Ask the doctors: Possibility of cattle eye worm infestation exceedingly rare

Dear Doctor: Please tell me more about the recent story in the news about the young woman who spent time on a ranch and then found worms living inside her eyes. We are cattle ranchers, and now I’m worried about my husband and kids.

Dear Reader: You’re correct – a 26-year-old woman from Oregon discovered that she was infected with Thelazia gulosa, a parasite commonly known as the cattle eye worm. Dogs and other canids, cattle and horses are the most common hosts. We hope that you’ll be reassured to learn that this is the first reported case of infestation of the cattle eye worm in a human host.

For those of you who missed the (fairly robust) media coverage of the story, here’s the gist. In 2016, a young woman who was working on a commercial fishing boat in Alaska thought she felt an eyelash in her eye. Although she rubbed her eye and flushed it with water, the scratchy, itchy sensation persisted. After a moment of looking in a mirror, she finally saw something above the lower lid of her left eye and lifted it out. Instead of one of her own brown lashes, she was looking at a slender white thread about a half-inch long – a living worm.

She went to an urgent care clinic and an ophthalmologist before seeing an infectious disease specialist. A team of parasitologists at the Centers for Disease Control and Prevention eventually nailed the diagnosis. In all, a total of 14 worms were removed from the young woman’s eye. It was by reading a research paper written in Germany in 1928 that a scientist from the CDC’s Parasitology Reference Diagnostic Laboratory finally identified the cattle eye worm.

So how did this happen?

Infestations of Thelazia, a genus of nematode worms that live as parasites in the eye and its surrounding tissue, are most commonly seen in animals and birds. Their spread to humans is extremely rare. In fact, just 10 other cases of eye worm infestation have ever been reported in North America. The most recent case in the United States was reported more than two decades ago. None of those 10 cases involved the cattle eye worm.

Although it’s impossible to know for sure, the thinking is that the young woman may have picked up the parasite while spending time at an inactive cattle ranch. The worms are carried by the flies that you see buzzing around the faces of cattle and horses. The worm larvae, which reach maturity inside the digestive tracts of the flies, move to their next host when the fly lands on the cow’s eyeball to drink. There, the eye worm grows to adulthood and produces more larvae, which are then picked up by new flies.

The good news is that because eye worms need the flies to reach maturity, their life cycle ends in a human host. With those 14 worms removed from her eyes, the young woman’s infestation was over. No permanent damage was done.

As for the rest of us, even if we spend time near cattle or horses, the chances of picking up the parasite are extremely small. Be sure to keep flies away from your face and eyes, and if one lands, remove it promptly.