JPC SYSTEMIC PATHOLOGY
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: Skin, oral cavity, and gills: Multifocally, there are nodular foci up to 500 µm within the epidermis, gill epithelium, and oral mucosa that are composed of hyperplastic epithelium that piles up to 8 cell layers. Hyperplastic epithelium surrounds up to 150 um diameter, irregularly round, single‑cell protozoal cysts with a 1‑2 µm thick hyaline wall, abundant, finely granular, to vacuolated basophilic cytoplasm that contains numerous host erythrocytes, 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 with numerous previously described intraepithelial protozoa.
MORPHOLOGIC DIAGNOSIS: Skin, oral cavity and gills: Hyperplasia, epithelial, nodular, multifocal, moderate, with numerous protozoal cysts, etiology consistent with Ichthyophthirius multifilis, Cardinal tetra (Paracheirodon axelrodi), piscine.
ETIOLOGY: Ichthyophthirius multifilis
ETIOLOGIC DIAGNOSIS: Cutaneous, oral, and branchial ichthyophthiriosis
CONDITION SYNONYMS: Freshwater white spot disease
- 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, carp and goldfish
- Non-encysting protozoan cause epidermal hyperplasia and hypertrophy due to feeding activity: Trichodina spp., Chilodonella spp., Ichthyobodo spp.
- 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
- 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.
- 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 (very prolireative) > 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
- 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
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 a C-shaped or horseshoe-shaped macronucleus
- Heavy infestations have lymphocytic, plasmacytic and neutrophilic inflammation
- 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
- Trichodina spp. (Trichodinosis): Non-encysting protozoan that infect marine or freshwater fish; causing mild disease similar to Ichthyophthirius multifiliis; wet mount identification of ‘scooting’ protozoan on affected tissue surfaces
- 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
- Amyloodinium (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
- 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)
- 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
- Epitheliocystis (Mucophilosis): Intracellular, Gram-negative infection of freshwater and marine fish with dermal and gill epithelial cell enlargement up to 400 µm in diameter
- Lymphocystis disease (piscine iridovirus): Similar gross appearance; variably sized, hypertrophied fibroblasts with basophilic intracytoplasmic inclusions on histology
- Abdel-Hafez G, Lahnsteiner F, Mansour N, Licek E. Pathophysiology of Ichthyophthirius multifiliis infection in rainbow trout (Oncorhynchus mykiss) and chub (Leuciscus cephalus). J Comp Pathol. 2014; 151(4):394-399.
- 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.
- Noga EJ. Fish Disease Diagnosis and Treatment. 2nd Ames, IA: Wiley-Blackwell; 2010:129-148.
- Roberts RJ. Fish Pathology. 4 ed. West Sussex, UK: Wiley-Blackwell; 2012: 84, 159, 309.
- Smith SA, Zimmerman K. Fish and chips. Vet Clin Path. 2016; 45(2):213-214.