Unembryonated eggs with an inconspicuous operculum, a small knob on the end opposite the operculum, and a size around 65 to 75 μm were seen in the stool of a traveler who often eats raw fish. What is the most likely identification?

• A. Fasciola hepatica
• B. Diphyllobothrium latum
• C. Paragonimus westermani
• D. Clonorchis sinensis

Answer: (B)

Recognizing egg morphology and how the parasite is acquired guides identification. These eggs are about 65–75 μm in length, are unembryonated when shed, and show an operculum with a small knob on the opposite end. This combination is classic for the fish tapeworm Diphyllobothrium latum. The size fits the typical range for its eggs, and the operculated, knobbed egg is a distinctive feature. The traveler’s history of eating raw fish fits the transmission route, since Diphyllobothrium latum is acquired from consuming undercooked freshwater fish.

Other parasites have different egg sizes and food exposures. For example, eggs of Clonorchis sinensis are smaller, while Fasciola hepatica eggs are larger and linked to ingestion of aquatic plants, and Paragonimus westermani eggs are typically associated with crustaceans and pulmonary symptoms. The combination of the egg size, unembryonated state, and fish-related exposure makes Diphyllobothrium latum the best fit.