Myiasis is a group of diseases caused by parasitization in human tissues and cavities of larvae of some species of flies. The clinical picture of myiasis is determined by the type of pathogen and the place of its parasitization. Intestinal myiasis is manifested by symptoms of enteritis and colitis; ophthalmomiasis – conjunctivitis, iridocyclitis, retinal detachment, death of the eye; cutaneous myiasis – the formation of skin infiltrates; urinal – urethritis, cystitis, urinary retention, etc. The diagnosis of myiasis is based on the detection of fly larvae. Treatment of cavity myiasis consists in washing the cavities with antiseptic solutions. With intestinal myiasis, laxatives are prescribed. Larvae are removed from the eyes and wounds of the skin with tweezers; with a deep location – surgically.
Myiasis are parasitic diseases associated with the penetration and stay in the body of animals and humans of arthropod insect larvae (mainly flies). The incidence of various types of myiasis is characteristic mainly for tropical countries of Africa and South America, but flies that can cause human entomoses also live on the territory of Kazakhstan, the Caucasus, and the southern regions of Russia. Taking into account the possibility of different localization of larvae in the human body, myiasis are of practical interest for surgery, gastroenterology, ophthalmology, otorhinolaryngology, urology. The course of myasis can be benign or malignant, leading to disability or death of the patient.
According to the type of parasitic larvae, the following types of myiasis are distinguished:
- accidental – caused by fly larvae that accidentally enter the human body. These include intestinal and genitourinary myiases.
- facultative – caused by larvae, usually developing in rotting substances, but sometimes flies lay eggs on open wounds and skin ulcers.
- obligate – caused by the larvae of flies that are parasites of humans and animals.
Depending on the place of parasitization of larvae, myiasis are divided into tissue and cavity. Taking into account the affected organs, there are skin, intestinal, genitourinary (urinal), nasal (nasal), ear and eye myiases (otomiasis and ophthalmomiasis). According to the depth of penetration of the pathogen and the nature of the course, surface (benign) and deep (malignant) myiasis are distinguished. The group of deep myiasis includes ordinary deep skin myiasis, African myiasis (cordylobiosis) and South American myiasis (dermatobiasis).
Typical pathogens of surface myiasis are larvae of meat, carrion, cheese, houseflies and house flies. They lay their eggs in festering abrasions, wounds, ulcers of the skin and mucous membranes, including eyes, nose, external genitals. The larvae that have emerged from the eggs abundantly seed the lesion. Superficial cutaneous myiasis proceeds benign. Larvae of pathogens of superficial myiasis feed only on pus and decomposed necrotic tissues, so patients are concerned about moderate itching in the wound and a feeling of “crawling goosebumps”. When examining the lesion, white swarming islands in the form of semolina grains are determined. After the end of the maturation cycle, the larvae fall out of the wound and pupate outside the human body.
Ordinary deep skin myiasis is caused by flies of the genus Wohlfortia (Wolfart’s fly) and Chrisomyia, which lay their eggs in purulent foci on the skin. The larvae of these flies are capable of destroying healthy tissues, penetrating into the dermis, subcutaneous tissue and deep-lying tissues. With deep myiasis of the skin, the patient’s body temperature rises, weakness and headaches occur. Unbearable acute pain in the wound area, where the larvae parasitize, can lead to fainting.
When localized on the conjunctiva, in the mouth and nose, the larvae corrode the mucous membranes and soft tissues, penetrating to the fascia and periosteum, destroying the cartilage and bones of the larynx, nasopharynx, sinuses, eye sockets, etc. Despite the short period of parasitization of larvae in the human body (3-6 days), this period is sufficient for the development of deep tissue lesions.
African myiasis (cordylobiasis) is a type of deep myiasis caused by the larvae of the fly Cordilobia anthropophaga. Female flies lay eggs on the surface of soil contaminated with organic residues and urine. The larvae that have emerged from the eggs are introduced into the human body in a percutaneous way when the skin comes into contact with dirty earth. After a few days, hyperemia of the skin occurs at the site of the larva’s introduction and a dense infiltrate forms, which soon turns into a furuncle-like node. Unlike a staphylococcal boil, which has a necrotic rod in the center, a small hole is located in the middle of the boil-like node, providing air access to the parasitic larva. After about 2 weeks, the larva emerges from the infiltrate opening and soon falls out of it into the external environment. After that, the wound usually heals, but with infection and suppuration of the node, an extensive abscess may occur.
