JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

November 2025

I-V15

Signalment: Striped bass

HISTORY: This fish had a wart-like growth on the dorsum.

HISTOPATHOLOGIC DESCRIPTION: Scaled skin: Multifocally, the superficial dermis is expanded up to 600 µm by hypertrophic fibroblasts that are up to 300 µm in diameter (cytomegaly) and are surrounded by a 10-30 µm eosinophilic, hyaline wall (lymphocyst). Lymphocysts have abundant finely granular, amphophilic cytoplasm, a central, irregularly round, vesiculate, or fragmented nucleus with coarsely clumped chromatin (degeneration and necrosis), and, often within the periphery of the cytoplasm, varying amounts of fibrillar, clumped basophilic material (intracytoplasmic viral inclusions). The epidermis is multifocally and mildly hyperplastic and there are low numbers of transmigrating neutrophils, and few dermal macrophages, lymphocytes, and plasma cells that rarely extend into the stratum compactum.

Ovary, liver, kidney, spinal column with notochord, pancreas, skeletal muscle, intestines: Essentially normal tissue.

MORPHOLOGIC DIAGNOSIS: Scaled skin, superficial dermis: Fibroblast hypertrophy, focally extensive, marked, with mild lymphoplasmacytic and histiocytic dermatitis and intracytoplasmic inclusions bodies, Striped bass (Morone saxatilis), piscine.

ETIOLOGIC DIAGNOSIS: Iridoviral dermatitis

CAUSE: Lymphocystisvirus, Lymphocystis disease virus

CONDITION: Lymphocystis disease (LD)

GENERAL DISCUSSION:

• Most common viral infection of aquarium fish
• Disease is chronic, disfiguring, self-limiting, and often spontaneously regresses; seen in freshwater and saltwater teleosts (over 125 species and 30 familes; does not affect salmonids, catfish, or cyprinids)
• Lymphocystisvirus is a double stranded DNA virus belonging to the Iridoviridae family (an iridovirus); Iridoviridae includes five genera: Iridovirus, Chloriridovirus, Ranavirus, Megalocytovirus, and Lymphocystisvirus
• Two lymphocystis viruses have been characterized – Lymphocystis disease virus 1 (LCDV-1) and Lymphocystis disease virus 2 (LCDV-2), which cause disease in flesus flounder/plaice and dab, respectively

PATHOGENESIS:

• Virus is viable in water for 1 week
• Virus infects dermal fibroblasts, resulting in tremendous hypertrophy
• Transmission not well characterized, but likely is horizontal via release of virus from rupture or sloughing of lesions into water, followed by infection of damaged skin
• Incubation period is long (weeks to months) and many fish are latent carriers, breaking with clinical disease after shipping or other stress

TYPICAL GROSS FINDINGS:

• Various sized, white to pink masses on the skin, less often gills, buccal cavity, and rarely on serosal surfaces of internal organs
• Early lesions: granular, discrete, pinpoint nodules or “sand-like dusting” on the body
• Later lesions: large, coalescing neoplastic-like masses

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Hypertrophied fibroblasts with extreme cytomegaly (“lymphocysts”) surrounded by a thick eosinophilic hyaline capsule, with basophilic intracytoplasmic viralinclusions and central nucleus; inclusions may be small, lacy and scattered (plaice type) or large and cord-like with blebs (mullet type)
• In later stages lymphocysts can rupture and be surrounded by numerous inflammatory cells (lymphocytes, histiocytes)

ULTRASTRUCTURAL FINDINGS:

• Forms paracrystalline arrays

ADDITIONAL DIAGNOSTIC TESTS:

• Wet mount of skin or gills – enlarged dermal fibroblasts

DIFFERENTIAL DIAGNOSIS:

• Epitheliocystis: Chlamydia-related bacterial disease; affects primarily gills; epithelial cell hypertrophy with peripherialization of nuclei by a granular basophilic inclusion with coccoid or coccobacillary bodies (cyst-like inclusions) that fill the entire cell
• Walleye dermal sarcoma virus: Type C retrovirus in the genus Epsilonretrovirus induces dermal sarcomas - solid masses of normal-sized cells with cytoplasmic inclusions
• “Ich” (Ichthyophthirius multifilis, I-P16): may grossly look similar to mild lymphocystis lesions; encysted ciliates within epithelial cells

