JPC SYSTEMIC PATHOLOGY
Signalment (JPC #2340404): A white sucker fish
HISTORY: Tissue from a white sucker fish taken at the annual Yellowstone Lake survey by gill net.
HISTOPATHOLOGIC DESCRIPTION: Eye: Circumferentially beneath the lens capsule there is a broad 500 um wide band that has loss of the normal lens fiber structure with replacement by a homogenous bright eosinophilic material (liquefaction of lens fibers) along with several elliptical, trematode larvae (metacercariae) at the posterior aspect of the lens. In the overlying lens tissue, there are multifocal nodules of enlarged, rounded, foamy, eosinophilic nucleated cells (bladder cells) and rare, rounded, brightly eosinophilic accumulations (Morgagnian globules) of liquefied lens material (cataractous change). Metacercariae are approximately 180 x 360 um long, with an oral sucker, a 5 um thick tegument, no body cavity with a moderately cellular parenchyma surrounded by a layer of regularly palisading epithelial cells beneath a layer of muscle. Within the choroid and extending into the iris are moderate to high numbers of granulocytes, histiocytes and fewer lymphocytes admixed with an eosinophilic, granular material (protein). Overlying the corneal surface, there is a focally extensive accumulation of erythrocytes, fibrin and necrotic debris. Focally, in the superficial corneal epithelium near the fornix, there are clusters of 40 um diameter, eosinophilic, glassy cells with irregular nuclei (club cells). Diffusely, within the scleral stroma, there are low numbers of lymphocytes and rare macrophages.
- Eye, lens: Cataractous change, subcapsular, multifocal, moderate, with intralenticular trematode metacercariae, white sucker fish (Catostomus commersoni), piscine.
- Eye, iris and choroid: Panuveitis, granulocytic, moderate.
ETIOLOGIC DIAGNOSIS: Lenticular diplostomatosis
CAUSE: Diplostomum spp.
SYNONYMS: Diplostomatosis, diplostomiasis, parasitic cataract, eye fluke disease
- Diplostomum spathaceum, the fish eye fluke, is a digenetic trematode that infects freshwater fish in North America and various areas of the world
- Salmon and lampreys can become infected during spawning migrations into fresh water
- Metacercariae (larval forms) may be found in the eyes, coelom, and central nervous system of fish, often causing very little host reaction
- Metacercariae (larvae) penetrate the skin (often along the lateral line) and can migrate to many different tissues; their preferred niche is the lens, which is the only location where larvae can mature
- They invade through the posterior lens capsule by means of anterior spines and the secretions of anterior penetration glands; larvae migrate to the anterior cortex (days to weeks) and cause generalized cortical liquefaction with proliferation of lens epithelium
- Proliferative epithelium may fill the lens or escape through the capsule perforation, and continue to grow in the vitreous and posterior chamber
- Usually there are no extralenticular sequelae when the lens capsule is penetrated, however if the lens ruptures, a severe endophthalmitis occurs
- As the numbers of parasites in the eye increases, the feeding efficiency of the fish decreases, and heavily infected fish spend more time in surface waters, which increases the likelihood that they will be eaten by an avian final host
- Adult trematode in intestine of definitive host (piscivorous birds) > eggs passed with feces > eggs embryonate in water > ciliated, free-swimming larvae (miracidium) hatch > within 24 hours, infect first intermediate host (gastropod mollusc - Lymnaea) > undergo asexual reproduction (sporocysts) > free swimming cercariae released > within 24-48 hours, penetrate the skin of second intermediate host (primarily fish, although amphibians, reptiles, and mammals reported) > migrate to lens where metacercariae develop in 50-60 days > definitive host ingests infected lens tissue > adult fluke develops in intestine within 5 days
TYPICAL CLINICAL FINDINGS:
- Diminished or lost visual acuity results in difficulties in feeding, stunted growth, poor response to stimuli, and increased susceptibility to predation
TYPICAL GROSS FINDINGS:
- Lens opacity, corneal ulceration, exophthalmos
- Fins and body: Red patches or swellings at the site of cercarial penetration
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Metacercariae may be found anywhere in the eye: Lens, choroid, retina, vitreous body, sclera, retrobulbar tissue; metacercariae have well-developed muscular pseudosuckers (cotylae) on each side of an anterior oral sucker, a ventral holdfast organ, and paired ceca
- Unless the larvae die, there is little inflammatory response; when larvae die, the host response ranges from localized granulomatous inflammation to massive panophthalmitis, depending on how many larvae die
- Cataractous changes include epithelial hyperplasia or metaplasia, degeneration and liquefaction of lens fibers, morgagnian globules, and bladder cells
- If the lens ruptures, severe perilenticular nonsuppurative endophthalmitis occurs
- Hemorrhage from penetration of the lens, retina, and iris
- Secondary bacterial or fungal infections may occur
Cataracts in freshwater fish
- Dietary deficiencies: tryptophan, thiamine, riboflavin, zinc, vitamin A, vitamin C
- Environmental factors: gas supersaturation, possible cold stress, excessive sunlight exposure, ultraviolet radiation
- Intralenticular trematodes: Diplostomum, Tylodelphys spp., Apatemon spp.
- Secondary to endophthalmitis
- Toxins: thioacetamide, crude oil extracts, aromatic hydrocarbons
- Direct trauma to lens
- Diplostomum spathaceum has been found in the lens of fish, amphibians, reptiles, birds, and mammals
- Humans - Two cases of Diplostomum infection of the lens have been documented
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