During a routine health survey of the pinniped colonies on the Galapagos Islands, eye infections by digenetic trematode parasites were found in five of the seven populations on five different islands. The parasite was identified as a member of the genus Philophthalmus, a common parasite found in the eyes of aquatic birds.
The parasites were in the conjunctiva of the eyes, usually bilateral, and normally complicated by secondary bacterial infection with a purulent secretion. Pups were more severely affected.
The prevalence of infection was as high as 90 percent, with 50 percent found with secondary bacterial infections.
This is the first report of an eye fluke from a marine mammal and transmission is speculative. However, most eye fluke species have a similar general pattern in their life cycles. Adults are hermaphroditic and produce eye-spotted miracidia within eggs, which hatch immediately upon exposure to water. Miracidia mimic the behavioral patterns of their snail hosts for enhanced localization and penetration. The miracidium injects a preformed redia stage into the snail. Redia produce three generations of cercaria, which leave the snail and quickly encyst on hard surfaces. Upon thermal stimulation in the throat of the host, the metacercarial stage excysts and migrates via the lacrimal duct to the orbit, where it matures to an adult. We are currently investigating whether the parasite is infecting the sea lions directly through cercarial eye contact during swimming, or by feeding on metacercarial stages encysted on food.
The author thanks Dr. Alberto Paras, Gerencia de Veterinaria, Puebla, Mexico, Sandie Salazar and Poly Robayo, Charles Darwin Research Center, Galapagos Island, for help during the fieldwork. Thanks also to the Marine Mammal Center for their continued support.