JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-P05

 

Signalment (JPC #2502479): 1-year-old rainbow trout (Oncorhynchus mykiss)

HISTORY: This trout was harvested from Willow Creek, MT, where trout numbers had recently decreased.

HISTOPATHOLOGIC DESCRIPTION: Head, parasagittal section: Affecting 20% of the cartilage, both of the calvarium and gill arches, are few multifocal variably-sized granulomas that infiltrate and efface the cartilage and are centered on numerous myxozoans admixed with moderate amounts of necrotic debris surrounded by many epithelioid macrophages and fewer lymphocytes and multinucleated giant cells which are occasionally further surrounded by concentric layers of reactive fibroblasts and fibrous connective tissue. The myxozoans are numerous, round to piriform, 8-10 µm diameter, and have a 1-2 µm thick, refractile wall, a 2 µm diameter nucleus, and two piriform, 2x4 µm polar capsules, each containing a 1 um diameter, basophilic nucleus. Multifocally, inflammatory cells expand perichondral and periosteal connective tissue, extend into the brain and skeletal muscle, and widely separate or replace neurons within extracranial and paravertebral ganglia. There is multifocal proliferation of fibrous connective tissue and muscle degeneration in affected areas.

MORPHOLOGIC DIAGNOSIS: Calvarium and gill arches: Chondritis, granulomatous, multifocal, moderate, with cartilage lysis, perichondritis, periosteitis, ganglioencephalitis, and numerous myxozoan spores, rainbow trout (Oncorhynchus mykiss), piscine.

ETIOLOGIC DIAGNOSIS: Myxozoal chondritis

CAUSE: Myxobolus cerebralis

CONDITION: Whirling disease

CONDITION SYNONYM: Black tail

GENERAL DISCUSSION:

• Myxobolus cerebralis, one of the most pathogenic myxozoans of fish, is an extracellular metazoal organism (subphylum Myxozoa, class Myxosporea) that induces a range of signs and lesions including whirling behavior, blackened caudal regions, and severe skeletal deformities
• A reportable disease that is a significant health and economic problem in hatcheries and wild populations of trout; has been implicated in the decline of wild trout populations in several western states and is a problem for several threatened and endangered species (bull trout, cutthroat trout, and steelhead)
• All salmonids (trout, salmon, and grayling) are susceptible to infection; rainbow trout are most sensitive; brown trout and Coho salmon are resistant
• The severity is inversely related to age at exposure with 100% mortality in newly hatched fry and fingerlings, and few or no clinical signs in fish older than 6 months
• Myxozoa features: Multicellular spores with a spore wall divided into two halves; polar capsules in spores; endogenous cell cleavage in trophozoite and sporozoite; all non-sexual stages multinucleate with enveloping (primary) cells that contain enveloped (secondary) cells
• Myxozoan infections in fish are categorized as coelozoic (vegetative stages occur in the body or organ cavities) or histozoic (vegetative stages occur in tissues either intercellular or intracellular)
• Myxozoans cause little or no inflammatory reactions in fish, though some incite severe chronic inflammation

LIFE CYCLE:

• Two-host life cycle: Myxospore in salmonid hosts; triactinomyxon (TAM) in the obligate host, the Tubifex tubifex worm
• Myxospores released from cartilage when infected fish decompose or are ingested by a predator > myxospores ingested by tubifex (oligochaete) worm > sexual and asexual replication in intestine > develop into free living TAM (74-120 days) > released in the feces of the worm > TAM attaches to skin, respiratory epithelium (gill), or buccal cavity of salmonids via polar filaments > penetrates through secretory openings of mucous cells at the attachment sites > asexual reproduction > migrate via peripheral nerves and CNS to cartilage as trophozoites > mature into large plasmodia which feed on cartilage > undergo sporogenesis > formation of myxospores
• Myxospores require several months in the mud to reach full infectivity and can survive in water for over 1 year; TAMs are fragile and short lived
• tubifex remain infected for life and release TAMs intermittently
• Separate cycles of proliferation can occur in epithelium or other organs besides the end target tissue; these extrasporogonic stages increase the number of parasites in the host without involving sporogenesis

PATHOGENESIS:

• Whirling behavior occurs because of multifocal constriction of the spinal cord and lower brain stem caused by inflammatory and necrotizing chondritis induced by the parasite lysing cartilage and feeding on chondrocytes within the axial skeleton resulting in loss of balance control and subsequent “whirling” behavior
• Black tail is secondary to inflammation in the posterior region of the spine, resulting in damage to the caudal sympathetic nerves responsible for controlling pigment deposition
• Older fish are often asymptomatic because their skeletons are more highly ossified with less cartilage on which the parasite can feed

TYPICAL CLINICAL FINDINGS:

• Symptoms develop 2-8 weeks post-infection (may be longer at cooler temperatures)
• Young fish (< 6 months): Blackened tail, erratic swimming, head and spine deformities, growth inhibition, lymphopenia, death
• Older fish: Spinal curvature; blunt nose; misshapen skulls, jaws, and opercula; often asymptomatic with low mortality

TYPICAL GROSS FINDINGS:

• Prominent scoliosis or kyphosis of the spine, often black discoloration of the tail
• Fish that survive develop deformities, including a blunt nose, misshapen head and jaws

