Contributor: Gordon K. Klintworth
Ophthalmomyiasis accounts for <5% of human myiasis. The nature of the botfly that invades the ocular tissue varies with the intermediate host, cattle (Hypoderma bovis), horse (Gasterophilus spp.), sheep (Oestrus ovis), and rodents (Cuterebra) and the prevalence varies with the geographic area. The larvae of certain flies can invade orbital myiasis [myiasis - orbital], external ocular structures [ophthalmomyiasis externa], internal ocular structures [ophthalmomyiasis interna], the skin of the eyelid [myiasis - dermal]. Causes of myiasis of the eyelid include Cuterebra and Dermatobia hominis. The eye can be invaded by maggots causing dermal myiasis. Ophthalmomyiasis caused by Chrysomya bezziana is particularly damaging. Scanning electron microscopy of the external features of the larvae can permit identification because of the characteristic external features of each specimen. This is most reliable when the entire larvae and especially its spiracles are well preserved. A pre-existing ocular lesion such as gonococcal conjunctivitis [gonorrhea] and periocular neoplastic ulcers, endophthalmitis may predispose to ophthalmomyiasis by attracting flies. In neglected patients, fly larvae may destroy orbital tissue and cause a marked inflammation and secondary bacterial infection. The larvae of the sheep botfly can often be seen wiggling beneath the transparent conjunctival epithelium. Certain fly larvae can penetrate through the conjunctiva and sclera and cause internal ophthalmomyiasis of the parasitic dipterous larvae. When this occurs usually a single larva gains access into the eye. Most affected individuals are asymptomatic, but severe visual loss may occur. Myiasis many affects farmers, laborers and beggars. Human involvement in some types of myiasis is favored by an association of poor personal hygiene and a proximity to infested animals.