Unexpected cause of conjunctival injection

Discussion

Ophthalmomyiasis externa is the infestation of conjunctiva by the larvae of Oestrus ovis, which is a large, yellowish-gray fly approximately 10-12 mm long, and mostly located in the sheep’s nasal cavity.1 The fly can eject the larvae in a milky fluid while in flight, and it is not necessary to hit the eye directly for infestation of humans, especially in sheepherding areas.2 Three types of ophthalmomyiasis are: external type - where the infestation is on the external ocular surface; internal type - where the larvae penetrate into the globe; and the third type, which is most destructive, is the orbital myiasis, where the larvae penetrate deep into the orbital cavity and destruct the tissues.3 Our case is the external type, where the larvae were seen on the conjunctival surface with normal fundus and orbita. The most common clinical presentation of external ophthalmomyiasis is acute catarrhal conjunctivitis associated with marked redness, lacrimation, pain, and foreign body sensation.3 Routine treatment is the removal of larvae with fine forceps after instillation of topical anesthetic drops.1 Different treatment options include topical tobacco dilution, ether, potassium permanganate, chloroform, pig’s fat, basil, chewing gum, and ivermectin.4 Irrigation of conjunctival sac with saline solution is not so effective alone because the larvae bite the conjunctival firmly by their hooks. It is essential to remove the larvae immediately, prevent residual larvae, and its potential complication of internal and orbital ophthalmomyiasis. As the larvae escapes from the bright light of slit lamp, the clinician should try to find the larvae in the folds of the conjunctival sac. The larvae bite the conjunctiva with their hooks and there may be conjunctival damage during the mechanical removal of the larvae. A comparison between removal after proparacaine eye drop and combination of cyclopentolate with proparacaine eye drop revealed that the larvae were moving more slowly, nearly inactive, and could not actively escape from the bright light in the latter.

Cyclopentolate is a parasympatholytic anticholinergic agent, such as atropine and scopolamine, and it blocks the muscarinic receptors, and induces relaxation of the sphincter of the iris and the ciliary muscles.5 We suppose that the drug may cause muscle paralyses of the larva. To the best of our knowledge, the effect of cyclopentolate on Oestrus ovis larvae had not been reported before.

As a novel finding in this report, topical cyclopentolate 1% drop instillation with pressure to medial canthus for a minute might affect the larvae, and make the collection of larvae easier, and we recommend to investigate this association with further in vitro studies. Topical cyclopentolate 1% drop, as a part of routine ophthalmic examination, as a part of ruling out internal ophthalmomyiasis, makes the collection of larvae easier, and safer.