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[Philophthalmus spp.]

Proglottids of Mesocestoides sp., collected from the stool of a dog.

Adult Philophthalmus sp., removed from the conjunctiva of a patient, stained with hematoxylin and eosin (H&E).

Causal Agents

Trematodes in the genus, Philophthalmus, the eye flukes. The three species recorded in humans are P. lacrymosus, P. gralli and P. palpebrarum.

Life Cycle

Life cycle of Philophthalmus spp.

Fully-embryonated eggs are shed into the water from the definitive host's eyes The number 1. Miracidia hatch almost immediately in water The number 2 and penetrate the snail intermediate host The number 3. 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 (The number 3a, The number 3b) and become cercariae. Cercariae are released from the snail The number 4 and encyst on aquatic vegetation or other solid objects in the water The number 5. The definitive host, which is usually an aquatic bird, becomes infected upon ingestion of metacercariae The number 6. Metacercariae excyst in the mouth and migrate to the eye where the adults reside The number 7. Humans rarely serve as incidental hosts, but may do so when they ingest metacercariae on aquatic vegetation The number 8.

Geographic Distribution

Presumed worldwide. The known human cases are from the United States, Central Europe, the Middle East and southeast Asia and Japan.

Clinical Presentation

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.

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  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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