A man came before his doctors with strangers living in his eyes, and left the medical literature with a case study. His tenants were adult worms of the species Thelazia callipaeda — males and females together, residing where they mated: directly on the surface of his eyes. Doctors plucked the whole colony out, as Beth Mole described in Ars Technica. Some of the females came away plump with embryonated eggs, and once removed, densely coiled larvae began bursting out of several of them — births attended by surgeons instead of midwives. The males announced themselves by their copulatory spicules, the needle-like structures the worms use to mate, which let doctors tell husband from bride on the instrument tray.
The cure was mechanical and complete. With every adult worm gone, the infection simply ended: no antiparasitic medication was required, even if some larvae had been left behind. At a follow-up appointment two months later, all the man’s eye symptoms had resolved and there were no more worms.
The courier is a fly
Eye worms do not travel by their own power. Their airline is the fly that feeds from animals’ eyes, slurping tears and secretions; from an infested eye it may pick up first-stage larvae, whereupon the insect serves as both incubator and air taxi. Inside the fly the larvae molt twice, emerging as third-stage larvae that migrate to the fly’s mouthparts and there await the next suitable watery eyeball. They disembark, settle in for good, and after one final molt become adults. The adults leave nothing to chance and nothing unoccupied: they live on the eye, and they spawn on it.
Unlike other worms, the female T. callipaeda is ovoviviparous — her embryos develop inside eggs carried within her own body. What she releases is not an egg clutch but a crowd: large broods of densely coiled first-stage larvae, waiting on the eye to be lapped up by the next thirsty fly and ferried to a stranger.
Consider the old geography of such journeys. To the fly, the eye is a refueling stop; to the worm, the fly is a vehicle; to the worm, the eye is a homeland complete with a marriage bed. Only the man, in this arrangement, was never consulted. The worms, in the end, were perfectly rational tenants. It was the lease that was monstrous.