External ophthalmomyiasis by Oestrus ovis larvae diagnosed in a Papanicolaou-stained conjunctival smear.

Human ophthalmomyiasis is an eye infection associated with the larvae of certain flies of the Order Diptera (Insecta). This parasitic infection is classified as internal, orbital or external in relation to the location of the larvae.1 External ophthalmomyiasis is caused by species of a number of families, which include Oestrus ovis. This species, also called 'sheep bot fly', is a parasite specific to ovines and can only incidentally affect man. Adult Oestrus ovis females normally project their larvae into the muzzle of sheep while flying. In humans, the face and particularly the eyes are affected, these larvae being responsible for most cases of human external ophthalmomyiasis.2,3 In sheep, the larvae climb into the nasal passages reaching the frontal sinuses. After a few months, they detach and complete their life cycle in the soil from which the adult emerges at the next favourable season. The cycle ends in the human host as the parasite dies after a few days or weeks. While the infection is uncommon and reported only sporadically in Northern Italy, shepherds, farmers and people living in rural areas where sheep are bred are particularly affected.4,5 The larva, a millimetre in length and equipped with two oral hooks and numerous rows of tiny spines, irritates the conjunctival layer provoking an acute foreign body sensation, photophobia, blepharospasm, a watery to mucopurulent discharge, a sometimes painful chemosis with oedema of the eyelids.2,4 The prognosis is normally benign. Careful conjunctival examination can demonstrate the typical larvae. We report a human case of this infection, which occurred in Northern Italy and was diagnosed by cytological examination of a Papanicolaou-stained conjunctival smear.