JPC SYSTEMIC PATHOLOGY
INTEGUMENTARY SYSTEM
August 2022
I-F14

Signalment (JPC# 1596989):  Juvenile Sumatran tigerfish.

HISTORY:  None

HISTOPATHOLOGIC DESCRIPTION:  

Scaled skin:  Multifocally, affecting 70% of the epidermis there are extensive areas of ulceration and erosion. Subjacent to erosions, epithelial cells are either swollen up to 18µm by increased clear space (hydropic degeneration) or hypereosinophilic, shrunken, with a pyknotic nucleus (necrotic). Diffusely overlaying and infiltrating the areas of ulceration are mats of innumerable basophilic, thin walled, non-septate, rarely non-dichotomous branching 5-8µm diameter hyphae that occasionally exhibit a bulbous terminal end (zoosporangia), that contain 2-4 um eosinophilic to basophilic globules (zoospores). Multifocally, previously described hyphae and zoospores infiltrate the cranium, are adjacent to the eye and subjacent skeletal muscle. Myocytes within the affected area are either degenerate with an enlarged pale eosinophilic sarcoplasm or necrotic with a hypereosinophilic, shrunken, fragmented sarcoplasm with loss of cross striations and a pyknotic nucleus. Previously described zoosporangia are located within the cranium and adhered to the eye. Multifocally, the perimysium and endomysium are expanded by increased clear space (edema) admixed with hemorrhage. 

MORPHOLOGIC DIAGNOSIS:  Scaled skin: Ulceration, regionally extensive, severe with skeletal muscle necrosis, degeneration, and numerous oomycete hyphae and zoospores etiology consistent with Saprolegnia, Sumatran tigerfish, pisces.

ETIOLOGIC DIAGNOSIS:  Cutaneous saprolegniasis

CAUSE:  Oomycetes

CONDITION:  Saprolegniasis

GENERAL DISCUSSION:

• Saprolegnia is an oomycete, also known as water mold, and acts as an opportunistic infection in aquatic environments
• Oomycetes are not true fungi, rather they are classified as Stramenopiles, which include diatoms and golden brown algae
  • Mycelial form resembles true fungi, but cell walls are composed of cellulose rather than chitin
• Within the order Saprolegniales are several genera, including Saprolegnia, Branchiomyces, Achlya, and Aphanomyces of which there are several well-known pathogens of fish, shellfish, and amphibians
  • Saprolegnia parasitica is one of the most destructive oomycete pathogens in fish (aka winter kill)
• Worldwide disease of wild and farmed, fresh water and estuarine fish and amphiabians, reported in over 95 different species. Is responsible for substantial loss economic loss in aquaculture
  • Infects egg and adult fishes
  • Can infect egg, larval, and aquatic post-metamorphic of amphibians

PATHOGENESIS:

• Infection primarily occurs after injury to the skin or gills, usually affecting only the epidermis, dermis, and more rarely the underlying superficial muscle
• Systemic infection is rare, but has been reported
• The zoospore is the infective form and is released from the zoosporangia which are located at the terminal end of the hyphae

TYPICAL CLINICAL FINDINGS:

• Death is a consequence of hemodilution (due to hemorrhage) and osmoregulatory failure
• Can cause massive die offs in egg populations of fish and amphibians
• Gill infections often lead to death within a short amount of time
• Marked lymphopenia due to impairment of lymphoid organs

TYPICAL GROSS FINDINGS:

• Tufts of white/tan “cotton like material” (when observed in water) often on the external surface of body, to include fins, gills, and opercula
  • Outside of water, tufts collapse and form soft granular plaques that overlay the erosions and ulcers

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Mats of thin-walled, non-septate, nonbranching hyphae affecting epidermis, dermis, and more rarely underlying muscle
• Zoosporangia contain round, biflagellated zoospores that are often present at the terminal end of the hyphae
• Underlying epidermis is often necrotic and ulcerated, hyphae can penetrate into deeper tissues
• Minimal inflammation is present

ADDITIONAL DIAGNOSTIC TESTS:

• Stain positive with Gomori methenamine silver (GMS)
  • Hyphae stain poorly with periodic acid Schiff (PAS)
• Hyphae can be seen with skin scrapings or wet-mounts
• PCR
• Culture

DIFFERENTIAL DIAGNOSIS:

• Other oomycetes in the family Saprolegniaceae:
• Saprolegnia (this entity)
• Aphanomyces: causes Epizootic ulcerative syndrome (EUS)
  • Form distinct granulomas causing extensive ulceration of the skin
    • Leads to a penetrating myopathy leading to myocyte necrosisà death by the primary organism or from other opportunistic oomycetes, fungi, or bacteria
• Branchiomyes: causes gill rot
  • Characterized by infarctive necrosis within the gills due to intravascular growth of the hyphae.
  • Also causes gill lamellae hyperplasia and fusion
• Other fungal and fungal-like organisms in fish:
• Exophiala: black yeast like fungià have melanized hyphae that produce yeast like cells via unilateral budding. Have three lesional patterns
  • Chromoblastomycosis: chronic subcutaneous mycosis forming verrucous plaques with sclerotic bodies
  • Eumycotic mycetoma: nodular granulomatous inflammatory condition
  • Phaeohyphomycosis: have melanized yeast, pseudohyphae and hyphae elements
    • Can cause cutaneous, subcutaneous, and systemic infections

COMPARATIVE PATHOLOGY:

• Amphibians: can be infected by all three genera of Saprolegnia, Aphanomyces, and Achyla
  • Affects captive and free-ranging larvae and aquatic post-metamorphic amphibians
    • In larvae will affect the keratinized mouth parts, nostrils, or gills
  • Similar gross and histological findings to fish
• Reported in an American alligator (Lau, J Comp Pathol 2020)

REFERENCES:

1. Bruno DW, West PV, Beakes GW. Saprolegnia and other Oomycetes. In: Woo TK, Bruno DW, eds. Fish Diseases and Disorders. Vol 3. 2^nd Cambridge, MA: CABI; 2011: 669-720
2. Frasca Jr. S, Wolf JC, Kinsel MJ, Camus AC, Lombardini ED. Osteichthyes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 984-986.
3. Lau CH, Snook ER, Swinford AK, Bryan LK. Achlya sp. Dermatitis in an American Alligator (Alligator mississippiensis). J Comp Pathol. 2020; 175: 1-4.
4. Noga EJ. Fish Disease: Diagnosis and Treatment. Ames, IA: Wiley-Blackwell; 2010: 156-162.
5. Pessier AP. Amphibia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 939-940.
6. Roberts RJ. The Mycology of Teleosts. In: Roberts RJ, eds. Fish Pathology. 4^th ed: Wiley-Blackwell; 2012: 383-392.

Click the slide to view.

---