The medical term myiasis refers to infestation of human tissues with larvae of flies. Maggots may infest wounds, but certain (not all) forms of myiasis require skin lesions.
Furuncular myiasis as caused by Dermatobia hominis and Cordylobia anthropophaga most frequently affects exposed body regions (e.g., face, arms and legs) in case of the former , but easily accessible, extensive skin areas (e.g., trunk, buttocks, thighs) in case of the latter. One site of infestation generally contains only one, rarely few larvae . However, one patient may present several sites of furuncular myiasis . Within one day after skin penetration, a small papule forms. It extends over the course of the following weeks and may reach diameters of several cm. There is a characteristic central punctum that may even allow seeing parts of the larva. Ill-smelling, purulent discharge may be observed in case of myiasis due to Cordylobia anthropophaga . Besides the dermatological lesion, patients often report pain.
Wound myiasis may sometimes be diagnosed upon visual inspection and detection of maggots. Some species burrow into deeper tissues, namely Cochliomyia hominivorax and Chrysomyia bezziana, and may not be visible upon first sight. They may form nodules in subcutaneous tissues .
Larvae of Hypoderma bovis and Gasterophilus intestinalis migrate through subcutaneous tissue, leave characteristic, raised tracks and cause inflammation, erythema, pain and pruritus. Most patients presenting with this form of myiasis will reveal close contact to life stock or equids during anamnesis.
Diagnosis is usually based on anamnesis, visual inspection and clinical examination. In some cases, maggots may be observed and myiasis can be diagnosed directly. If this is not the case, laboratory analyses of blood samples and imaging diagnostics may be employed to reveal the cause of the patient's symptoms. Similar to other parasitic infestations, myiasis may lead to eosinophilia and leukocytosis. Inflammatory markers may be elevated in moderate to severe cases.
Sonographic examinations may be of help to detect larvae in furuncular lesions. Doppler ultrasound may display the presence of body fluids within the parasite and may therefore serve to distinguish larvae from inanimate foreign bodies . Infestation of the nasopharyngeal, ocular or auricular cavities, internal organs or the central nervous system may be revealed by magnetic resonance imaging or computed tomography scans .
Different therapeutical approaches have been proposed for furuncular myiasis, wound myiasis and migratory myiasis. With regards to the former, occlusion of the central punctum of the dermatological lesion may be an easily available, albeit not less effective form of treatment. Maggots require oxygen and may spontaneously emerge from their host if parasitic hypoxia is triggered by covering their breathing hole with paraffin, beeswax or oil. Upon emergence, larvae may be removed with tweezers. It may be helpful to enlarge the breathing hole before covering it . Of note, occlusion of the breathing hole should not cause the larva to die. In this case, it cannot emerge spontaneously and needs to be removed surgically.
This is actually the second way to remove insects causing furuncular myiasis. Because they are embedded in surrounding tissues, larvae cannot be removed by mere pulling through the breathing hole and a small surgical procedure is required to reach that aim. Local anesthesia is usually sufficient for it to be carried out. It is important to remove larvae completely since remaining parts of the parasite may cause further inflammation. Subsequent antiseptic wound management should go without saying. Infected wounds require at least topical application of antibiotics.
The latter also applies to cases of wound myasis. Thorough debridement, complete removal of maggots and antiseptic dressings are the mainstays of therapy in these patients. Similar to what has been mentioned before, they should be performed under local anesthesia.
Parasites may not be easily accessible in non-dermatological lesions. These patients require systemic anti-parasitic treatment which can be provided, for instance, by administration of ivermectin.
Myiasis is most frequently self-limiting and is associated with very low morbidity and mortality. In general, skin lesions heal spontaneously after larvae emerged from their host . Severe sequelae are to be expected in severe cases where maggots reach the central nervous system .
Myiasis may, however, cause significant pain and mental stress. Also, the probability of an infested wound to become infected is increased. This observation may also be due to poor wound management that allows for myiasis in the first place.
Myiasis is most commonly triggered by the human botfly (Dermatobia hominis) and the tumbu fly (Cordylobia anthropophaga). Both principally trigger furuncular myiasis, i.e., maggots do not migrate from where eggs were laid on or into the skin. Ophthalmomyiasis due to infestation with human botfly larvae has been described.
The New World screw-worm fly (Cochliomyia hominivorax) and the Old World screw-worm fly (Chrysomyia bezziana) are both causative agents of wound myiasis. Both are obligate parasites.
Maggots that may infest the nasopharyngeal, ocular or auricular cavities or internal organs are usually primarily animal parasites. In this context, the bovine botfly (Hypoderma bovis), the horse botfly (Gasterophilus intestinalis) and the sheep botfly (Oestrus ovis) generally affect those animal species they are named after. Maggots of bovine and horse botflies may migrate towards the central nervous system and the gastrointestinal tract, respectively, but usually remain within subcutaneous tissue. Sheep botfly larvae develop inside of eyes or nostrils.
The above mentioned, single species causing hematophagous myiasis is the Congo floor maggot Auchmeromyia senegalensis, formerly known as Auchmeromyia luteola.
The humid, warm to hot climate of tropical and subtropical regions is where most obligate parasitic flies can be encountered. However, maggots may need days or even weeks to develop inside a human host and may not cause symptoms until a traveler who visited such areas turned back home. Also, there are some species that are distributed in temperate climates. In detail, the following species may be encountered in these regions (or in people who traveled there):
Distinct species have different life cycles, may infest human tissue if the opportunity arises or may be obligated to do so in order to complete certain developmental stages.
