Clinostomum

Clinostomum is a genus of parasitic trematodes, commonly known as yellow grub worms or louse-flukes. These flatworms are found worldwide and infect a variety of aquatic organisms, playing a role in complex life cycles. The adult worms are typically found in the digestive tracts of piscivorous birds, such as herons, egrets, and cormorants. The larval stages, however, are what are most often encountered by humans and are responsible for the common name "yellow grub."

The life cycle of Clinostomum is indirect, involving at least three hosts. The first intermediate host is usually a freshwater snail, where the parasite undergoes asexual reproduction. The second intermediate host is typically a fish, in which the parasite encysts as a metacercaria, or grub, in the flesh, often appearing as a yellowish spot. This is the stage that can be accidentally ingested by humans.

When fish containing Clinostomum metacercariae are consumed raw or undercooked by paratenic hosts or definitive hosts, the metacercariae excyst and develop into adult worms. In humans, accidental ingestion of metacercariae can lead to a condition called halzoun, a temporary pharyngeal inflammation, or more commonly, intestinal clinostomiasis, where the worms mature in the digestive tract. While infections in humans are generally mild and self-limiting, they can cause discomfort. Control measures focus on proper cooking of fish and preventing birds from defecating in water sources.