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Three species of Philophthalmus trematodes (eye flukes) have been recorded parasitizing humans: P. lacrymosus, P. gralli. and P. palpebrarum. The geographic distribution of the genus is presumed to be worldwide. The known human cases are from the United States, Central Europe, the Middle East, and southeast Asia and Japan.
Fully-embryonated Philophthalmus eggs are shed into the water from the definitive host’s eyes. Miracidia hatch almost immediately in water and penetrate the snail intermediate host. Several snail genera may serve as intermediate hosts, including Thiara spp. and Melanoides spp. Inside the snail host, the miracidia (which contain a pre-formed redia) undergo a series of stages and become cercariae. Cercaria are released from the snail and encyst on aquatic vegetation or other solid objects in the water. The definitive host, which is usually an aquatic bird, becomes infected upon ingestion of metacercariae. Metacercariae excyst in the mouth and migrate to the eye where the adults reside. Humans rarely serve as incidental hosts, but may do so when they ingest metacercariae on aquatic vegetation.