In November 2023, an adult, female American black bear was legally harvested in Coolbaugh Township, Monroe County, Pennsylvania. During processing of the bear for taxidermy preparation, multiple linear nematodes were observed behind the third eyelid. Nematodes were extracted and submitted for identification. Two additional harvested bears from Monroe and Pike Counties, Pennsylvania, were also reported to have similar ocular nematode infections, but specimens from those bears were not collected.

Figure 1

Figure 1. Morphologic features of adult female Thelazia callipaedaeyeworm isolated from an American black bear in Coolbaugh Township, Monroe County, Pennsylvania, USA, 2023. A) Anterior end showing the large, deep,...

We identified 9 female and 4 male adult nematodes from the bear as T. callipaeda on the basis of morphologic and morphometric features (8). The nematodes were characterized by the presence of a cup-shaped buccal capsule and cuticular transverse striations, as well as the location of the vulvar opening anterior to the esophageal-intestinal junction on the female worms (Figure 1). Female nematodes were 1.16–1.46 cm long and 0.36–0.42 mm wide; male worms were 0.82–1.06 cm long and 0.31–0.42 mm wide. The number of transverse cuticular striations ranged from 160 to 400/mm in the cephalic, midbody, and caudal regions.

Figure 2

Figure 2. Phylogenetic relationship of Thelazia callipaeda isolate from an American black bear in Coolbaugh Township, Monroe County, Pennsylvania, USA, 2023 (GenBank accession no. PP739308), and other species of ...

We extracted genomic DNA from a midbody fragment of a female adult worm and amplified, sequenced, and analyzed the partial cytochrome oxidase c subunit I (cox1) gene, as previously described (2). We generated a 623-bp cox1 sequence (GenBank accession no. PP739308), which showed 99%–100% maximum identity with T. callipaeda sequences available in GenBank. Phylogenetic analysis was performed by using the maximum-likelihood method and confirmed the taxonomic identification of T. callipaeda. The isolate clustered with all previous isolates from domestic animals in North America and with some isolates from Europe (Figure 2), indicating circulation of the newly introduced pathogen in wildlife habitats and transmission from domestic animals to wildlife.

The presence of adult T. callipaeda eyeworms in an American black bear suggests the establishment of a sylvatic transmission cycle in the United States and expansion of the number of definitive host species used by the zoonotic nematode. In the past decade, wild carnivores have been identified as primary definitive hosts associated with the sylvatic cycle in thelaziosis- endemic and non–thelaziosis-endemic areas of Europe and Asia (7). American black bears are the most widely distributed species of bear in North America, inhabiting diverse regions throughout Mexico, Canada, and the United States (9). Given the bears’ extensive geographic distribution and frequent and close interaction with humans and pets (10), thelaziosis in the black bear population raises concerns about the rapidly increasing incidence and geographic range of T. callipaeda eyeworms in the United States. Although further research into the extent to which black bears play a role in the maintenance of the sylvatic cycle and transmission of T. callipaeda eyeworms is needed, the presence of the zoonotic nematode in such a wide range of hosts implicates exposure and risk for transmission to threatened and endangered species and direct or indirect risk for transmission to humans and domestic animals.