Introduction

In traditional medicine, methods like rinsing, surgery, enzymatic debridement are among therapies for chronic wounds. But these methods have disadvantages such as pain, damage to healthy tissue, prolonged manipulation, etc [1].

An alternative therapy with fast and selective wound cleansing is maggot therapy [2]. These larvae (maggots) remove necrotic parts and bacteria from wounds. They also assist in forensic fields [3].

Lucilia sericata (L. sericata) belongs to blowfly species and Lucilia cuprina (L.cuprina) is also the L. sericata’s related species. Like L. sericata, L. cuprina has clinical usage³. Because of mass proliferation possibility, ease of cultivation, good viability and ability to withstand several damaging causes in part associated with necrocoprophagy, blow flies are important in physiological and ecological research [4].

Besides, one of the problems of some diseases such as diabetes mellitus is the wound infections. The process of wound healing is affected in diabetes mellitus. An approach in treating wound infection in diabetes mellitus is using L. sericata larvae. Studies have shown that these larvae can improve wound healing with different mechanisms [5].

In this study, different aspects of maggot therapy, for instance research results related to maggot therapy, providing favorable situation for maggots, influential factors in maggot therapy, and wound healing mechanisms of maggot therapy were reviewed.

Methodology

Search terms including «larval treatment», «maggot therapy», «larval therapy», and «L. sericata» were used. Google scholar was the search database. Articles that were included in this study were about maggot therapy, or contained information associated with the subject. Article’s information that was used in this review was up to and including the year 2021. These articles were original as well as review studies. Information about research findings related to maggot therapy, favorable situation for maggots, influential factors in maggot therapy, and wound healing mechanisms of maggot therapy was extracted from selected articles, and used for clarifying different aspects, and the importance of this underutilized therapeutic method.

Maggots and wound healing

A 2017 study of Tantawi et al. showed that number of bacteria in wounds significantly decrease after larval therapy. Their research was conducted on recumbent patients with pressure ulcer [6].

It has been observed that maggot therapy can reduce proinflammatory cytokines like TNF-alpha, IL-12. This acts against formation of proinflammatory macrophages and produce angiogenic growth factors which enhance wound healing [7].

It has been shown that maggot therapy has anti-MRSA properties in vitro and in clinical case reports [8]. Malekian et al. study found less growth of MRSA and Pseudomonas aeruginosa with maggot therapy in the case of refractory wounds [9].

Ability of L. sericata larvae to grow on necrotic wound shows that they can be appropriate to colonize contaminated environments. This theory is supported by Rat-tailed maggots life properties that can survive in highly contaminated aquatic habitats, which reflects the production of antimicrobial peptides in these maggots [10].

The concept of combining maggot debridement therapy (MDT) with genetic engineering to promote wound healing has been studied. It has been shown that transgenic L. sericata larvae can express and secret human platelet derived growth factor-BB (PDGF-BB). PDGF-BB of humans induces increase in the number of cells, promote treatment of wounds. Furthermore, it has been evaluated as a compound that can treat non-healing wounds [11].

L. sericata germ-line transformation using piggybac/vector helper combination has been reported. Facilitation in investigations on genes of L. sericata can be achieved through an efficacious transformation method [12].

By mechanisms like necrotic tissue removal and digestion, MDT promotes wound healing. Debridement is an important part of healing of wounds [13—15]. MDT can also increase the pH of environment of wound that can cause inhibition of bacteria growth [16]. Proteases of maggots have been shown to be up-regulated as a result of immune challenges [17].

These larvae don’t digest healthy tissues. It has been revealed that they ingest fluorescent bacteria in vitro [18].

Basicity of wounds that are treated with maggot along with derivatives of urea like allantoin, are supposed to stimulate healing of wound [19, 20]. Another mechanism of wound healing is inhibition of complementary activation [21].

Specific bacteria and pathways of their metabolism have role in growth of flies [21]. One bacterium of digestive tract of L. sericata is Proteus mirabilis that its sensitivity to various antibiotics was evaluated. Antibiogram sensitivities showed diversity [22].

