Correspondence

Eye (2006) 20, 1088–1090. doi:10.1038/sj.eye.6702120; published online 21 October 2005

Ophthalmomyiasis externa caused by Dermatobia hominis: a successful treatment with oral ivermectin

No financial support was received

T H Wakamatsu1 and P T P Pierre-Filho1

1Department of Ophthalmology, Faculty of Medical Sciences, State University of Campinas (UNICAMP), Campinas, São Paulo, Brazil

Correspondence: Tais Hitomi Wakamatsu, Avenida Imperatriz Teresa Cristina 946 apartment 62, CEP 13100-200, Brazil. Tel/Fax:+55 19 325 47141; E-mail: taiswakamatsu@yahoo.com.br

Sir,

Ophthalmomyiasis refers to infestations of the eye and/or ocular annexa by larvae of the order Diptera and represents less than 5% of the cases of human myiasis. When larvae remain outside of the eye, it is termed ophthalmomyiasis externa, while penetration of the eye itself is termed ophthalmomyiasis interna, a severe condition that may lead to blindness.1, 2, 3

Dermatobia hominis may very occasionally cause ophthalmomyiasis externa, with eyelid, and conjunctival involvement.4, 5, 6 Conventional treatment consists of the removal of the larvae from the affected sites, although sometimes access is difficult to such areas.4, 7 This paper describes a case of ophthalmomyiasis externa caused by D. hominis in a child successfully treated with oral ivermectin, making surgical extraction by incision and exploration unnecessary.

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Case report

An 11-year-old boy, with a complaint of 5 days of pain, slight pruritus, and oedema in his right inner canthus, was brought to the Department of Ophthalmology, State University of Campinas, Brazil. There was no history of previous ocular surgeries, and he was not taking any ocular medications. He was in good general health with no systemic symptoms. There was no history of exposure to animals other than household pets.

On examination, he had a visual acuity of 20/20 with no spectacle correction, a normal extraocular motility and fundus findings. A small erythematous lesion with a well-demarcated punctum in the centre, and periorbital oedema were noted in the right inner canthus. On slit-lamp examination, small yellowish organism and a serous purulent fluid drained from the punctum (Figure 1) were observed. Since attempt to remove the larva by means of a forceps was unsuccessful, the patient was given a single oral dose of ivermectin (200 mug/kg). In the following day, a portion of the larva appeared at the punctum and remained there (Figure 2). The larva was then easily grasped with forceps and gently removed without making any incision into the lesion. The larva measured about 15 mm and showed slow and erratic movements. It was identified as D. hominis larva based on the morphologic features and the pattern of its spinous rings (Figure 3). Topical treatment with a steroid-antibiotic ointment allowed the patient to heal within a few days.

Figure 1.
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Lesion in the right inner canthus with the larva visible as a white spot within.

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Figure 2.
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Emergence of the larva. A forceps allowed us to remove the larva intact.

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Figure 3.
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Intact Dermatobia hominis larva after extraction.

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Comment

Our patient was infected with D. hominis, the most common cause of cutaneous myiasis in tropical and neotropical regions of Central and South America.3, 6, 7 Several reports of myiasis infestation by this parasite have been described worldwide as a result of international travel.4, 5, 6, 7, 8, 9 To initiate human infection, the female botfly first attaches her eggs to an insect such as a mosquito. When the mosquito lands on a human and deposits the egg, the warmth of the skin causes the eggs to hatch. Within minutes the larva penetrates in the skin, usually using the mosquito entrance bite or hair follicle as a canal through the skin.4, 5 The presence of the larva within the skin incites a local inflammatory reaction. A furunculous lesion is formed, in which the larvae remain for up to 90 days.9

Patients most often infested with only one larva, complain of pruritis and pain, and they may sense movement of the larva. Commonly, there is serous purulent exsudate from the lesion, as in our case. If the larva dies within the cavity, the lesion may be very similar to a chalazion.3

Several methods of treatment have been reported. Extraction with forceps is difficult because of its depth within the skin and because of the presence of rows of backward spines that attach it to subcutaneous tissues. Occlusion of the punctum with agents as pork fat, raw meat, petroleum jelly, liquid paraffin, nail polish, adhesive tape, chewing gum, bee's wax, and mineral oil or other heavy oils have been successfully used.7, 8 This suffocates the larva, causing it to come out of the burrow in search air, where it can be grasped. A technique recently described involves injecting a local anaesthetic under the larva itself so the pressure forces the larva out of the skin.8 This may not be useful when removing multiple larvae due to the possible toxicity of the amount of anaesthetic required. Often surgical excision with debridement of the cavity is required if the above methods fail or for removal of dead larvae.

Recently, indications for topical and oral use of ivermectin in the treatment of myiasis have been found in the literature.2, 10 Ivermectin was used successfully in a case of orbital myiasis caused by Cochliomyia hominivorax.2 As in our patient, a single oral dose of ivermectin (200 mug/kg) led spontaneous emigration of the larvae. It is assumed that ivermectin blocks nerve impulses on the ending nerve through the release of gamma aminobutyric acid (GABA), linking to the receptors and causing palsy and death. Acetylcholine, which is the main peripheral neurotransmitter in mammalians, is not affected by ivermectin. Also, ivermectin does not penetrate the central nervous system of mammals easily, where GABA acts as a neurotransmitter, maintaining a security margin when it is used at the recommended dose.10

We suggest that oral ivermectin may be considered as an efficient and safe method of treatment of human ophthalmomyiasis. Early detection of ophthalmomyiasis and management are important in preventing complications.

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References

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