Diplostomids

Diplostomids are a group of digenean trematodes in the family Diplostomatidae, within the order Diplostomida. They are best known for parasitizing fish as eye flukes, but the group comprises multiple species that use similar multi-host life cycles. Adult worms reside in the definitive hosts, typically birds, while juveniles infect intermediate hosts such as snails and fish.

The life cycle is complex and usually involves three hosts. Eggs released in the feces of the final host hatch into free-swimming miracidia that infect freshwater snails. Inside the snail, the parasites develop into sporocysts and produce cercariae, which leave the snail and invade fish. There they encyst as metacercariae, frequently in the eyes or other tissues. When a fish is consumed by a bird, the metacercariae mature into adults in the definitive host, and eggs are produced to continue the cycle.

Pathology and ecological impact vary by species. In fish, diplostomid metacercariae in the lenses can cause diplostomiasis, leading to ocular damage, reduced vision, impaired predator avoidance, slower growth, and increased susceptibility to predation. In definitive hosts, infections are often asymptomatic or of minor clinical significance. Diplostomids are of particular concern in aquaculture and wild fish populations, where they can influence survival and fitness.

Distribution is worldwide in freshwater systems where suitable snail and fish hosts occur. Diagnosis relies on detecting metacercariae in fish tissues, eggs in feces of definitive hosts, or molecular methods. Control measures in captive settings include snail management, minimizing exposure of fish to infected snails, and, where appropriate, anthelmintic treatment of definitive hosts.