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