WASHINGTON (WASHINGTON POST) – The woman had just rounded a corner while running along a steep trail in coastal California in February 2018 when she charged face-first into an unpleasant surprise: a swarm of flies.

The pesky bugs quickly engulfed her, forcing her to swat them away from her face and even spit some out of her mouth. But little did she know, things were about to get much worse.

A month later, her right eye started to bother her. She rinsed it with water and out came the source of the irritation – only it wasn’t an errant eyelash or a wayward dust particle.

It was a live worm, roughly half an inch long, transparent and wriggling. And it wasn’t alone.

Soon after the first worm revealed itself, the 68-year-old plucked another one of the squirming critters from her eye, where it had been living in the space between her lower eyelid and eyeball.

In a rare occurrence, of which there is only one other documented case, experts say the Nebraska woman was infected by a parasitic eye worm known as Thelazia gulosa , a species normally found in cattle, according to a recent paper published in the journal Clinical Infectious Diseases.

The parasites often spread among cows – their preferred hosts – through certain types of face flies that eat eye secretions, such as tears, the Oct 22 paper said.

The flying insects carry the worm’s young, and when they’re feeding, they expel the larvae onto the surface of the new host’s eye, according to the Centres for Disease Control and Prevention (CDC).

The flies the woman ran through were likely larvae carriers, and at least one managed to come in contact with her eyes long enough to leave the parasites behind, Mr Richard Bradbury, the paper’s lead author, told Gizmodo. The trail she was running on is located near Carmel Valley, an area south-east of Monterey known for cattle ranching.

“Normally people would shoo any flies near their eyes away before they could do this, but in this case the patient had run into so many flies at once that she could not shoo them all away before one expelled larvae onto her eye,” Mr Bradbury, a former member of the CDC’s Division of Parasitic Diseases and Malaria, wrote in an e-mail.

Once the woman, who was not named, discovered the two worms in March 2018, she went to an ophthalmologist in Monterey, California, near where she was staying at the time. The doctor extracted a third worm, which was preserved for analysis.

Still, her eye irritation persisted, so when the woman returned to Nebraska, she consulted another doctor. No worms made an appearance during that visit, but the woman was informed that both her eyes were inflamed.

It didn’t take long for the woman to find and pull out what would be the fourth and final worm herself. Her symptoms finally cleared up about two weeks later, the journal article said.

Meanwhile, the worm sample was making the rounds. It was first sent to the California State Public Health Laboratory before getting forwarded to the CDC, where researchers nailed down the exact species and noticed a significant detail about the eye worm.

The worm was an adult female and her eggs contained developed larvae, “indicating that humans are suitable hosts for the reproduction of T. gulosa”, the paper said.

The Nebraska woman’s horrific experience was preceded by an eerily similar case involving a 26-year-old woman, who also became infected with the worms in 2016 after spending time in cattle fields near her native southern Oregon, The Washington Post’s Lena Sun reported. In that instance, the woman had 14 of the tiny translucent worms removed from her eye.

Though there have been only two occurrences of the parasite showing up in humans, researchers say the relatively short time period between the first and second case suggests that “this may represent an emerging zoonotic disease in the United States”, according to the October article.

“While it may just be a fluke event that two cases have occurred within a year or two of each other, it does raise the possibility that something might have changed in the ecology of T. gulosa in the USA to cause it to start occasionally infecting humans,” Mr Bradbury told Gizmodo.

Other infectious disease experts say that it is still too soon to call it a trend.

“You have to have the right fly, with the right bug and the right timing,” Ms Erin Bonura, an assistant professor of infectious diseases at Oregon Health & Science University who was the treating physician in the 2016 case, told The Post. “It’s just chance, and that’s why we don’t see it very often.”

It is also unlikely that a parasite would suddenly decide to switch hosts, such as going from an animal to a human, Mr William Gosnell, a programme director with the University of Hawaii at Manoa’s department of tropical medicine, medical microbiology and pharmacology, told The Post.

“Parasites are eukaryotic organisms like us, so they don’t change very quickly,” Mr Gosnell said.

In fact, much remains unknown about this particular parasite and its interactions with people, Ms Bonura said. The second case could help change that by providing additional data, leading to a better understanding of the worm and ideas for how to prevent future infections, she said.

Ms Bonura stressed that while “nobody wants to have a worm in their eye that they have to take out”, the parasites aren’t dangerous and the chances of experiencing the nightmarish ordeal are slim.

“It’s unlucky,” she said about becoming infected. “We don’t want people to worry about getting eye worms every time they go running.”

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