First instar larva of Oestrus ovis: 'Caught in action'

First instar larva of Oestrus ovis: ‘Caught in action’

A 42-year-old man presented with complaints of foreign body sensation and redness in his left eye after being reportedly struck by an insect during a wind storm. Slit-lamp examination revealed a tiny translucent larva slithering over the bulbar and palpebral conjunctiva [Figure - 1]a, [Figure - 1]b. There was no evidence of any corneal involvement. The larva was retrieved from the eye and its microscopic examination revealed that it had an oval-shaped segmented body with prominent oral hooks connected to the internal cephalopharyngeal skeleton [Figure - 1]c, [Figure - 1]d. The cause of ophthalmomyiasis externa was identified as the first instar larva of the sheep botfly, Oestrus ovis. Ophthalmomyiasis externa caused by Oestrus ovis is a rare disease with only a handful of cases reported, mostly from rural settings. The larvae pupate in the soil for 4-8 weeks, form a chrysalis, in which they morph into adults and then emerge. Human beings are an intermediate accidental host. The larva can be distinguished by sharp, dark brown oral hooks [Figure - 1]d and each segment is seen covered by tufts of numerous hooks with spinose tips on the anterior margin and spiny bifid posterior end [Figure - 1]c. The common presenting features are itching, redness and foreign body sensation, usually in a single eye, which get resolved following removal of the larvae. It is imperative to consider it in the differential diagnosis of unilateral conjunctivitis and to recognize and treat the condition effectively as any delay may lead to potential complications such as penetration into the eye leading to blindness.

Figure 1: A. Slit-lamp examination: intraocular tiny translucent larva over the bulbar conjunctiva, near the inner canthus of eye; B. Slit-lamp examination: intraocular larva (closer view); C. Photomicrograph showing the larva covered by tufts of numerous hooks with spinose tips on the anterior margin and spiny bifid posterior end ((100); D. Photomicrograph showing oral hooks connected to the internal cephalopharyngeal skeleton ((400).