South American myiasis (dermatobiasis) is caused by the larval stages of a human gadfly, which lays eggs on the body of blood-sucking insects (mosquitoes, flies, horseflies). When insects attack a person, the hatched larvae penetrate into the skin, where they begin their further development. An infiltrate with a fistulous course forms around the invaded larvae, from which a serous-purulent fluid is released. After 1.5-2 months, having reached a length of 20-25 mm, the mature larvae fall out and pupate in the soil. The course of South American myiasis is accompanied by a moderate sensation of pain at the site of infiltration.
Intestinal myiasis develops as a result of accidental ingestion of fly larvae with food. When parasitizing larvae in the stomach, the main manifestations include nausea, vomiting, pain in the epigastric region. The presence of parasites in the intestine causes a typhoid condition, symptoms of long-term enteritis and colitis (abdominal pain, diarrhea, tenesmus, admixture of blood in the stool, etc.).
A severe course of myiasis is observed when larvae parasitize on the mucous membrane of the eyes, nose, and ears. Genitourinary myiasis occurs when eggs get on dirty underwear and bed linen, from which the larvae penetrate the mucous membrane of the genitals and into the urethra. With urogenital myiasis, a clinic of vulvitis, vaginitis, urethritis, cystitis can develop.
In the ocular form of myiasis (ophthalmomiasis), gadfly larvae parasitize in the conjunctival sac, causing the phenomena of conjunctivitis, iridocyclitis, the formation of corneal ulcer. In particularly severe cases, they can penetrate into the lacrimal sac and inside the eyeball, causing its destruction. Cases of fatal encephalitis have been described. Parasitization of larvae in the nasal passages is accompanied by fever, headache, mucopurulent rhinitis. Otomiasis caused by the larvae of the wolfart fly can be accompanied by perforation of the eardrum and penetration of parasites to the meninges. Pathogens of abdominal myiasis can parasitize in the oral cavity, paranasal sinuses, anal canal.
Diagnosis and treatment
The diagnosis of cutaneous myiasis is established by careful examination of the lesion with a magnifying glass and with side lighting. At the same time, the movement of larvae located in colonies can be seen in the wound. The basis for the recognition of intestinal myiasis is the detection of fly larvae in the study of vomit, feces, urine, urethral and vaginal smear. Patients with suspected otomiasis need to consult an otolaryngologist; to exclude ocular myiasis, an ophthalmologist is examined.
Treatment of skin myasis consists in removing the larvae from the lesion by washing the wound with antiseptic solutions (potassium permanganate, furacilin, etc.). To facilitate the removal of the larva, a few drops of sterile oil can be poured into the infiltrate hole; having lost air access, the parasite shows on the surface of the skin and can be easily captured with tweezers or a clamp. After that, the vacated cavity is treated with disinfectants and closed with an aseptic dressing. With the development of purulent complications, antimicrobial ointments or systemic antibiotics are used.
In the intestinal form of myiasis, gastric lavage, laxatives and antiparasitic drugs are prescribed. Treatment of genitourinary myasis consists in washing the urethra to remove the larvae. Larvae are extracted from the conjunctival cavity with pointed eye needles after instillation of anesthetic and antiseptic drops. In other cases, the larvae must be surgically removed.
Prognosis and prevention
The prognosis of superficial skin myases is favorable. With deep tissue lesions, multiple parasitization of larvae, the addition of purulent complications, disabling and fatal outcomes are possible. To prevent skin myiasis, it is necessary to use repellents that repel flies and gadflies. In order to prevent the deep penetration of parasites, timely and rational treatment of all skin lesions is necessary. It is important to destroy flies and their breeding foci; storage of products in places inaccessible to flies; protection of wounds from invasion by fly larvae; prevention and treatment of myiasis in animals.