JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

November 2022

I-P16

 

Signalment (JPC# 1647900): Cardinal tetra (Paracheirodon axelrodi)

 

HISTORY:  This is one of ten Cardinal tetras obtained from a pet store.  Nine days later, multiple raised, pinpoint to 3 mm white spots appeared.  Within days, all ten tetras were dead.

 

MICROSCOPIC DESCRIPTION: Head and rostral body, parasagittal section: Multifocally within the epidermis, gill epithelium, and oral mucosa are nodular foci up to 500 µm composed of hyperplastic epithelium that piles up to 8 cell layers. Epithelial cells within foci of hyperplasia are occasionally markedly hypertrophied and contain intracytoplasmic, up to 175µm diameter, irregularly round, single‑celled protozoal cysts with a 1‑2 µm thick hyaline wall; abundant, finely granular to vacuolated, basophilic cytoplasm that contains abundant membrane-bound, phagocytosed material; and a 30 x100 µm, crescent-shaped, deeply basophilic macronucleus (trophont). Multifocally, there is mild hyperplasia of gill epithelium with blunting and fusion of secondary lamellae in addition to the numerous previously described intraepithelial protozoa.  

 

MORPHOLOGIC DIAGNOSIS:  Skin, oral cavity, and gills: Epithelial hyperplasia, nodular, multifocal, moderate, with numerous protozoal cysts, Cardinal tetra (Paracheirodon axelrodi), piscine. 

 

ETIOLOGY:  Ichthyophthirius multifilis 

 

ETIOLOGIC DIAGNOSIS: Cutaneous, oral, and branchial ichthyophthiriosis

 

CONDITION:  Ich

 

CONDITION SYNONYMS:  Freshwater white spot disease

 

GENERAL:  

• One of the most common parasitic protozoans of freshwater fish worldwide
  • Less pathogenic for cold water species (e.g. salmon)
  • Scaleless fish are especially vulnerable: Catfish, sunfish 
• Economically important ectoparasite of captive commercial fish 
• Infected fish can develop immunity to re-infection that can last for several months
• Cryptocaryon irritans is saltwater equivalent.  

 

LIFE CYCLE: 

• Adult ciliated protozoa (theronts) adheres to epidermis > burrows into epidermis or gills > nodular epidermal hyperplasia around the trophont  > trophont erupts through the epithelium, drops off host, forms a capsule (tomont) adhering in environment  > undergoes binary fission > producing hundreds to thousands of tomites > infective, motile theronts > adhere to epidermis of fish 
  • Theront: Free-swimming, non-feeding, infective stage 
  • Trophont:  Host (fish)-feeding stage 
  • Tomont:  Encysted, dividing stage 
  • Tomite: Daughter cells produced by a tomont
• Duration of life cycle and number of tomites produced is temperature dependent
  • Warmer water (59 – 77 ° F) improves proliferation

 

PATHOGENESIS:  

• Encysting protozoans (such as this) induce focal epidermal hyperplasia and hypertrophy at attachment sites due to burrowing and/or encysting
• Protozoa penetrates the epithelium > entering and exiting of the epithelium results in erosions and ulceration > prone to secondary bacterial infections
• Parasites cause intense pruritus; additional epidermal injury from self trauma
• Severe gill hyperplasia leads to hypoxia and possibly respiratory disease
• Can be fatal if severe epithelial destruction due to osmoregulatory failure

 

TYPICAL GROSS FINDINGS:  

• Small 1.0 mm grayish-white raised multifocal to coalescing nodules on the surface of the body, fins and gills with epidermal hyperplasia  
• In advanced severe cases, cysts coalesce and form mucoid masses
• Epithelial erosion and ulceration
• Often cause behavioral changes such hyperactivity and chronic irritation

 

TYPICAL MICROSCOPIC FINDINGS:  

• Oval, holotrich (evenly dispersed cilia over entire body) ciliated parasite encysted within host’s epithelium is pathognomonic
• Protozoa have a small circular mouth opening at one end, numerous small contractile vacuoles throughout the body, opaque granules, and an oval, C-shaped, or horseshoe-shaped macronucleus  
• Heavy infestations have lymphocytic, plasmacytic and neutrophilic inflammation 