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Cartilage: Cartilage degeneration/lysis, granuloma formation, ovoid to ellipsoidal spores with two piriform-shaped polar capsules at the anterior end which house an extrudible filament
• Sporoplasm (group of infective germ cells enclosed in an envelope) contains an iodinophilous (glycogen) vacuole (best seen in fresh spores) that is characteristic of the genus Myxobolus; spores are 6-10 µm in diameter

ADDITIONAL DIAGNOSTIC TESTS:

• Diagnosis usually made via histopathology of the head, gill, or vertebral cartilage; for cerebralis, a cross-sectional sample should be taken from just behind the eye to include cartilage of auditory capsule (a very common site of infection)
• PCR, in situ hybridization, cytology (wet mount and Wright’s), electron microscopy
• Polar capsules stain blue with Giemsa; mature spores are strongly acid fast
• For asymptomatic infections, can enzymatically digest head cartilage and concentrate spores by sedimentation

DIFFERENTIAL DIAGNOSIS:

Spinal Deformities:

• Hypovitaminosis C ("broken back syndrome"): Can cause spinal deformities and darkening of the skin, hemorrhages, soft enlarged vertebrae, and deformed gill filaments
• Mycobacterium: All species of fish susceptible, more common in older fish and in tropical saltwater fish; can sometimes cause vertebral deformities, miliary tubercles in liver, kidney, and spleen with acid-fast organisms (usually in high numbers)
• Ichthyosporidiosis (Ichthyophonus hoferi): A protozoal (previously thought fungal) disease that can cause scoliosis; white foci (encysted "spores" surrounded by thick connective tissue wall) in heart, liver, and most other organs; black granulomas in the skin
• Infectious hematopoietic necrosis (rhabdovirus): Necrosis of submucosal eosinophilic granular cells (pathognomonic); can cause scoliosis
• Tryptophan deficiency: Can cause scoliosis and/or lordosis, calcium deposits in the kidneys and on bones surrounding the notochord
• Vitamin A toxicity: Pathogenesis unknown, may be related to differential development of the vertebral growth plates
• Organophosphates & organochlorines: Cholinesterase inhibition leads to muscle hypercontraction and eventual spinal deformation
• Trifluralin: Herbicide; causes vertebral hyperostosis and dysplasia in salmonids and other fish

Other Causes of Abnormal Swimming Behavior:

• Viral hemorrhagic septicemia (rhabdovirus): Serious disease in Europe; spiraling swimming behavior in the acute form
• Infectious pancreatic necrosis virus (birnavirus, D-V26): Rotation of body along long axis during swimming; necrosis of pancreas; petechial hemorrhages of viscera (esp. pyloric ceca); clear gelatinous material in stomach
• Hypovitaminosis E: Can cause irregular swimming; muscle atrophy, skin darkening, steatitis, thickening of swim bladder wall with formation of constriction bands
• Flexibacter psychrophilis (“cold water disease”): Skin darkening

COMPARATIVE PATHOLOGY:

Other pathogenic myxozoans:

• Henneguya ictaluri (P-P17): Most commonly seen as a problem in farmed catfish; usually causes necrotizing branchitis (also known as proliferative gill disease, hamburger gill disease) and dermatitis
• Myxosoma cartilaginis: Common in the eastern U.S.; localizes in the cartilage at the base of fin rays and gill arches of bluegill, sunfish, and black bass; does not cause deformation or impaired movement
• Myxobolus acanthogobii-like myxozoan: Found in a recent study to cause scoliosis and lordosis in numerous aquatic species in Japan; examined blue-spotted and marbled flathead fish (commercially significant species); myxozoal cysts were most common over optic lobe or cerebellum
• Myxozoanosis occasionally seen in amphibians; also rarely seen in mammals, birds, and reptiles
• Myxobolus hendricksoni reported in the brain of fathead minnows with no clinical sign; demonstrated how extensive space-occupying lesions can have little effect in fish
• Myxobolus neurophilus is a serious pathogen in farmed yellow perches and associated with focal encephalomalacia and meningitis

REFERENCES:

1. Feist SW, Longshaw M. Phylum myxozoa. In: Woo PTK ed. Fish Diseases and Disorders. Vol 1. 2nd ed. Oxfordhsire, UK: CAB International; 2006:230-296.
2. Frasca 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. San Diego, CA: Elsevier; 2018:988-990.
3. Gardiner CH, Fayer R, Dubey JP. Myxozoa. In: An Atlas of Protozoan Parasites in Animal Tissues. 2nd ed. Washington, DC: American Registry of Pathology; 1998:14-15.
4. Ngo Al, Go J, Spiers AB, Jenkins C. Scoliosis and kyphosis in blue-spotted and marbled flathead fish associated with a Myxobolus acanthogobii-like parasite. J Vet Diagn Invest. 2024. 36(3):380-388.
5. Noga EJ, ed. Fish Disease: Diagnosis and Treatment. 2nd ed. Ames, IA: Wiley-Blackwell; 2010:229-236, 242-243.
6. Roberts RJ, ed. Fish Pathology. 4^th Ames, IA: Wiley-Blackwell;2012:117-118, 120-121, 138, 295, 301, 326-327, 463

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