Sample Review of Related Literature MDT

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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 healing 3. 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-1, 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 octahydrodipyrrolo[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.

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