JPC SYSTEMIC PATHOLOGY
MUSCULOSKELETAL SYSTEM
APRIL 2022
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 um diameter, and have a 1-2 um thick, refractile wall, a 2 um diameter nucleus, and two piriform, 2x4 um polar capsules, each containing a 1 um diameter, basophilic nucleus.  Multifocally, inflammatory cells expand perichondral and periosteal connective tissue, extend into the brain, and widely separate or replace neurons within extracranial and paravertebral ganglia.

 

MORPHOLOGIC DIAGNOSIS:  Calvarium and gill arches:  Chondritis, granulomatous, multifocal, moderate, with cartilage lysis, perichondritis, periosteitis, ganglioencephalitis, and numerous myxozoan trophozoites, 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; 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

 

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 feeding on cartilage
• Black tail is secondary to inflammation in the posterior region of the spine resulting in damage to the caudal 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:

• 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 um in diameter

 

ADDITIONAL DIAGNOSTIC TESTS:

• PCR, in situ hybridization, cytology (wet mount and Wright’s), electron microscopy
• Polar capsules stain blue with Giemsa; mature spores are strongly acid fast

 

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
• Case report of renal myxozoanosis in 2 crown river turtles (Hardella thurjii) (Garner, Vet Pathol. 2005)
• Myxozoanosis occasionally seen in amphibians; also rarely seen in mammals, birds, and reptiles
• Myxobolus albi diagnosed in cartilage of captive lumpfish resulting in exophthalmos and grossly visible white to tan scleral nodules (Calvin, J Vet Diagn Invest. 2012)

 

REFERENCES:

1. Baldwin TJ, Vincent ER, Silflow RM, Stanek D. Myxobolus cerebralis infection in rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) exposed under natural stream conditions. J Vet Diagn Invest. 2000; 12(4): 312-321.
2. Cavin JM, Donahoe SL, Frasca S, et. al. Myxobolus albi infection in cartilage of captive lumpfish (Cyclopterus lumpus).  J Vet Diagn Invest.  2012; 24(3): 516-524.
3. Densmore CL, Blazer VS, Waldrop TB, Pooler PS. Effects of whirling disease on selected hematological parameters in rainbow trout. J Wildl Dis. 2001; 37(2): 375-378.
4. 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.
5. 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.
6. 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.
7. Garner MM, Bartholomew JL, Whipps CM, Nordhausen RW, Raiti P. Renal myxozoanosis in crowned river turtles Hardella thurjii: description of the putative agent Myxidium hardella n. sp. by histopathology, electron microscopy, and DNA sequencing. Vet Pathol. 2005;42(5):589-95.
8. Gilbert MA, Granath WO. Whirling disease of salmonid fish: Life cycle, biology, and disease. J Parasitol. 2003; 89(2): 658-667.
9. Noga EJ, ed. Fish Disease: Diagnosis and Treatment. 2nd ed. Ames, IA: Wiley-Blackwell; 2010: 229-236.
10. Nolan MW, Roberts HE, Zimmerman KL, Smith SA. Pathology in practice. 2010; 236(6): 631-633.

 

Click the slide to view.

---