Myiasis is a parasitic disease caused by the larvae of botflies (Diptera hominis) infesting skin wounds and mucous membranes such as the eyes, ears, mouth and nasal cavity. This can cause severe infection, pain, odor, discharge and psychosocial distress.
Myiasis can be prevented by proper wound care, personal hygiene and screening for flies in endemic areas. Oral ivermectin and albendazole have proved effective in the treatment of myiasis.
Symptoms
Myiasis refers to the infestation of a living animal by fly (dipterous) larvae, which feed on the living tissue or liquid body substances and/or ingested food of the host [1]. It can occur in all vertebrate animals including humans. The infestation can be classified as either “faculative” or “obligatory” [2]. The former refers to larvae that enter a human or animal voluntarily through open wounds, natural orifices, and other traumatic sites, whereas the latter occurs due to ingestion of the larvae from dead or decaying meat or other organic substrates, or even spoiled food.
Myiosis is a common problem in people traveling to developing or tropical countries, especially Africa and South America. It occurs due to a combination of factors, including poverty, lack of hygiene, advanced age, psychiatric illness and physical disabilities that limit an individual’s ability to discourage flies. Wound myiasis is most common, involving the cutaneous (skin) and mucous membranes such as oral, genital and nasal. It is also seen in the ear and eye sockets.
Flies such as Dermatobia anthropophaga, Cordylobia rodhaini and others lay their eggs on damp fabric or on the ground. The larvae penetrate the skin by means of phoresis, which involves the action of the fly’s mouthparts and the retraction of the outer covering of the egg to expose a pore.
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The flies infest the wounded skin and eventually form maggots, which grow to be the size of a pea. Initially, the maggots are not visible and only a small bump is present that may resemble a pimple or a bacterial furuncle. Later the bump enlarges and drains clear or yellowish fluid. Itching and the sensation of movement are often reported by patients, as well as stabbing pain, which can be severe in some cases.
Oral myiasis is usually caused by the oviparous fly (Cynomolgus) O. ovis and affects domestic sheep, goats and some wild ruminants worldwide. It can be a serious health problem in these animals, as it causes the clinical condition known as oestrosis, in which it results in reduced mental status and extrapyramidal symptoms such as salivation and a false sense of movement.
Diagnosis
Myiasis is the infestation of live human or vertebrate animals with fly (dipterous) larvae, which at least for a time, feed on dead or living tissue of the host. Depending on the relationship between the infesting fly species and its host, myiasis may be obligatory or facultative. Obligatory myiasis occurs when the flies lay their eggs in or on live tissue, while facultative myiasis happens when the eggs or larvae reach living tissues by opportunistic penetration through natural orifices, traumatic wounds, or ingested food. The infection can affect any organ system including the digestive tract, genitals, respiratory tract, skin, lungs, heart, arteries, blood vessels, and muscles.
Flies are the most common cause of myiasis in humans. The primary dipteran families responsible are botflies (Oestridae), blowflies (Calliphoridae), and flesh flies (Sarcophagidae). There are about 100 myiasis causing fly species.
People are more likely to develop myiasis when they are unable to keep their skin clean and covered, especially in poor countries. They are also more likely to develop myiasis if they have open wounds that are not properly cared for, particularly indigent people who have limited access to health services and resources. Poor sanitation is a significant risk factor for myiasis because it leads to exposed organic waste and attracts flies.
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The most important step in the diagnosis of myiasis is to determine the specific infesting flies that are present. This can be done by examination of the lesions and/or obtaining larvae for microscopy and culture. In addition, blood tests (such as erythrocyte sedimentation rate, peripheral eosinophilia, and immunoglobulin E) and imaging studies (MRI or CT scan) can be helpful.
Manual extraction of the larva is the most common treatment of cutaneous myiasis. However, it can be difficult because of the tapered shape and rows of spines that are on the body segments. The insertion of an occlusive agent (petrolatum, animal fat, bacon, hair gel, beeswax, mineral oil, or even turpentine solution) over the lesion enables the larva to be crushed and killed or pulled upward and removed. Oral or intravenous ivermectin has been used to kill the larvae and make their extraction easier. Other treatments include antibiotics, antiseptic dressings, thorough cleansing and debridement, and tetanus immunization.