It was shown that amputation decreased by 40—50% because of larval therapy [23]. In another research a woman with perforated intestine and a diffuse infection in abdomen which caused intestine wall damage, was referred. Because of dangers related to surgery using to remove dead parts of intestine, they applied larval therapy. After two days, larvae were removed. They found that no dead tissue left there and the patient recovered with no need for another surgery [24, 25].

Proteases that are secreted by maggots are responsible for degradation of extracellular matrix compounds such as fibronectin and laminin [13]. This causes the stimulation of cell division and reconstruction of extracellular matrix compounds [26].

The maggots of most of the fly species are invasive and can infect healthy tissues. Therefore, usually maggots of L. sericata or Phaenicia sericata (belong to Calliphoridae family) which remove dead tissues and cause no damage to healthy ones are applied for larval therapy [24, 27]. The maggots that used for larval therapy breed in a humid condition.

Young maggots should either be used within 8 hours or be kept in refrigerator between 8 to 10°C to decrease metabolic rate [13]. Maximum activity of maggots is achieved by providing a condition that results in optimum body temperature and supplies needed oxygen and humidity.

Excessive humidity can kill maggots. Survival and growth of maggots can be decreased by hydrogel dressing and propylene glycol, but growth of maggots is not affected by systematic antibiotics [13]. The maggot dressing can occasionally become loose, especially if being left for 48 hours. The maggots, if not collected, become pupae and get out of their place as adult flies 1—2 weeks later. The flies and the maggots that were used before are regarded as factors for causing infection. Thus, the maggots should be collected at the end of the therapeutic cycles and discarded [25]. Drying of maggots is the most common cause of maggot therapy failure. Usage of a pad moistened with saline solution on top of the dressing can prevent drying. Existence of eschar covering the entire surface of wound also dry the maggots as a result of blocking the maggots’ access to the wound below the eschar. To solve this problem, we can soften the edges of eschars with hydrogel dressing a few days before the maggot therapy [24].

Because of risk of septicemia in use of maggots improperly and with no precautions [28], 20% sugar solution is given to the maggots to be applied in therapy. For stimulation of ovulation, pieces of liver and meat are used. The eggs are separated and disinfected and by a combination of 3% Bakto agar and crushed live put to sterile media. The maggots are removed from medium after approximately 24 to 36 hours and transferred to sterile container for therapeutic use. At 5—8°C, the maggots can live up to five days with no loss of vitality [29].

Pain in maggot therapy is likely because of proteolytic enzymes which have effect on nerve endings and larvae’s movement on wound surface [30]. Pain at the wound site is the most common complication of maggot therapy and starts when the maggots reach the age of 30 hours [31]. Pain can be due to stimulation of nerves and painkiller use or maggot removing are methods for relieving. After two or three days, the wound becomes smelly and watery [32].

There has been the suggestion that phantom pain is related to maggot therapy [33]. Furthermore, other than wounds, maggot therapy is helpful in treatment of inflammation of temporal lobe, face necrotic tumors and genital tract gangrene [32].

Removing of dead tissue using non-surgical methods is slower than surgical ones, however, At the time that surgery is not wanted, maggot therapy is more effective and better alternative method [32]. In a study, it was found that all of the wounds created after the injury were healed by maggot therapy but joint infection wounds were not treated and half of them ended up with amputation [23].

Orkiszewski stated that maggot therapy is the best choice in leg ulcers, pressure ulcers, carbuncles and infected traumatic wounds [34]. Moreover, a number of studies showed successfulness of maggot therapy on animals [35].

There is controversy about maggot therapy and it is usually not preferred by clinicians for cleaning surgical wounds, but, this method is a safe, cheap and simple therapy with good outcomes [25, 36]. The amount of removed necrotic tissue by maggots in 24 hours is up to 25mg [36]. The maggots are left on the wounds usually from48 hours to 4 days [38, 39].

