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.
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).
- Mass (inflammatory mass harboring the larva).
- Crawling sensation
- 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).
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.