JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

November 2025

I-V14

Signalment (JPC Accession# 1497357): California sea lion (Zalophus californianus)

HISTORY: This sea lion presented with multiple vesicular and ulcerated skin lesions over various parts of the body.

HISTOPATHOLOGIC DESCRIPTION: Haired skin: There are multifocal intracorneal vesicles and pustules, up to 3 mm in diameter, filled with intact and necrotic neutrophils, cellular and karyorrhectic necrotic debris, fibrin, and granular to amorphous, eosinophilic proteinaceous fluid. The epidermis is hyperplastic characterized by acanthosis, prominent rete ridges, and orthokeratotic hyperkeratosis; there is pigment in all layers (hyperpigmentation), and prominent intercellular bridging (spongiosis). Within the superficial dermis, there are low numbers of perivascular lymphocytes, plasma cells, and macrophages. Lymphatics are mildly dilated (edema), and the endothelium of superficial small vessels is prominent (reactive).

Haired skin, pinna: There is a focally extensive 1.5 cm diameter area of ulceration and erosion bounded by remaining ends of a ruptured vesicle. The ulcerated and eroded epithelium is replaced and overlain by a serocellular crust composed of fibrin, necrotic cellular debris, necrotic neutrophils, and enmeshed macrophages, lymphocytes, and plasma cells. The epithelium adjacent to the ulcer is hyperplastic with rete ridges and moderate orthokeratotic hyperkeratosis and prominent intercellular bridging (spongiosis). The underlying dermis contains low numbers of previously described inflammatory cells, granulation tissue characterized by numerous small caliber vessels oriented perpendicular to the ulcer, and edema characterized by ectatic lymphatics and increased clear space between dermal collagen fibers.

MORPHOLOGIC DIAGNOSIS:

1. Haired skin: Epidermitis, vesiculopustular, subacute, multifocal, mild, with orthokeratotic hyperkeratosis and mild superficial perivascular lymphohistiocytic dermatitis, California sea lion (Zalophus califonianus), pinniped.

2. Haired skin, pinna: Dermatitis, ulcerative, subacute, focally extensive, moderate, with orthokeratotic hyperkeratosis.

ETIOLOGY: San Miguel sea lion virus (SMSV) (calicivirus)

ETIOLOGIC DIAGNOSIS: Caliciviral dermatitis

GENERAL:

• The Caliciviridae, a family of nonenveloped RNA viruses, is divided into five genera: Vesivirus, Lagovirus, Sapovirus, Nebovirus, Norovirus; the first two contain most of the animal caliciviruses
• SMSV and vesicular exanthema of swine virus (VESV) are considered a single species/genotype of calicivirus with many serotypes
• SMSV belongs to the genus Vesivirus, and is endemic in pinnipeds and some cetaceans
• Overcrowding of pinnipeds (rookery/breeding site) produces the ideal environment for direct contact spread of the virus or viral contamination of the water
• Potentially zoonotic (vesicular dermatitis and flu-like symptoms)

PATHOGENESIS:

• Biophysically and morphologically similar to VESV; damaged skin allows entry of the virus, followed by local viral replication in epidermal cells and viremia
• Vesicles form due to viral replication in epidermal cells leading to cell lysis, inflammatory cell infiltration, and separation of the epidermal layers by fluid transudate

TYPICAL CLINICAL FINDINGS:

• There are vesicles up to 3 cm in diameter on the nonhaired skin of flippers and in the oral cavity; vesicles may rupture leading to erosion/ulceration
• Disease can be self-limiting, with a rapid recovery

TYPICAL GROSS FINDINGS:

• Vesicles, ulcers, or plaque-like lesions on the flippers or oral cavity
• Ulcerative and nodular dermatitis of the lips, nasal planum, and chin reported
• Abortion and premature birth reported, but causation not proven

TYPICAL MICROSCOPIC FINDINGS:

• Spongiosis of the stratum spinosum leading to subcorneal vesicles
• Viral inclusion bodies are not seen

ULTRASTRUCTURE:

• Caliciviruses are nonenveloped, single stranded RNA, 35-39 nm in diameter with “cuplike cavities” (indentations) on the surface of the virion

ADDITIONAL DIAGNOSTIC TESTS:

• Cell culture, RT-PCR

DIFFERENTIAL DIAGNOSIS:

• Sealpoxvirus: Verrucous, self-limiting dermal nodules of skin and oral mucosa; histologic findings include acanthosis, ballooning degeneration, and eosinophilic intracytoplasmic viral inclusion bodies

COMPARATIVE PATHOLOGY:

• Caliciviruses in the SMSV group have been identified in many species of sea lions, seals, dolphins, whales, and terrestrial animals (feral swine, feral donkeys, gray foxes, domestic ruminants, and mink); SMSV serotype 7 isolated from opaleye fish

Other Caliciviridae viruses:

• Feline- Feline calicivirus (Vesivirus): Causes feline respiratory disease with oral vesicles that ulcerate; conjunctivitis, stomatitis, gingivitis, rhinitis, tracheitis and pneumonia
• Lagomorph- Rabbit hemorrhagic disease (RHD), Viral hemorrhagic disease, or Rabbit calicivirus disease (RHD virus): Causes necrotizing hepatitis, pulmonary edema, fibrin thrombi, and lymphoid necrosis; similar to the calicivirus that causes European brown hare syndrome (EBHS)
• Porcine- VESV (Vesivirus, Caliciviridae): Biophysically and morphologically similar to SMSV; thought to have originated from the feeding of uncooked contaminated (sea food) garbage; disease of historical significance only, eradicated from the US swine population in 1959; microvesicles to vesiculopustules to healing erosions with a marked perivascular inflammation in the underlying dermis of the snout, mouth, and nonhaired skin and feet; difficult to distinguish gross lesions from foot and mouth disease; reportable disease
• Avian- chicken calicivirus: Stunting and high mortality in chicks
• Non-human primate- primate calicivirus: Produces vesicles and persistent infections
• Mouse- murine norovirus: Hepatic inflammation and necrosis, also variable lesions in lung, gastrointestinal tract, mesenteric lymph nodes, brain, spleen

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Wiley Blackwell; 2016:264-6.
2. Colegrove KM, Burek-Huntington KA, Roe W, Siebert U. Pinnipediae In: Terio KA, McAloose D, Leger JS. Pathology of Wildlife and Zoo Animals. London, United Kingdom: Elsevier; 2018:579.
3. Owen H, Flint M, de Wit M. Sirenia. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:600.
4. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:121.

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