Parasitic Eyeworm Spreads in the US—and Bears Are the Latest Victims

An invasive species of eyeworm is spreading widely in the US and finding new species. In a study published this week, scientists revealed the first known case of this Thelazia callipaeda infection in an American black bear from Pennsylvania. The researchers warn that the findings could indicate a rapid increase in the population and range of roundworms, which are also known to infect pets and people.

Thelazia worms are spread by a variety of flies that look a lot like your regular flies. But these flies love to feed on the body fluids of animals—especially their tears. Worm larvae will ride flies into the eyes of their hosts. The larvae will reach the next stage of life in the fly’s gut, pass through its mouth, and be spit into the eyes of another host. Once there, the worms will fully mature, mate, and lay larvae to begin the complete life cycle again.

There are three known types of Thelazia which can infect people. T. callipaeda is the most common cause of these diseases, although its confirmed distribution was restricted to Europe and Asia until recently. In 2018, officials reported the first US local case T. callipaeda from a pet dog from New York. Other cases in cats and dogs have been seen since, usually from the Northeast. This latest issue is detailed by researchers in a report published Wednesday in the journal Emerging Infectious Diseases.

According to the report, the virus was found in a black bear legally harvested in Monroe County, Pennsylvania, in November 2023. As the bear was being prepared for taxidermy, someone noticed many nematodes (worms) on the back of one of the third bear’s skins. The worms were examined and sent to researchers for testing. The scientists ended up identifying nine women and four men T. callipaeda worms from the bear’s eye. Further genetic testing confirmed the identity of these larvae, and also showed that these larvae were closely related to larvae removed from all other recent cases seen in North America and very close to other cases seen in Europe. There are two other bears recently harvested in this area that are reported to have eye worms although these worms were not collected for study.

The findings suggest, the researchers say, that T. callipaeda has successfully made the jump from domestic animals to wild animals in North America. The discovery of this case in Pennsylvania also suggests that its scope is expanding. And the occurrence of larvae in black bears is of great concern, as these animals are widely distributed throughout the continent.

“Due to the widespread distribution of bear habitats and frequent and close interactions with humans and domestic animals, theoziosis in black bears raises concerns about increased incidence and geographic scope. T. callipaeda eyeworms in the United States,” the researchers wrote.

More research will be needed to determine the extent to which bears and other local wildlife play a role in the circulation and spread of the worm here. But these worms are already known to infect a large number of animals in Europe, including carnivores, omnivores, and lagomorphs (rabbits and rabbits). So if T. callipaeda has established a foothold in the US, it will have many sweet targets to infect, including vulnerable animals, humans, and pets. Don’t worry, though, T. callipaeda it’s not even an eye worm to worry about these days. A few years ago, the first human cases of Thelazia grocery store infection was reported in the US

Personally, I swat any flies that come near my face as fast as possible from now on.