A woman in the US has become the first person ever to be infected with a rare microscopic eye worm that was previously only found in cattle.

In 2016, 26-year-old Abby Beckley from Oregon, felt an itching sensation in her eye for over a week before she pulled a half inch worm from her eyeball, said a report by the Centres for Disease Control (CDC) published in the American Journal of Tropical Medicine and Hygiene this week.

Beckley visited a local doctor, who removed two more worms. An ophthalmologist found three more and over the course of a 20-day stay at the CDC, where the parasite was identified as a member of the Thelazia family, 14 more worms were extracted from her eye. According to CDC researcher and lead author of the report, Richard Bradbury, the worms could not be removed all at once, only as they became present and visible.

It was only months later that the researchers realised that the worm was a Thelazia gulosa – a type of parasite previously only found in cattle. The worm is spread by flies, which carry the larvae in their mouth parts and drop them into the fleshy part of the eye while feeding. From there, the worms subsist on the host’s eyeball lubrication. Researchers believe Beckley contracted the worm during time spent horseback riding in rural south Oregon.

The thelazia gulosa parasite (Credit: Centres for Disease Control)

Other worms from this family have infected humans before, but are still extremely rare. Only 160 cases linked to the species have been reported in humans in Europe and Asia, and only 11 have been reported in the US.