, disinfection, and enhancement of healing or tissue

growth6. Maggots are applied to the wound as they digest necrotic tissue and pathogenic

bacteria. They are highly precise in debriding only necrotic tissue over one or two days

and derive nutrients through a process known as "extracorporeal digestion." They secrete

a broad spectrum of substances, including allantoin, urea, phenylacetic acid,

phenylacetaldehyde, calcium carbonate and proteolytic enzymes11, which have

antimicrobial properties and liquefy necrotic tissue thus allowing maggots to absorb dead

tissue in a semi-liquid form over the course of several days. Also, extracts of Lucilia

sericata contained p-hydroxybenzoic acid, p-hydroxyphenylacetic acid and octahydro-

dipyrrolo[1,2-a;1',2'-d] pyrazine-5,10-dione molecules which showed antibacterial

activity against Micrococcus luteus and Pseudomonas aeruginosa13. However, studies by

Robinson and Norwood, and Mumcuoglu et. al., found out that disinfection was more of

a function of larval ingestion of wound bacteria which are killed as they pass through the

maggot's digestive tract14,15 as opposed to the phenylacetic acid and phenylacetaldehyde

secretions of Proteus mirabilis, commensals present in the midgut of Phaenicia sericata.

Also, the excretion of ammonia, a waste product, by Phaenicia sericata was also believed

to be responsible for combating bacterial infections by increasing the pH resulting to an

alkaline condition unfavorable for many bacterial species1.

In an optimum wound environment, maggots molt twice, increasing in length

from 1-2 mm to 8-10 mm, within a period of 3-4 days leading to the efficient removal of

necrotic tissue and simultaneous disinfection of the affected area. Maggots used in

maggot therapy do not damage healthy tissue16. They precisely operate at the junction

between healthy and necrotic tissue.

Early theories suggest that maggots’ crawling motion enhance wound healing

through physical stimulation of viable tissue in the wound and also oxygenation in

chronic wound. It is also suggested that the actions of allantoin (2,5-Dioxo-4-

imadazolidinyl urea) or ammonia bicarbonate could be responsible for the abundant

growth of granulation tissue as demonstrated by the study of Robins on Lucilia17. They

excrete their nitrogenous waste as 10% allantoin and 90% ammonia which stimulated

growth of local granulation tissue due to the increase in wound pH from acid to neutral or

slightly alkaline at pH 7 or 8. Also, Robinson performed successful clinical tests using 1–

2% solutions of ammonia carbonate and ammonia bicarbonate elucidating the promotion

of healing in purulent and indolent wounds1.

The most commonly used maggot species in MDT are Phaenicia sericata, and

Lucilia sericata, since they are the ones capable of digesting necrotic tissue while others

invade healthy ones.

Phaenicia sericata is a common species of yellowish or metallic green blowfly

(family Calliphoridae, order Diptera). It is an abundant scavenger feeding on carrion or

excrement and lays their eggs almost exclusively in dead or rotting flesh. They are

usually the first insects attracted to a fresh carcass, sometimes within minutes of death

because of the organic odors of decomposition. The eggs are most often laid around

natural body orifices or open wounds. Tiny maggots hatch from eggs in 6 to 48 hours and

produce a mixture of proteolytic enzymes including collagenase that breaks down the

dead tissue to a semi-liquid form which is then reabsorbed and digested. The larvae tend

to congregate into groups and feed, initially on small defects in the tissue10. The larvae

increase in size very rapidly. They develop through three instars on carrion for 3 to 9

days before leaving the food source to pupate in soil. After 2 to 7 days in a prepupal

stage, they form a puparium from their last larval stage skin. A fourth larval stage occurs

within the puparium before pupation. Adult flies emerge 10 to 17 days after the formation

of the puparium. Development from egg to adult occurs in 16 to 35 days, and is

dependent on temperature and environmental conditions12,18.

There are several ways to breed maggots often it involves exposing raw meat to

open air for several hours. After mating, a female fly is highly attracted to meat by her

sensitive scent organs. She lays her off-white eggs in clusters (25-500) near a wound or

opening in the flesh. Larvae length can range from 2mm (one day old) and almost

transparent to 1 cm (3-4 days old)19. Larvae reach their full size within five days to a

week. During this period, maggot skins change from being translucent to a bright red to a

grey hue of its insides, then finally to a creamy white. The maggots are available for use

when a spot of black is visible at the rear end of the larva20. Upon reaching full size, or

when the available food supply is exhausted, the maggots leave the meat, clean

themselves and prepare for pupation. The maggots contract and cease moving. Their

outer skin hardens into a crisp shell, turning from a creamy white hue to a darker red-

brown one. After a few weeks of pupation, the adult fly inside the cocoon hatches and

metamorphosis is complete. Ideally, there should be minimal disturbance during the

breeding process.