Dermatobia hominis, for instance, uses hematophagous arthropods to deliver their eggs. The female fly usually catches a mosquito, less frequently a tick, attaches its eggs to the vector's body and thus assures that larvae that hatch upon detection of body heat can easily enter the human skin through the vector's bite. Larvae pass distinct developmental stages and reach a total length of up to 20 mm. Most frequently, they do not move from the original site of infestation and thus cause furuncular myiasis, but cases of ophthalmomyiasis triggered by this species have occasionally been reported. After five to ten weeks, these fully-developed larvae leave their human host, drop to the ground and continue their life cycle .
Cordylobia anthropophaga lay their eggs on clothes or soil. They hatch here and larvae may survive up to two weeks waiting for contact with a suitable host. Similar to the human botfly, body heat is the main indicator of proximity of such a host. These larvae dispose of sharp mandibles that allow for penetration of the intact skin . In human skin, larvae grow to a final size of up to 15 mm. They also cause furuncular myiasis and leave their host after a maximum of two weeks of development.
Cochliomyia hominivorax and Chrysomyia bezziana both infest poorly managed wounds . Very small skin lesions are sufficient to attract these flies. Eggs are laid into these wounds and larvae burrow into their host's tissue in a screw-like manner, which is why these species are commonly called screw-worm flies. Maggots prefer necrotic tissue to feed on and leave their host after approximately one week of development.
Humans are accidental hosts for Hypoderma bovis and Gasterophilus intestinalis, i.e., parasites are unable to complete their current developmental stage in human beings. The majority of patients presenting with migratory myiasis due to infestation with these species live or work in close proximity to live stock and equids. Hypodermis bovis larvae penetrate human skin and subsequently migrate through subcutaneous tissue. They may spontaneously emerge or die within these tissues. While damage to the spinal cord is a major problem of hypodermosis in cattle, this complication rarely occurs in humans. Maggots of Gasterophilus intestinalis behave similarly in humans, wander through subcutaneous tissue and usually die here. Both kinds of larvae may advance with velocities of up to one cm per hour .
Poor hygienic conditions, often associated with a low socioeconomic status, are the main risk factors for myiasis. High prevalence rates among poorly educated, rural populations in underdeveloped countries argues for this conclusion. Adequate sanitation would be the most effective way towards less cases of myiasis.
Travelers should not expose themselves by sleeping outdoors, possibly even on the floor or by using short-sleeved clothing. They should prevent contact to flies and vectors by using repellents, window screens and mosquito nets.
Myiasis is a general term that describes infestation of living or necrotic tissue with larvae of flies. This condition may be caused by a variety of fly species and due to their overall geographical distribution, it may be diagnosed worldwide. However, certain species that are obligate parasites mainly inhabit tropical and subtropical regions. Here, myiasis is more of a health concern than in temperate climates.
Both opportunistic and obligate parasites may infest human tissues. In general, the former do not dispose of any tools to penetrate intact skin. They may infest wounds and preferentially feed on necrotic tissue. The latter, in contrast, require protein to complete determined developmental stages. They will actively seek a host, penetrate their skin and move towards their target tissue. Of note, there are obligate parasitic flies that cause wound myiasis, too.
Clinically, wound myiasis is distinguished from furuncular myiasis, but both are forms of cutaneous myiasis . Other species penetrate the skin and thus initially cause cutaneous myiasis, but subsequently migrate to other tissues. This form of the condition is therefore called migratory myiasis. Besides the skin, maggots may infest the nasopharyngeal, ocular or auricular cavities as well as internal organs such as the gastrointestinal or urogenital tract  . Other tissues may rarely be targeted. Hematophagous myiasis is rare and up to date, only one species distributed in sub-Saharan Africa has been described to obligatorily feed on human blood .
Myiasis is the medical term for infestation of human tissues with larvae of flies.
Distinct fly species may cause myiasis. While some merely take advantage of an opportunity that may arise in form of a poorly managed wound, others obligatorily need to pass certain developmental stages within human or animal tissues to complete their life cycle. Flies whose maggots feed on living or dead human tissues exist all over the world. Most obligatorily parasitic species, however, are distributed in humid and warm climates of tropical and subtropical regions.
As has been mentioned above, some flies lay their eggs in wounds that may be as small as insect bites. Larvae may be visible in this wound or may form small nodules under the skin.
One species is known to attach their eggs to mosquitos or ticks and have hatched larvae penetrate human skin through their respective bites. Others actively penetrate it with sharp mandibles. These species cause furuncular myiasis, i.e., larvae develop at the site of infestation and don't move from here. A papule that is increasing in size over the course of the following weeks can be observed.
Migratory myiasis is caused by other species whose larvae usually develop in life stock or horses. In humans, maggots wander aimlessly through subcutaneous tissues and leave raised tracks behind.
Visual inspection and clinical examination usually allow for diagnosis of myiasis. If this is not the case, diagnostic imaging can be employed to visualize larvae in subcutaneous and other tissues.
Wounds infested with maggots need to be thoroughly cleaned, parasites have to be removed and antiseptic dressings should be applied.
Maggots are in need of oxygen. If breathing holes characteristic for furuncular myiasis are occluded, larvae may emerge spontaneously and can easily be removed. This procedure should be carried out with care in order to avoid that the parasite dies inside the patient's skin. The latter would require surgical removal under local anesthesia. In some cases, this therapeutic approach may be the first option.