Philophthalmiasis

Causal Agents

Zoonotic trematodes in the genus Philophthalmus, also called “eye flukes”, have infected humans on very rare occasions. The three Philophthalmus species conclusively identified in humans are P. lacrymosus (also spelled P. lachrymosus and P. lacrimosus), P. gralli, and P. palpebrarum. Note that in most previously reported cases, species could not be ascertained.

Life Cycle

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Adult flukes in the avian definitive host’s ocular orbit release embryonated eggs, which are then introduced into water sources by the host (e.g. during diving, aquatic foraging, or drinking)  .  Miracidia hatch almost immediately in water and penetrate the snail intermediate host  . Inside the snail host, pre-formed sporocysts within the miracidia produce generations of rediae, which become cercariae (  ,  ). Cercariae are released from the snail and encyst on stationary objects in water (e.g. vegetation, shells of mollusks and crustaceans) and become flask-shaped metacercariae; no second intermediate host is involved, as in many other trematodes  . The definitive host, which is usually an aquatic bird, becomes infected either upon ingestion of metacercariae or by direct exposure of the eye to cercariae or metacercariae in water  . When ingested, metacercariae excyst almost instantly in the mouth and migrate through the nasal passages to the ocular orbit over few days. Adult flukes are found in the conjunctival sac, either around the lachrymal glands or under/on the surface of the nictitating membrane  . Human infections are extremely rare, and the route of infection is not entirely clear. Direct entry of cercariae or metacercariae into the orbit following exposure during bathing or swimming in fresh water seems likely based on some case histories, although oral inoculation cannot be ruled out  .
 

Hosts

The majority of Philophthalmus species are associated with waterbirds and wading birds (e.g., gulls, ducks, geese, herons) although other avian species are capable of being infected under experimental conditions. Aberrant infections have also been achieved experimentally or observed naturally among some mammals, such as rabbits, rats, and non-human primates. All species that have been identified in human infections (P. lacrymosus, P. gralli, P. palpebrarum) use avian hosts.

Snail intermediate hosts are diverse and include freshwater and marine species. Introduced, invasive snail species may be responsible for the spread of Philophthalmus spp. to new locations. For many species, the snail host range is not known. Melanoides tuberculata appears to be the primary intermediate host for P. gralli.

Geographic Distribution

Various Philophthalmus species occur worldwide in avian hosts. Human cases have been reported from North America, Central Europe, the Middle East, East Asia, and South Asia.