 

DIFFERENTIAL DIAGNOSIS:  

• Protozoan: 

• Non-encysting protozoan cause epidermal hyperplasia and hypertrophy due to feeding activity:  Trichodina spp., Chilodonella spp., Ichthyobodo spp. 
  • Trichodina spp. (Trichodinosis): Non-encysting protozoan that infects marine or freshwater fish; causing mild disease similar to Ichthyophthirius multifiliis; wet mount identification of ‘scooting’ protozoan on affected tissue surfaces
  • Ichthyobodo spp. (Ichthyobodosis, costiosis, Ichthyobodo necator complex):  smallest ectoparasite of fish (approximately the size of red blood cell) ; infects immunosuppressed and young fish; primary freshwater disease yet can infect saltwater;  two forms: detached and mobile;  grossly causes epithelial hyperplasia with increased mucus production, giving fish a bluish cast (slime)
• Encysting protozoans induce focal epidermal hyperplasia and hypertrophy at attachment sites due to burrowing and/or encysting: Ichthyophthirius spp., Cryptocaryon spp., Amyloodinium spp., Piscinoodinium spp
  • Cryptocaryon irritans (‘Marine Ich’):  Penetrates the epithelium; same life cycle and same gross lesions in saltwater fish; readily diagnosed when seen under the skin or gills Chilodonella spp. (Chilodonellosis): Similar same life cycle and gross pathology as Ichthyophthirius multifiliis; yet more insidious often with severe tissue damage due to strong cellular response.  Is smaller in diameter compared to Ichthyophthirius (up to 450 um)
  • Amyloodinium spp. (amyloodiniosis, marine velvet disease, marine oodinium disease, oodiniosis): One of the most important diseases of warm water marine fish affecting elasmobranches (sharks, rays) and teleost (ray fin fish); highly adapted dinoflagellate with similar life cycle and gross pathology as Ichthyophthirius multifiliis; primarily affects gills and  can infect skin and eyes; diagnosis base on larger 50 to 350 µm trophonts
  • Piscinoodinium spp. (freshwater velvet disease, rust disease, gold dust disease, Pillularis disease, freshwater Oodinium):  Freshwater analogue of Amyloodiniosis with same life cycle and gross pathology 

• Fungal / Algae: 
  • Saprolegniales (typical water mold infection, winter kill):  One of the most common freshwater infections of fish (especially estuarine tropical fish); causes cottony, proliferative growth on the skin or gills 
• Bacterial: 

• Epitheliocystis (Mucophilosis): Common gill disease of marine and freshwater fish; intracellular, Gram-negative, Chlamydia­-related bacteria that causes dermal and gill epithelial cell enlargement up to 400 µm in diameter

• Viral:

• Lymphocystis disease (I-V15, piscine iridovirus):  Similar gross appearance; variably sized, hypertrophied fibroblasts with basophilic intracytoplasmic inclusions on histology

 

REFERENCES:  

1. Borel N, Polkinghorne A, Pospischil A.  A review on chlamydial diseases in animals: still a challenge for pathologists?  Vet Pathol. 2018;55(3):374-390.
2. Frasca S, Wolf JC, Kinsel MJ, et al. Osteichthyes. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. Cambridge, MA: Elsevier; 2018: 962-963, 975-976, 981-993. 
3. Gardiner CH, Fayer R, Dubey JP.  An Atlas of Protozoal Parasites in Animal Tissues, 2nd ed. Washington, DC: Armed Forces Institute of Pathology; 1998: 16-17. 
4. Noga EJ. Fish Disease Diagnosis and Treatment. 2^nd ed. Ames, IA: Wiley-Blackwell; 2010:129-148.
5. Roberts RJ. Fish Pathology. 4 ed. West Sussex, UK: Wiley-Blackwell; 2012: 84, 159, 309. 
6. Smith SA, Zimmerman K. Fish and chips. Vet Clin Path. 2016; 45(2):213-214.

 

 

 

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