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A woman who was trail running on the coast of California didn’t think much of it when she ran through a huge swarm of flies. A month later, her right eye started to bother her and out came a live parasitic worm.
The unnamed jogger’s disturbing experience, documented in a recent issue of the journal Clinical Infectious Diseases, didn’t end there.
It all began in February 2018 while the 68-year-old Nebraska woman was spending the winter in the Carmel Valley. During a jog on a local trail, she came across the swarm of flies so large that she was surrounded by them and tried swatting them away and spitting some out of her mouth. When her eye started bugging her a month later, she rinsed it out, thinking there was an eyelash or dust particle in it. That’s when she discovered a half-inch-long worm, still wriggling. Soon after, she found another.
The woman went to see an ophthalmologist in Monterey where the doctor extracted a third worm.
Upon returning to her home state, her eye continued to bother her. She went to another doctor, who couldn’t find anything but noted that both of her eyes were swollen. Once she returend home, she discovered and extracted the fourth and final worm herself and her eyes cleared up about two weeks later.
It turns out that the that the worm was a parasite called Thelazia gulosa, which is normally found in cattle. According to the Centers for Disease Control and Prevention, cows are the organisms’ preferred hosts. The parasites are spread through certain types of face flies that eat eye secretions, such as tears. The flies carry the worm’s young and then transmit the larvae onto the surface of a new host’s eye when they’re feeding.
The trail the woman was running is in an area known for cattle ranching. The flies she ran through were likely larvae carriers, with at least one coming into contact with her eyes long enough to leave the parasites behind, Richard S. Bradbury, the paper’s lead author, told Gizmodo.
“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,” said Bradbury, a former member of the CDC’s division of parasitic diseases and malaria.
One of the worms was analyzed and identified as an adult female whose eggs contained developed larvae, “indicating that humans are suitable hosts for the reproduction of T. gulosa,” the journal said.
Typically, symptoms of Thelazia infection include inflammation, foreign body sensation and excessive lacrimation (tearing), according to the journal. However, less commonly, the worms can migrate across the surface of the eye and cause corneal scarring and blindness.
Only one other case of a human host has been documented. Last year, the American Society of Tropical Medicine and Hygiene reported a case of a 26-year-old woman who became infected after spending time horseback riding near cattle fields in southern Oregon in 2015. Over a period of 20 days, she had a total of 14 live, translucent worms removed from her eye.
“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,” Bradbury told Gizmodo.
Despite these two cases, people who like to hike and run trails don’t need to be worried about being infested by eye worms, according to a handful of experts who talked to Yahoo Canada.
“I have seen some cases of ‘worms in eyes’ when working in West Africa with the Orbis NGO, but even there they are extremely rare,” says Dr. Simon Holland, clinical professor in the department of ophthalmology at the University of British Columbia in Vancouver. Orbis is a charity that prevents and treats blindness in 18 countries in South America, Africa, and Asia.
Dr. Hunter Cherwek, a global ophthalmologist who’s currently working with Orbis in Ghana, says tremendous progress has been made in treating infectious blindness all over the world, including river blindness, trachoma (a contagious bacterial infection of the eye), Cytomegalovirus retinitis (a serious viral infection of the retina), and “worms”.
“But still, extremely rare cases unusual eye infections can come up anywhere,” he said via email. “I think the most sight threatening risk for joggers, especially trail runners, would be trauma, like a branch, so my biggest recommendation would be to wear protective sunglasses and practise contact lens hygiene.”
Dr. William Bowie, professor in the division of infectious diseases at UBC and former acting director of the BC Centre for Disease Control, says that fly-transmitted parasitic infections are not uncommon, particularly in people who walk or sit in sand in the tropics or get bites into which insects deposit eggs which hatch.
“Flies are well-known to be vehicles for lots of infections,” Bowie says. “In terms of worry about being infected while jogging it would be an absolute non-consideration. If I was worried about acquiring an infection, which I am not, there are many others to consider before worrying about this likely rare infection. Most of these other infections are less common or not present here [in Canada].”