COMPARATIVE PATHOLOGY:

• Systemic iridoviruses – cause significant mortality and economic loss in food and ornamental fish; disease influenced by stressors; transmission via contaminated water and ingestion of infected tissue
  • Genus Megalocytivirus – affect only fin fish; fish are lethargic, anemic; grossly have brachial hemorrhage and splenomegaly; histologically, cytomegalic cells with intracytoplasmic inclusions in multiple organs, usually adjacent to the endothelium of vessels
    • Red sea bream iridovirus (RSIV) – OIE listed disease
    • Infectious spleen and kidney necrosis virus (ISKNV)
    • Banggai cardinalfish iridovirus (BCIV) – only Megalocytivirus in marine fish
      • Recent paper on ocular pathology in Syngnathidae and Apogonidae aquarium fish found 13 cases of ocular lesions due to iridovirus in Banggai cardinal fish and two in other Apogonidae species (out of 608 inflammatory lesions (Kumaratunga, J Comp Pathol 2023)
  • Genus Ranavirus – affect fin fish, amphibians (OIE reportable), and reptiles; cause systemic and necrotizing disease centered on blood vessels (endotheliotropism) and often affect the skin (ulcerative, necrotizing hyperkeratotic dermaitits); causes intracytoplasmic inclusions, especially in hepatocytes, that do not expand the cytoplasm
    • Epizootic haematopoietic necrosis virus (EHNV) – necrosis in renal hematopietic interstitium, liver and spleen in redfin perch and rainbow trout
    • European catfish virus (ECV)
    • Singapore grouper iridovirus (SGIV)
    • Frog virus-3 – fish, frogs
    • Turtle virus-3 – various turtles and tortoises; cause necrotizing glossitis, tracheitis, pneumonia, splenitis; liver hemorrhage and lipidosis; ulcerated skin abscesses; widespread fibrinoid vasculitis
    • Bohle iridovirus – detected in over 12 reptile families; recent paper in Kreffts freshwater turtles (Wirth, Vet Pathol 2022); infection starts in the spleen and spreads to liver, kidney, pancreas, mucosa, and skin; lesions similar between the two –dermatitis, fibrinoid necrosis, and necrosis in the liver, spleen and other organs; PCR is effective for early detection and screening
• Invertebrate iridoviruses (Iridovrius and Chloriridovirus genera) have been isolated from the lung, liver, and intestine of few lizards with pneumonia and pox-like skin lesions
• An additional group of unclassified irodoviruses called “reptile erythrocytic viruses” or “intraerythrocytic iridoviruses” have been reported in various reptiles, amphibians, and few sharks; result in plaques on the tongue, anemia, pallor, and weight loss; see intracytoplasmic inclusions in 40-50% of erythrocytes (previously classified as a protozoal organism)

References:

1. Frasca S Jr., Wolf JC, Kinsel MJ, et al. Osteichthyes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press; 2018:962-963.
2. Kumaratunga V, Adams VJ, Donaldson D, Pont RT, Stidworthy MF. Ocular pathology in aquarium fish with a focus on the Syngnathidae and Apogonidae families. J Comp Pathol. 2023;200:1-11.
3. Noga EJ. Fish Disease Diagnosis and Treatment. 2nd ed. St. Louis, MO: Mosby; 2010:171-174.
4. Origgi FC. Lacertilia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press; 2018:881-882.
5. Origgi FC. Serpentes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press; 2018:908.
6. Pessier AP. Amphibia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press; 2018:933.
7. Rodriguez CE, Duque AMH, Steinberg J, et al. Chelonia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press; 2018:841-842.
8. Stedman NL, Garner MM. Chondrichthyes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press; 2018:1012.
9. Wirth W, Forzan MJ, Schwarzkopf L, et al. Pathogenesis of Bohle iridovirus infection in Krefft’s freshwater turtle hatchlings (Emydura macquarii krefftii). Vet Pathol. 2022;60(1):139-150.

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