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Myiasis

What Is It, Causes, Signs and Symptoms, and More

Author:Maria Emfietzoglou, MD

Editors:Alyssa Haag,Emily Miao, PharmD,Kelsey LaFayette, DNP, ARNP, FNP-C

Illustrator:Jessica Reynolds, MS

Copyeditor:Stacy Johnson, LMSW


What is myiasis?

Myiasis is a parasitic infection caused by an infestation of fly larvae, also referred to as maggots. Common fly species that may cause myiasis include Dermatobia hominis and Cordylobia anthropophaga. The most common clinical type of myiasis is cutaneous myiasis, which provides for several clinical subtypes, including furuncular, wound, and migratory, or creeping, myiasis. Aside from cutaneous myiasis, there is also nasopharyngeal myiasis, which involves the nose, sinuses, and pharynx, and ophthalmomyiasis, which affects the eyes. Rare types of myiasis include intestinal myiasis, which involves the gastrointestinal tract, and urogenital myiasis, which affects the urogenital system. Myiasis is relatively uncommon in the United States and typically occurs in tropical and subtropical regions in Mexico, South America, Central America,  sub-Saharan Africa, and the Caribbean islands. 

Knee wound with fly larvae infestation.

What causes myiasis?

Myiasis occurs when Diptera flies lay their eggs in humans. Common flies that cause myiasis, specifically furuncular myiasis, include Dermatobia hominis and Cordylobia anthropophaga. Less commonly involved flies include Cochliomyia hominivorax and Chrysomya bezziana, which mainly cause wound myiasis; Hypoderma bovis and Gasterophilus intestinalis, which mainly cause migratory myiasis; and Oestrus ovis, which generally causes ophthalmomyiasis. 

Myiasis cannot be transmitted from person-to-person. However, fly larvae can be transmitted to humans in several ways. For instance, flies can deposit their eggs near a wound or sore, and the hatched larva can burrow into the skin. Therefore, individuals are at an increased risk of developing myiasis when traveling to endemic areas with untreated injuries or open wounds.  Alternatively, Diptera flies can attach their eggs to mosquitos or ticks, and when these insects bite an individual, the eggs of the Diptera fly can then enter the skin through the insect bite. Additionally, flies can lay their eggs on the ground or on objects that may come into contact with people’s skin, such as damp clothing. Flies can also infect animals, like horses and cattle, and cause infection in humans who come in close contact with infected animals. 

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What are the signs and symptoms of myiasis?

Signs and symptoms of myiasis depend on the specific type. Individuals with myiasis may have general symptoms, including fever or lymphadenopathy (i.e., swollen glands). Furuncular myiasis typically presents as non-healing, boil-like lesions that may be purulent, painful, pruritic, and tender. In wound myiasis, the larvae deposit in open wounds and may cause systemic symptoms like fever and chills. In migratory myiasis, the larvae tend to migrate, and therefore, there is a tortuous, thread-like red line marking the passage of larva through the skin. In nasopharyngeal myiasis, signs and symptoms include epistaxis (i.e., nosebleed); foul-smelling nasal discharge; nasal passage of worms; nasal obstruction; facial pain, headache, and dysphagia. In the case of ophthalmomyiasis, individuals usually experience severe eye irritation, redness, foreign body sensation, pain, lacrimation, and eyelid edema. Intestinal myiasis can cause abdominal pain, vomiting, and diarrhea, while urogenital myiasis can cause dysuria and hematuria

How is myiasis diagnosed?

Diagnosis of myiasis may involve the individual's travel history and a thorough physical examination. A complete blood count (CBC) may also be performed and typically shows leukocytosis, specifically, eosinophilia. Imaging tests, such as ultrasound, MRI, or CT scans, may also be ordered to determine the size of the larvae and establish the diagnosis. Biopsy of the lesions is rarely performed, but it may reveal inflammatory infiltrates. 

How is myiasis treated?

Myiasis commonly resolves on its own as the larvae are sloughed within weeks. In cutaneous myiasis, however, larvae extraction may be aided by applying topical substances, like petroleum jelly and liquid paraffin, to assist the migration of the larvae out of the skin. A few hours after applying the substances, tweezers may be used to pull out the larvae that have emerged. For all types of myiasis, antiparasitic medications, such as ivermectin, to kill the fly larvae, are commonly used. In severe cases, larvae need to be surgically removed using topical anesthesia. After surgery, the use of antiseptic dressings and antibiotics to prevent secondary infection may be necessary. In wound myiasis, maintaining proper hygiene of the wound is very important. 

What are the essential facts to know about myiasis?

Myiasis occurs when a skin infestation of developing fly larva (maggot), most commonly of the Dermatobia hominis and Cordylobia anthropophaga species. The most common clinical type is cutaneous myiasis, which includes the clinical subtypes of furuncular, wound, and migratory myiasis. Most cases of myiasis occur in tropical and subtropical regions of Africa and America. Individuals commonly become infested when traveling to endemic areas with untreated injuries or open wounds. There are several ways of transmission to humans, such as deposition into an open wound. Diagnosis is usually based on a thorough history, physical examination, blood tests, imaging tests, and rarely, biopsy. Although myiasis commonly resolves on its own, treatment can include substances, like petroleum jelly, to aid larvae extraction; antiparasitic medications, like ivermectin, and surgery in rare cases. 

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Resources for research and reference

Centers for Disease Control and Prevention. Parasites - Myiasis. Retrieved 18/07/2022 from https://www.cdc.gov/parasites/myiasis/index.html

Centers for Disease Control and Prevention. Myiasis - Frequently Asked Questions. Retrieved 18/07/2022 from https://www.cdc.gov/parasites/myiasis/faqs.html

DermnetNZ. Myiasis. Retrieved 18/07/2022 from https://dermnetnz.org/topics/cutaneous-myiasis

Medscape. Myiasis. Retrieved 18/07/2022 from https://emedicine.medscape.com/article/1491170-overview

MSD Manual. Cutaneous Myiasis. Retrieved 18/07/2022 from https://www.msdmanuals.com/home/skin-disorders/parasitic-skin-infections/cutaneous-myiasis

Science Direct. Myiasis - an overview. Retrieved 18/07/2022 from https://www.sciencedirect.com/topics/medicine-and-dentistry/myiasis