1 : ip16.jpg
2 : ip16aa02.jpg
3 : ip16aa02.jpg
4 : ip16aa02.jpg
5 : ip16aa10.jpg
6 : ip16aa40.jpg
7 : ip16ab10.jpg
8 : ip16ab40.jpg
9 : ip16ac10.jpg
10 : ip16ac10h.jpg
11 : ip16ac40.jpg
12 : ip16ac40h.jpg
Read-Only Case Details Reviewed: Oct 2010



October 2019



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



·      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



·      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



·      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

·      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



1.    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.

2.    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.

3.    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: 982.

4.    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.

5.    Noga EJ. Fish Disease Diagnosis and Treatment. 2nd ed. Ames, IA: Wiley-Blackwell; 2010:129-148.

6.    Roberts RJ. Fish Pathology. 4 ed. West Sussex, UK: Wiley-Blackwell; 2012: 84, 159, 309.

7.    Smith SA, Zimmerman K. Fish and chips. Vet Clin Path. 2016; 45(2):213-214.

Click the slide to view.

Click on image for diagnostic series.

Back | Home | Contact Us | Links | Help |