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Trematodes in the genus, Philophthalmus, the eye flukes. The three species recorded in humans are P. lacrymosus, P. gralli and P. palpebrarum.
Fully-embryonated 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. Cercariae 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 .
Presumed worldwide. The known human cases are from the United States, Central Europe, the Middle East and southeast Asia and Japan.
There are two forms of ocular philophthalmiasis. One is an external form with follicular conjunctivitis and superficial keratitis. Patients with this form often have watery discharge from the palpebral conjunctiva of the upper and lower eyelids. The second is a sub-conjunctival form consisting of a mild edema with minimal cellular reaction.