The parasitic nematode Thelazia californiensis uses flies that feed on tears and eye secretions to complete its life cycle. The flies carry the worms between hosts, where the nematodes mature and reproduce in the eye. In the reported case, doctors removed adult worms measuring 11–20 mm from a man’s eye; his symptoms had disappeared and no worms remained two months later.
A life cycle carried by flies
The nematode’s life cycle depends on a fly that feeds through an animal’s eye and consumes its tears and other secretions. After a fly feeds on an infected eye, it takes up first-stage larvae. The insect serves as both an incubator and a flying means of transport.
Inside the fly, the larva molts twice and develops into a third-stage larva. It then moves to the fly’s mouthparts, where it waits until the insect reaches another suitable, watery eye. The larva leaves the fly and settles in the new host’s eye, where it molts further before becoming an adult. Adult worms live directly in the eye and mate there.
Females release larvae inside the host
T. calipeda is ovoviviparous, meaning that embryos develop inside eggs retained within the female worm’s body. The females release large numbers of tightly coiled, first-stage larvae, which can then be consumed by thirsty flies.
Some of the female worms removed from the man’s eyes were coiled around developed eggs. Doctors observed that, after removal, some of the coiled larvae began emerging. They identified male worms by their mating spines—needle-like structures that help facilitate mating.
The infection resolved after removal
Fortunately for the man, doctors removed all the adult worms from the eye, ending the infection. No antiparasitic drugs were needed, even though some larvae might have remained. At a follow-up examination two months later, all of his eye symptoms had resolved, and doctors found no additional worms.
Source: arstechnica.com
