A 68-year-old woman returned from a trail run on the California coast only to discover the culprit of the itching sensation in her right eye was a half-inch long worm she proceeded to pull out of her eyeball.
Upon closer inspection, the woman found another worm in her eye, which she was also able to remove by hand after rinsing her eyeball with tap water.
When she went to the eye doctor, the clinician found yet another of the parasitic worms, known as "nematodes," in the woman's eye and told her to continue flushing it with water, according to the nightmarish new case study published October 22 in Clinical Infectious Diseases.
About a month later, the woman pulled a fourth and final nematode out of the same eye. By that time, she'd returned to her home state of Nebraska and been dismissed by doctors who couldn't find a cause for her continually itchy eyes.
It wasn't until the woman received an anti-parasitic drug called ivermectin, and continued flushing her eye with water, that the infection finally resolved.
Fortunately, this type of parasite, which typically infects cattle, is extremely rare in humans. But the case illustrates the surprising and unexpected ways parasites can cross over from animal to human hosts.
Most human cases of 'eyeworms' occur in rural communities with poor living conditions
Certain species of Thelazia, a genus of parasites sometimes called "eyeworms," have previously been known to infect humans in the U.S. and worldwide, as well as their feline and canine friends.
The parasites can cause serious eye damage and even blindness, causing scarring of the cornea as they move across the surface of the eye.
Most human cases occur in rural communities in Europe and Asia with poor living conditions, according to previous research. Children and the elderly are most at risk, since they can have a harder time keeping flies that are linked to the parasites away.
When it comes to T. gulosa specifically, though, the recent case is only the second known human diagnosis. In 2016, A woman in Oregon was infected and at least 14 worms were found in her eyes. Doctors could actually see the worms crawling in the membrane over the surface of her eye.
Although visible, the worms are small, between a quarter and three-quarters of an inch long, and very thin, about half as wide as the edge of a credit card.
The parasites seem to be spread by a species of fly that feed on eyeball secretions, usually on cattle
T. gulosa have been endemic in American cattle since the 1940s. It's not clear how the worms made the leap from cows to people, but scientists have linked the parasites to face flies, which feed on the fluids secreted by cow or human eyeballs to lubricate them.
While the cause of the most recent patient's infection remains unknown, she did report running into a swarm of flies, which she had to swat away from her face, including her eyes and mouth, according to the case study.
The park where the woman was running is located in a cattle ranching area.
The authors speculate that T. gulosa may be on the rise in livestock, resulting in a "zoonotic spillover" to humans and possibly other mammalian hosts.
While alarming, the worms can typically be dealt with, as in this case, by having them removed (ideally by a doctor) and providing an anti-parasite medication.