In maggot therapy, L. sericata rather than L. cuprina is used and the reason is that the former is regarded safer; however, there are studies that showed safety and effectiveness of L. cuprina [40, 41]. It has been reported that some of other flies (Calliphora vicina, Calliphora vomitoria, Phormia regina, Chrysomya albiceps, Sarcophaga carnaria and Hermetia illucens) possess favorable properties [29]. Besides L. sericata and L. cuprina, other flies have shown effectiveness in wound healing, e.g. Lucilia caesar, Phornia regina, Calliphour erythrocephala, Cynomyia eachverina and Lucilia eximia [42].

Wound debridement by maggots before surgery was observed to decrease rates of infection after surgery [31]. Additionally, in people with resistance to treatment with meglumine antimonate, larval therapy was shown to be effective by enhancement of healing of cutaneous lesions and less time to achieve it [43].

Application of larval treatment when the number of patients who require treatment is overwhelming and resources for therapy are limited (for example in low-income countries), can be very useful [36]. Recently, larval treatment has been applied for healing fungating wound related to Kaposi’s sarcoma [44]. However, the regeneration of skin applying L. sericata larvae is unstable and over the period of time with MDT it slows [45].

Effects of some factors on maggot therapy

Achieving an adequate response in use of larval therapy is only possible when the rearrangement of microflora in wound occur [46].

It was found that one week was needed for debridement of this eschars and it took two weeks for thicker eschars regardless of origin of ulcer [47].

In a study, using maggot therapy, the time for debridement and achievement of that was affected by area of ulcer and duration of wound [48]. In another study of 116 wounds, gender, diabetes mellitus and smoking didn’t seem to change the result of maggot therapy [23]. In spite of that, in some research it has been observed that diabetes mellitus, chronic limb ischemia and septic arthritis made debridement and healing duration longer [49].

Moreover, Kecici et al. did not find that age had any effect on maggot therapy success [48]. But in another study, age (6 years or older) was seemed to have negative effect on wound healing by maggot therapy [23]. Some evidence shows that more amount of larval extractions is needed when bacteria are growing [9].

Sensitivity pattern of symbiotic bacteria may be important in healing of wound and formulating rational antibiotic policy [21].

It has been shown that microbial volatile organic compounds (MVOCs) have role in activation regulation of blow flies; and these compounds include dimethyl disulfide, dimethyl trisulfide, ethanethiol, indole, isobutylamine, p-cresol, phenol, phenylacetic acid, skatole, together with phenylacetaldehyde [50—54]. Type of MVOCs produced by bacteria and response manner of flies are dependent on strain and bacterium [55].

Cruz-Saavedra et al. showed that no significant difference existed between L. sericata and Sarconesiopsis magellanica in parameters for evaluation of Leishmania panamensis related ulcer cicatriation [42].

24 hours has been found to be the best period of therapy with extract and after 72 hours it showed inhibitory effect. 12.5 mg per ml is the best concentration for therapy and higher concentrations are related to toxicity and cell death [56]. Besides, enzymatic debridement will probably be unsuccessful in patients with acidic exudate, because debridement enzymes activity decreases in acidic pH [57].

A formulation that presented in Telford et al. study, showed its adequacy as coating for non-level, level or hollowed areas with no retraction or running off. Lucilia enzymes are inherently resistant to decomposition by inflammatory enzymes of humans [58], and inhibition of maggot enzymes is less than bovine or human similar enzymes [59].

Conclusion

Impact of larval treatment on wound healing has been investigated and efficacy of this method has been shown. Diabetic ulcers are one of important types of wounds that can be treated with maggot therapy.

Maggot therapy has several advantages. One of them is the existence of different mechanisms associated with wound therapy. Secretion of antimicrobial compounds, reduction of pro-inflammatory cytokines and production of pro-angiogenic growth factors, ingestion of bacteria, increasing PH, inhibition of complementary activation, stimulation of cell division and reconstruction of extracellular matrix compounds have role in wound healing.

Besides, maggots utilized for larval treatment do not digest healthy tissues and selectively remove dead tissues. Additionally, maggot therapy is a safe, cheap and fast method.

Larval treatment may be especially useful when the number of patients who require treatment is overwhelming and resources for therapy are limited.

Moreover, genetic engineering is a promising approach for enhancing beneficial properties of maggots.

Altogether maggot therapy can be applied as a favorable alternative method for wound healing.