Eye Fluke (Diplostomum) and Cataracts in Fish

Diplostomum larvae enter the lens of a fish's eye and cause cataract and blindness. The fluke cycles through snails and gulls, cannot be cured in the fish, and is controlled at the water source.

Eye fluke is infection of the fish eye by larval flukes of the genus Diplostomum, which settle in the lens and turn it opaque (cataract), sometimes causing blindness. The parasite reaches fish only from infected snails and matures only in fish-eating birds, infected fish cannot be cured, and control means breaking the snail–bird cycle and protecting the water supply.

Cause

Diplostomum is a genus of digenean trematodes in the family Diplostomidae. The Alaska Department of Fish and Game (ADF&G) manual names Diplostomum spathaceum as a common lens parasite, while other species live in the vitreous chamber. A Danish field study confirmed this split: D. pseudospathaceum, D. mergi and D. paracaudum were found in the lens, whereas Tylodelphys clavata and D. baeri occupied the vitreous humour. The lens has no blood supply, so immune cells cannot reach the larvae there, which helps explain the parasite's wide range of fish hosts.

How the larvae make the eye opaque

Adult flukes live in the intestine of gulls and other fish-eating birds; eggs pass into the water with the droppings.
Miracidia hatch and infect pond snails, mainly Lymnaea species such as Lymnaea stagnalis, as well as Radix, where the parasite multiplies asexually.
Snails release free-swimming cercariae, which penetrate the fish, usually through the skin of the flank.
The young larvae migrate to the eye, generally within 24 hours depending on temperature, enter the lens and develop into metacercariae.
The metacercariae can stay in the eye for a long time, and the damage they cause to the lens produces cataract and, often, blindness.
Blind or visually impaired fish are easier for birds to catch, which completes the cycle; the parasite is also reported to alter host behaviour in ways that favour this.

Host range

Diplostomum species are common throughout North America and Europe. Many salmonids and other freshwater fish are susceptible, including warmwater species such as channel catfish; rainbow trout is a recorded host, and eye flukes were found in roach, perch, bream and bleak in Danish lakes. The Danish study found the snail stage to be highly host-specific but the fish stage much less so, with several fish species carrying the same eye fluke species. ADF&G states that this parasite poses no human health concern, while the fishpathogens.net database notes isolated reports of infection in amphibians, reptiles and humans.

Signs and effects

Opaque, milky lens (cataract); in advanced cases lens dislocation, rupture of the lens capsule and inflammation inside the eye.
Blindness with darkened body colour, inability to feed, weight loss and, in chronic cases, emaciation.
If both eyes are affected, complete blindness may follow and the fish will probably die from predation or starvation; with one eye affected it may survive indefinitely.
Massive invasion by cercariae can kill fish quickly, with haemorrhages resembling a bacterial infection; in a laboratory study, 1,000 cercariae per fish killed all rainbow trout fry within 24 hours.
In aquaculture the parasite reduces feeding, growth and marketability, and blindness has been recorded in hatchery coho salmon.

Telling it apart from other eye cloudiness

A cloudy lens is not proof of eye fluke. A veterinary case series on cataract surgery in aquarium fish lists osmotic and nutritional imbalances, unsuitable temperature, excessive ultraviolet radiation, toxins, trauma and genetic predisposition as recorded causes of cataract, notes that age-related cataract has not been reported in fish, and states that in wild fish cataracts are often due to Diplostomum. Because infection requires cercariae from snails, eye fluke is most likely in fish that have lived in ponds, lakes or farms supplied with untreated surface water. Diagnosis is confirmed by finding metacercariae in the lens or vitreous humour on a wet-mount or squash preparation under the microscope, or by histology. Other causes of a cloudy or bulging eye are covered in the Cloudy Eye and Popeye (Exophthalmia) articles.

Control by breaking the snail–bird cycle

Infected fish cannot be cured, and cataracts from causes other than acute osmotic imbalance have not been reported to reverse with husbandry changes or medication.
Control snail populations in ponds and supply channels and exclude fish-eating birds from the site.
On fish farms, WikiVet lists filtering incoming water to 30–40 microns and drawing water from depth to reduce exposure to cercariae.
Cercarial shedding is strongly temperature-dependent and peaks above 20 °C, so the infection pressure is highest in warm summer periods.
Veterinary cataract surgery (lens removal) has been performed on fish in a public aquarium to restore feeding, and phacoemulsification is used for cataracts caused by intraocular parasites; this is a specialist procedure, not a home treatment.