Treatment
There are a number of treatment options for myiasis. In cases of severe infection suffocation can be used, where a doctor covers the affected area with petroleum jelly and other gel like substances that suffocate the parasites. This is a relatively safe procedure that can save lives in the most severe infections. Some patients may also be prescribed medications such as Ivermectin which can be used to treat this condition. However, surgery is generally the best option for treating myiasis.
Wound myiasis occurs when fly larvae infest open wounds in a living host, especially in mucous membranes (nasal, oral, and vaginal) and body cavity openings (ears, eyes, and nose). Infection is often accompanied by pain, itching, and bleeding from the site of the infestation, as well as fever, chills, and headache. Other symptoms include systemic inflammation (e.g., increased eosinophilia and peripheral neutrophilia), massive tissue destruction, the formation of granulomas, and secondary infection.
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Infection with tumbu fly (Cordylobia anthropophaga) and human botfly (Dermatobia hominis) is common in tropical areas and among travelers to these regions. Infection with the screwworm flies Lucilia caesar and Phaenicia sericata is less frequent. These flies breed in carrion and are known to carry the deadly Clostridium botulinum bacteria. Infections with these flies have been reported in people who have eaten the maggots of Lucilia caesar or Phaenicia sericata or who have inhaled the larvae from a decaying carcass.
Cutaneous myiasis is associated with poor social conditions, advanced age, psychiatric illness, and alcoholism in addition to physical disabilities that restrict the ability to discourage flies. It is more common in men who work outdoors, where a lack of clothing can lead to exposure to insects. Infection can also occur in the context of traumatic injuries, particularly head trauma, which is often accompanied by loss of consciousness and subsequent brain damage.
Treatment focuses on the removal of the larvae, surgical debridement of the infected tissues, intensive rinsing with antiseptic solutions, and wound dressing changes on a daily basis. A layer of petrolatum over the wound helps to immobilize the maggots, facilitating extraction. In some cases, the use of a 15% solution of chloroform in olive oil or another oil or ether can facilitate maggot removal.
Prevention
Intestinal myiasis is a common problem in countries with poor socioeconomic conditions. It is probably due to the ingestion of food or water contaminated with dipteran fly larvae. The flies are commonly found in these countries because of inadequate sanitation, which leads to the accumulation of organic waste and attracts them. The human gastrointestinal tract is particularly vulnerable to myiasis because it allows the flies to gain access through natural orifices such as the mouth, eyes, nose, and genitourinary passages.
Wound myiasis is usually the result of obligatory parasitism by flies of the suborder Calliphoridae (the “filth flies”). However, several other families, including Muscidae and Sarcophagidae, are also capable of causing this kind of infestation. Flies of the suborder Culicidae may also cause accidental myiasis, although less frequently than flies of the infraorder Calliphoridae.
The oviposition of the larvae depends on the species of the flies, but most often they are deposited in opportune locations in living tissue. The flies that cause obligatory myiasis generally develop in organic substrates of animal origin, such as decaying flesh and excreta. Flies that are capable of causing accidental myiasis, on the other hand, attack and feed on necrotic wounds and ulcers in living host tissues.
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Occlusive salves and dressings, followed by manual removal, are useful treatments for cutaneous myiasis. Oral ivermectin is effective in oral and nasal myiasis, and the use of a mild cathartic agent is recommended for intestinal myiasis. Cavitary myiasis, which is a severe form of the disease, may require antibiotic treatment or surgery.
Preventive measures for myiasis involve controlling the source of the flies, especially their breeding habitats. Proper sanitation can help, but the most important measure is to eliminate the flies in areas where they might be attracted to humans, such as by installing window screens, keeping rooms and habitations clean and tidy, and using repellents containing DEET. In hospitals and nursery homes, the use of sterile disinfectants to sterilize surfaces and equipment is an excellent preventive measure. Also, a strict policy should be implemented to ensure that patients are not kept in rooms or areas where there is evidence of the presence of flies and their eggs or larvae.
