An invasive species of parasitic eyeworm is spreading further in the U.S. and finding new types of animal hosts. In a study published this week, scientists revealed the first known case of Thelazia callipaeda infection in an American black bear from Pennsylvania. The researchers caution that this discovery could indicate a rapid expansion in the population and range of the roundworm, which is also known to infect pets and humans.
Thelazia worms are spread by species of flies that look very similar to your typical house fly. But these flies have a preference for feeding on animals’ bodily fluids—particularly their tears. Worm larvae will hitch a ride onto the flies from their hosts’ eyes. The larvae will reach the next stage of life in the fly’s guts, move to its mouth, and then get spit out into another host’s eyes. Once there, the worms will fully mature, mate, and lay larvae to start the gross life cycle all over again.
There are three known species of Thelazia that can potentially infect humans. T. callipaeda is the most common culprit of these infections, though its confirmed distribution was confined to Europe and Asia until recently. In 2018, officials reported the first local U.S. case of T. callipaeda in a pet dog from New York. Other cases in cats and dogs have been seen since, usually from the Northeast. This latest case was detailed by researchers in a report published Wednesday in the journal Emerging Infectious Diseases.
According to the report, the infection was found in a black bear legally harvested in Monroe County, Pennsylvania, sometime in November 2023. As the bear was being prepared for taxidermy, someone noticed multiple nematodes (roundworms) behind one of the bear’s third eyelids. The worms were examined and sent to the researchers for examination. The scientists ultimately identified nine female and four male T. callipaeda worms from the bear’s eye. Further genetic testing confirmed the identity of these worms, while also showing that the worms were closely related to worms pulled from all other recent cases seen in North America and somewhat close to other cases seen in Europe. There were two other bears recently harvested from the area that were also reported to have eyeworms, though 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 wildlife in North America. The discovery of this case in Pennsylvania also suggests its range is expanding. And the worms’ emergence in black bears is especially troubling, since these animals are widely distributed throughout the continent.
“Given the bears’ extensive geographic distribution and frequent and close interaction with humans and pets, thelaziosis in the black bear population raises concerns about the rapidly increasing incidence and geographic range of T. callipaeda eyeworms in the United States,” the researchers wrote.
More research will be needed to confirm the extent to which bears and other local wildlife are playing in the worm’s circulation and transmission here. But these worms are already known to infect a wide range of hosts in Europe, including carnivores, omnivores, and lagomorphs (rabbits and hares). So if T. callipaeda has truly established a foothold in the U.S., it will have plenty of juicy targets to infect, including endangered animals, people, and their pets. Don’t worry, though, T. callipaeda isn’t even the only eyeworm to worry about these days. Just a few years ago, the first human cases of Thelazia gulosa infection were reported in the U.S.
Personally speaking, I’m swatting any flies that come near my face as quickly as possible from now on.
