JPC SYSTEMIC PATHOLOGY
SPECIAL SENSES
April 2021
S-P01 (NP)
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 subjacent to the lens capsule there is a broad 500 um wide band characterized by loss of the normal lens fiber structure with replacement by a homogenous, brightly eosinophilic material (liquefaction of lens fibers). Within this band at the posterior aspect of the lens are several elliptical trematode larvae (metacercariae). Between this band and the lens capsule are multifocal nodules of enlarged, rounded, foamy, eosinophilic nucleated cells (bladder cells) and rare, rounded, brightly eosinophilic accumulations of liquefied lens material (Morgagnian globules) (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, adherent to the corneal epithelium 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.
MORPHOLOGIC DIAGNOSIS:
1. Eye, lens: Cataractous change, subcapsular, multifocal, moderate, with intralenticular trematode metacercariae, white sucker fish (Catostomus commersoni), piscine.
2. Eye, iris and choroid: Panuveitis, granulocytic, moderate.
ETIOLOGIC DIAGNOSIS: Lenticular diplostomatosis
CAUSE: Diplostomum spp.
SYNONYMS: Diplostomatosis, diplostomiasis, parasitic cataract, eye fluke disease
GENERAL DISCUSSION:
- Diplostomum spathaceum, the fish eye fluke, is a digenetic trematode that infects freshwater fish in North America and various areas of the world
- A primary pathogen of the eye of fish; metacercariae can be found in the lens, vitreous humor, and retina
- Cyprinids and rainbow trout are particularly susceptible
PATHOGENESIS:
- 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
LIFE CYCLE:
- Adult trematode in intestine of definitive host (piscivorous birds) > eggs passed with feces > eggs embryonate in water > ciliated, free-swimming larvae (miracidia) 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, cataract, corneal ulceration, exophthalmos
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Unless the larvae (metacercariae) 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
DIFFERENTIAL DIAGNOSIS:
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
COMPARATIVE PATHOLOGY:
- Diplostomum spathaceum has been found in the lens of fish, amphibians, reptiles, birds, and mammals
REFERENCES:
- Noga, EJ. Fish Disease: Diagnosis and Treatment, 2nd ed. Ames, IA: John Wiley & Sons.; 2010: 219.
- Paperna I, Dzikowski R. Digenea (Phylum Platyhelminthes). In: Woo PTK, ed. Fish Diseases and Disorders, 1, 2nd ed., Cambridge, MA: CAB International; 2006:374-375.
- Wootten R. The parasitology of teleosts. In: Roberts RJ, ed. Fish Pathology, 4th ed., Ames, IA: Blackwell Publishing Ltd.; 2012:322-323.
