Etiology

There are four stages in the life cycle of a fly.

• The egg develops within the female adult.
• Parthenogenesis is asexual reproduction with offspring developing from unfertilized eggs.
• The larva is implanted by the female into a suitable host.
• Pupa are mature larvae that usually fall to the ground and mature in the soil.
• Adult flies develop from pupa.

Humans are mostly secondary hosts, depending on the fly.

When flies implant larvae on the external surface of the eyelid and/or conjunctiva, including deeply embedded larvae that extend into the orbit, it is called external ophthalmomyiasis.

Internal ophthalmomyiasis refers to invasion of the globe.

The vector in acquiring external ophthalmomyiasis can be a finger after contact with an infested definitive host.

• A shepherd or rancher can transmit eggs and larvae to the eyelid of Oestris ovis, a fly whose definitive host is sheep or goat.
• The larvae frequently infest the nostril of the animal.
• The animal will have rhinorrhea as a manifestation of the infestation.
• In a 1983–1987 study of sheep from the Rocky Mountain area, including Wyoming, Colorado, Idaho and Nebraska, over 90% of 400 sheep studied were infested, mostly in the nostril (University of Wyoming, Cooperative Extension Service, April, 1992).

The fly can also directly implant the larva into the eyelid or conjunctiva.

A third vector is blood-feeding insects such as mosquitoes.

• The female fly captures the mosquito and attaches eggs to its surface.
• The fly egg is implanted in the human by the mosquito bite.

Larva can penetrate intact skin or conjunctiva.

In clinical practice, most cases of orbitopalpebral myiasis are infestations of necrotic tissue, such as neglected tumors.

• Urban cases of ophthalmomyiasis frequently are neglected basal and squamous cell carcinomas.

Epidemiology

External ophthalmomyiasis is mostly in tropical areas where botflies are more common and in regions with poor hygiene

However, cases of myiasis are observed in rural and urban United States, also frequently in patients with poor hygiene.

Flies are mostly active during warm months, therefore that is when these cases are more commonly seen.

• In the Southern United States flies are active in all but the coldest months.
• In the Northern United States flies are active in the summer and early fall.

Dermatoba hominis, is a human botfly found in Mexico, Argentina, Chile and Uruguay, whose definitive hosts include humans.

• Human botfly infestation has be seen in the Southern United States and in travelers from endemic countries.

The sheep gadfly, Oestris ovis, is a common cause of external ophthalmomyiasis in the United States.

The common housefly Musca domestica can rarely produce myiasis in any geographic region (Indian J Ophthalmol 2013; 61:671).

History

• Pain
• Bleeding
• Mass (inflammatory mass harboring the larva).
• Crawling sensation
• Fever
• Anorexia
• Headache
• Tearing

Clinical features

• Tan or brown color
• The respiratory pore of a burrowed larva in healthy tissue can be seen at the skin surface.
• Edema of the eyelid
• Ulceration and necrosis
• Visible larva (burrowed beneath the skin or conjunctiva)
• Visible crawling pupa (white and shiny) or adult flies — video courtesy Richard C. Allen, MD, PhD, FACS.
• Foul smell (from decay of nonviable tissue)
• Necrosis of periocular tissue
• Rhinorrhea — in humans, infestation of the nose is common.
• Can appear in a chronically exposed hydroxyapatite implant (OPRS 2004; 20:395).

Testing

There are two clinical contexts in which external ophthalmomyiasis is seen.

• Necrotic tissue can have crawling maggots (larvae) identified on the external surface.
• Larvae can be embedded in healthy tissue.
• In the skin, only the external respiratory orifice might be visible at the surface in photos of ophthalmomyiasis.
• In the conjunctiva, larvae might be visible crawling on the external surface or be embedded in an inflammatory mass.

The larva can be submitted to the laboratory for typing.

Testing for staging, fundamental impairment

Deep orbital infestation might be revealed by CT or MRI.

Secondary infection should be confirmed by culture.

Primary etiology of tissue necrosis is confirmed by biopsy.

Larvae infest healthy tissue, even with proper hygiene.

The adult female fly is attracted to the odor of necrotic tissue.

• Therefore, poor hygiene invites the fly to implant the larva.
• The nasal cavity emits the odor of necrosis and infection, in animals and humans, encouraging larval implantation by the female fly.

• Furuncle
• Molluscum
• Acute conjunctivitis

Natural history

Mild Oestris ovis infestation of periorbital tissue can be common and self-limited (Arch Ophthalmol 1958; 60:1107).

With necrotic tumor in the orbit, the potential for intracranial extension makes orbital myiasis a life threatening condition.

Medical therapy

Treatment is mostly surgical removal of the larvae, but medical treatment is helpful for secondary bacterial infection and to devitalize the larvae.

Medical treatment with an antiparasitic such as oral ivermectin becomes important when early infestation is recognized on a neglected tumor.

• With elderly patients who refuse treatment for advanced periocular tumors, clinical vigilance and appreciation for possible myiasis might limit morbidity.

Systemic antibiotics are administered with broad spectrum coverage of aerobic and anaerobic organisms to treat secondary infection.

• Intravenous clindamycin offers aerobic, anaerobic and MRSA coverage.

A single dose of oral ivermectin (200 μg/kg) can facilitate resolution of the infestation (Eye 2005; 19:1018).

• Oral ivermectin can also make it easier to surgically eradicate the infestation, making the larvae nonviable.
• Ivermectin is usually used in veterinary medicine for parasitic infestation.

Radiation therapy

The primary eyelid malignancy might not be resectable and might best be treated with radiotherapy.

Surgery

Treatment is removal of the necrotic tissue and removal of the infesting larvae.

When larvae have invaded deeply, approaching the orbital apex, exenteration might be appropriate to prevent intracranial extension (Ophthalmology 1986; 93:1228).

Other management considerations

Permethrin is an insecticide used to treat scabies and lice.

Compounded as liquid (shampoo) or cream (5%)

• Moderately toxic to the cornea, most useful when the eye is nonviable
• Pack the necrotic wound with petroleum compounds to promote healing.

Irrigation with potassium permanganate solution to reduce the foul smell.

Hydrogen peroxide will immobilize the maggots and sterilize the tissue surface.

Chloroform, ether or ethanol have been used to immobilize maggots.

Occlude the respiratory hole of a burrowed larva with ointment, asphyxiating the larva.

Common treatment responses, follow-up strategies

Once clinically resolved there should be minimal incidence of dormant infection and recurrent infestation.

There are no data on side effects of ivermectin for ophthalmomyiasis.

Ivermectin used to treat onchocerciasis (river blindness) can cause arthralgia, myalgia, tachycardia and headache.

Early aggressive intervention can limit loss of vital ocular tissue including potential invasion of the eye.

Ocular myiasis was described in 1930, in a man who felt a foreign body sensation in the eye and 6–8 hours later had 25 larvae evident beneath the conjunctiva. (Am J Ophthalmol 1930; 13:801).

• This was an example of infestation in healthy tissue.
• In a similar case described in 1935, a man was struck in the eye by the sheep gadfly Oestris ovis and ten hours later had ten larvae removed from the conjunctiva. (Am J Ophthalmol 1935; 18:547).
• The sheep gadfly can deposit larva into the eyelid or conjunctiva while flying, with less than a second's contact with the tissue (Am J Ophthalmol 1939; 22:1253).

There are numerous reports in the literature of ophthalmomyiasis in necrotic tumors (Ophthalmology 1986; 93:1228).

1. Video: Maggot to Fly Transformation
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