These tiny worms live in eyes, feed on tears and could transmit to humans

Scientists have found over a dozen parasitic worms in the eyes of a black bear, renewing fears that humans could face an emerging danger from the tiny worms that can cause blindness.

The black bear was killed in rural Pennsylvania in November of last year. The researchers extracted DNA from the tiny, less-than-2-centimeter worms, connecting them with Thelaziosis, an "emerging zoonotic disease," they wrote in the paper.

Caroline Sobotyk, the principal author of the study and an assistant professor of clinical parasitology at the University of Pennsylvania, said the discovery marked the first time the parasite was found in a wild animal in the U.S.

The worms have previously been found in pets in the U.S. Researchers say finding the worms in a wild bear furthers concerns that the eyeworm could expand across the U.S.

"This does increase the risk for us humans to get this parasite as well, and for our pets and other animals," she said.

The worms pose a risk of "transmission to humans and domestic animals," according to a new research paper published in the Centers for Disease Control and Prevention's Emerging Infectious Diseases journal on Wednesday.

"It wouldn't come directly from any of our wildlife species into people. But wildlife could act as what we call a reservoir" of indirect infection, said Kevin Niedringhaus, an assistant professor of wildlife pathology at the University of Pennsylvania who co-authored the study.

The worms are transmitted by infected flies, which pass them to animals and potentially humans by landing near their eyes and feeding on their tears, according to the CDC. In serious cases, they can trigger ulcers on the surface of the eye and even blindness.

"Offspring of the worm is probably being collected by the fly, and then that's being carried over to another host, whether it's another animal or a human," Niedringhaus said. Flies carrying the parasite then lay eggs on the face of the host, transmitting the worm to its eye.

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Worms identified in American pets in 2018

The broad range of land inhabited by black bears in the U.S. and their frequent interactions with humans and pets has scientists worried that the worm could spread. The worm also poses a threat to infect endangered and threatened species, according to the researchers.

"We do not know the impact that these worms can cause in wildlife," Sobotyk said.

The first case was identified in the U.S. when the worms were found in a Labrador retriever in 2018, with more cases identified in American pets in the years since, mostly in the Northeast. A research paper published in February found cases of the worm in two cats and 11 dogs that had not traveled outside American borders, leading researchers to believe "this parasite is emerging in North America."

"Not only are dogs and cats probably playing a role in the maintenance of this new parasite here," Niedringhaus said. "Wildlife were probably playing a role as well."

The worms were first widely identified in southeast Asia, before cases started to pop up in Europe, beginning in the early 90s, according to an article from the academic journal Trends in Parasitology. Niedringhaus said human cases have been reported in Asia.

Since then, cases have been reported in almost all European countries. Red foxes "seem to play a large role" in their transmission, according to the paper.

Sobotyk said people shouldn't be worried about severe or fatal health consequences of the worms. They are restricted to the eyes, meaning they won't migrate to other parts of the body like organs.

"No one's going to die because of eyeworms," she said.

"Worst case scenario, they can cause blindness," she added. Still, someone infected would likely visit a doctor before it escalated to that level, she said.

Niedringhaus said pet owners can stay on the alert for the emerging parasite by taking their pets for regular check-ups and keeping an eye open for any signs of disease around their eyes, like excessive wateriness.

"The best thing to do is to keep an eye probably on your pets first and foremost, because they're probably going to act, more than likely, as the means of potential transmission," he said.