JPC SYSTEMIC PATHOLOGY
Signalment: Striped bass
HISTORY: This fish had a wart-like growth on the dorsum.
HISTOPATHOLOGIC DESCRIPTION: Scaled skin: In a focally extensive area, the superficial dermis is markedly thickened up to 2 mm by numerous basophilic, round to polygonal fibroblasts that are up to 300 um in diameter with a basophilic, vacuolated, finely granular cytoplasm that is surrounded by a 10-30 um amphophilic, hyaline wall (lymphocystis cells). The cells have a markedly enlarged central nucleus with coarsely clumped, fibrillar chromatin and a variably distinct nucleolus. Frequently, the nucleus is replaced by an amorphous eosinophilic material admixed with a basophilic fibrillar material (degeneration and necrosis). Frequently lymphocystis cells contain a fibrillar, basophilic material within the peripheral cytoplasm (inclusion material). Lymphocystis cells are separated by thin bands of connective tissue that contain low numbers of macrophages, lymphocytes, plasma cells and karyorrhectic necrotic debris that extends into the subjacent stratum compactum.
MORPHOLOGIC DIAGNOSIS: Scaled skin (per contributor): Fibroblast hypertrophy, focally extensive, marked, with cytoplasmic inclusions and lymphoplasmacytic and histiocytic dermatitis, Striped bass (Morone saxatilis), piscine.
ETIOLOGIC DIAGNOSIS: Iridoviral dermatitis
CAUSE: Lymphocystis disease virus (LDV) or piscine iridovirus
CONDITION: Lymphocystis disease (LD)
- Most common viral infection of aquarium fish
- Lymphocystis disease virus (iridovirus) is an intracytoplasmic, 130-330 nm, double-stranded DNA virus
- Worldwide distribution; affects a wide variety of higher teleosts
- Self-limiting disease which affects fish compromised by underlying problems, stress, crowded environments
- Transmission is primarily horizontal via direct contact, surface wounds and contaminated bait; skin and gills are the main portals of viral entry
- Infects dermal fibroblasts causing cellular hypertrophy (up to a 100,000 fold increase); cells do not divide, but the nucleus and cytoplasm expand, and develop a thick (5–10 um) hyaline laminar capsule
- Nuclei are enlarged and vesiculate with a distorted or indistinct nucleolus
- Viral G protein-coupled receptor on fibroblasts may inhibit apoptosis to induce cellular transformation
- Incubation is host-dependant; warm water species have shorter cycles (2 weeks) than cold water species (up to a year)
- Wild fish tend to have localized chronic lesions; aquarium fish have generalized skin involvement and higher mortality
- 8 kDa protein is a cellular receptor for LCDV and is located in the gill, stomach, intestine, skin have the greatest expression of the receptor; receptor is also located in the heart, brain liver, spleen, head kidney, kidney and gonad
TYPICAL CLINICAL FINDINGS:
- Relatively benign disease, with some reports of weight loss
- Nodules take weeks to years to develop and will eventually heal with little scarring
TYPICAL GROSS FINDINGS:
- Early stages appear as salt-like dusting on skin
- Later stages are characterized by macroscopic cream-colored wart-like nodules on the skin and fins (occasionally on the gill filaments, pharynx, intestinal wall, mesentery, liver, spleen and ovary), measuring 0.3–2.0 mm in diameter
- Nodules can become reddened depending on vascularity
- Nodules consist of individual lymphocystis cells or lymphocysts
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Large non-neoplastic hypertrophied dermal fibroblasts up to 2 mm in diameter with a 5-10 um thick hyaline laminar capsule, enlarged centrally located nucleus, nucleolus and basophilic cytoplasmic inclusion material is unique and diagnostic
- Basophilic intracytoplasmic inclusion material may appear as lacy, web-like fragments (plaice type) or dense vacuolated bodies (mullet type)
- In later stages lymphocysts can rupture and be surrounded by numerous inflammatory cells
- Lymphocystis is the largest icosahedral virus at 130-330 nm in diameter
- Forms paracrystalline arrays
- Dense viral core within two capsid unit membranes; globular surface subunits with long filaments
- Nucleus and nucleolus of infected cells enlarge proportionally
ADDITIONAL DIAGNOSTIC TESTS:
- Wet squash prep
- Epitheliocystis: Affects primarily gills; nuclei of epithelial cells are pushed peripherally by a granular basophilic inclusion with coccoid or coccobacillary bodies
- Dermal sarcoma virus: Solid masses of normal sized cells with cytoplasmic inclusions
- Trematode cysts: Movement of trematodes observed in cysts
- Ich (Ichthyophthirius multifilis): Encysted ciliates within epithelial cells
- Lymphocystis disease has been described in 125 teleost species of 42 families and 9 orders and includes species from marine, brackish, and freshwater environments
- Iridoviruses are mainly pathogens of insects, fish, amphibians and reptiles
- The genus Ranavirus, family Iridoviridae, affects amphibians
- Bollinger TK, Mao J, Schock D, Bringham MR, Chinchar VG. Pathology, isolation, and preliminary molecular characterization of a novel iridovirus from tiger salamanders in Saskatchewan. J Wildl Dis. 1999; 35(3): 413-429.
- Ellis, AE. Genome analysis of fish lymphocystis disease virus. In: Anders K, Darai G, eds. Fish and Shellfish Pathology. Orlando, FL: Academic Press; 1985: 301-305.
- Huang Y, Huang X, Zhang J, Gui J, Zhang Q. Subcellular localization and characterization of G protein-coupled receptor homolog from lymphocystis disease virus isolated in China. Viral Immunology. 2007; 20(1):150-159.
- Noga EJ. Fish Disease Diagnosis and Treatment. 2nd ed. St. Louis, MO: Mosby; 2010:171-173.
- Roberts RJ. The virology of teleost. In: Fish Pathology. Philadelphia, PA: WB Saunders; 1989: 192-300.
- Stoskopf MK. Fish Medicine. Philadelphia, PA: WB Saunders Company; 1993: 276-288.
- Wolf K. Lymphocystis disease. In: Fish Viruses and Fish Viral Diseases. Ithaca, NY: University Press; 1988: 268-286.
- Wu RH, Tang XQ, Sheng XZ, Zhan WB. Tissue distribution of the 27.8 kDa receptor and its dynamic expression in response to lymphocystis disease virus infection in flounder. J Comp Path. 2015;153(4):324-332.