Diplostomum spathacaeum
Also known as: Eye Fluke
Introduction
The eye fluke is a parasite of fish. Its scientific name is Diplostomum spathacaeum but other Diplostomum species may also be involved.
In the fish the parasite migrates to the eye where the metacercariae can be found. This leads to blindness and an inability to find food, especially in those species that rely on their sight to hunt, such as Salmonids.
When entering the fish in large numbers, the parasites can lead to an 'acute cercarial attack syndrome' leading to very severe damage and mortality.
Life Cycle
The life cycle of Diplostomum is complex and involves two more hosts: the definitive host which is a gull or fish-eating bird and a snail intermediate host (Lymnea species usually).
There is sexual reproduction in the intestine of the bird and shedding of eggs via the bird's faeces into the water. The eggs then hatch into free-swimming ciliated miracidia which infect and undergo asexual reproduction in the snail intermediate host. Cercariae are released from the snails and actively seek out fish to act as the second intermediate host. Millions of cercariae can be released over a very short period of time.
The cercariae mostly penetrate through the flank of the fish, often along the lateral line and they migrate through the tissues to the eye where the metacercariae develop.
The life cycle is complete once a fish-eating bird eats the fish and the metacercariae develop into adults in the intestine of the bird.
Clinical Signs
Fish will be blind, which will appear as fish becoming dark, unable to feed, losing condition and having an increased susceptibility to predation.
The acute cercarial syndrome appears as acute mortality with haemorrhages, resembling a serious bacterial infection.
Metacercariae may be visible in the eye of the fish if observed closely.
Diagnosis
There may be cataracts and metacercariae visible in the eye of fish.
A squash preparation of a fish eye may reveal the organisms through microscopic examination.
Gross pathology signs: in recently infected fish, petechiae and exophthalmia are observed. Chronically infected fish frequently show dark colouration, exophthalmia with emaciation and loss of condition, lens dislocation and capsular rupture. The site of entry is marked by tiny capsular perforation through which cortical lens fibres exude.
Histopathology: acute infections cause subcapsular cataract formation with varying capsular change. Occasionally, the perforations lead to lens rupture and severe endophthalmitis, followed by a generalised cortical liquefaction as flukes migrate to the anterior cortex with proliferation of the lens epithelium.
Treatment and Prevention
Once infected, fish cannot be cured.
Control of an outbreak may involve filtration of the water to 30-40 microns to eliminate cercariae from the water.
Prevention involves exclusion of birds from the farm and control of snail populations but this can be difficult to implement.
The cerceria are buoyant and there has been some success in taking water from a depth to feed a fish farm.
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References
Noga, E. (2010) Fish Disease: diagnosis and treatment John Wiley and Sons
Roberts, R. (2001) Fish Pathology Elsevier Health Sciences
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