A species of worm that can infect human eyes has done it again, and this time it chose a 68-year-old woman from Nebraska. The woman is thought to be only the second human victim of these worms ever documented. But the incident signals they could become an emerging parasitic disease in the U.S.
The woman’s doctors described the disturbing details of the case in a paper published this October in the journal Clinical Infectious Diseases. Researchers from the U.S. Centres for Disease Control and Prevention also contributed to the paper.
According to the report, Dianne Travers Gustafson, a sociologist from Creighton University in Nebraska, was visiting California in February 2018. One day, as she was running along a trail, she went straight into a swarm of flies. Unbeknownst to her, she had likely run into a particular kind of fly known as the face fly, or Musca autumnalis. That’s because, a month later, an irritation in her right eye led her to the horrifying discovery of a tiny roundworm.
“The vector fly will expel larvae onto the surface of the eye or the conjunctiva while feeding on lacrimal secretions (tears, etc.). This can happen very quickly, so the fly would not have had to sit on the eye for more than a few seconds to expel the larvae,” lead author Richard Bradley, a former member of the CDC’s Division of Parasitic Diseases and Malaria, told Gizmodo by email. “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.”
Gustafson washed out the worm with tap water. Soon, though, she found and extracted a second one. A visit to the ophthalmologist the day after then yielded a third wiggler. Weeks later, she returned home to Nebraska but continued to experience inflammation in both eyes. A subsequent visit to another ophthalmologist turned up nothing, but lo and behold, she eventually pulled out a fourth worm. Thankfully, that was seemingly the last hitchhiker, as her eyes quickly returned to normal afterward.
When Bradley, now a visiting professor of public health at the Slovak Tropical Institute, St. Elizabeth University, Bratislava, and his team at the CDC Parasitology Reference Laboratory studied one of the worms taken from Gustafson, they discovered it was a member of the species Thelazia gulosa, better known as the cattle eye worm. Scarier still, it was an adult female worm, complete with a full cache of “eggs containing developed larvae.” In other words, we—and the surface of our eyes—seem to be perfectly suitable hosts for them. Luckily, Bradley said, these larvae can only compete their life cycle and become adults if they’re picked up anew by face flies and are spat back into another animal’s eyes. So there’s no danger of having your eyes become a breeding ground for generations upon generations of these worms.
Gustafson isn’t the only person to have suffered this ignoble infection. In 2016, an Oregon woman ended up playing mum to a whopping 14 cattle eye worms, which made her the first person known to be infected by this particular species (Bradley’s team also helped uncover the identity of the worms in that case). And while both women escaped with nothing more than a very scary story to share, the authors noted that in animals, these infections can cause permanent eye damage, even blindness, if left untreated.
Species of Thelazia are known to infect our eyes, thankfully only rarely. But up until last year, T. gulosa had never been fingered as a potential culprit for infection. And the fact that these two infections have been documented so close together in time suggests that this could become a relatively rare but established trend, the authors noted.
“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,” Bradley said.
It’s possible, for instance, that the species (and the flies that spread them) are now spreading far and wide among animal populations in the U.S., creating more opportunities for spillover transmission to people. But the only real way to solve this very creepy mystery, Bradley said, is to start tracking the prevalence of these infections among animal hosts across the country, as well as among face fly populations.
“It is also really important that anyone who thinks they might have worms in their eye should go and see a qualified medical doctor for help,” Bradley added. “The doctor can arrange to send any worms recovered from the eye to a parasitology reference laboratory for identification to see if they are T. gulosa. This way, we can know if any more human infections like this happen.”
Here’s hoping they never do.