JPC SYSTEMIC PATHOLOGY
Musculoskeletal System
April 2019
M-P05

Signalment (WSC 95/96: 11-1):  1-year-old rainbow trout (Oncorhynchus mykiss)

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

HISTOPATHOLOGIC DESCRIPTION:  Head:  Multifocally within the cartilage of the calvarium and gill arches, there are few variably-sized granulomas that surround and replace chondrocytes and cartilaginous matrix.  Centrally, the granulomas contain numerous, round to piriform, 8-10 um diameter myxozoans and moderate amounts of necrotic debris.  The central core is surrounded by many epithelioid macrophages, fewer lymphocytes and multinucleated giant cells, which are occasionally further surrounded by concentric layers of reactive fibroblasts and fibrous connective tissue.  The myxozoans 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:  Head and gill arches, cartilage and bone:  Chondritis, granulomatous, multifocal, moderate, with perichondritis, periosteitis, ganglioencephalitis and numerous myxozoan organisms, rainbow trout (Oncorhynchus mykiss), piscine.

ETIOLOGIC DIAGNOSIS:  Myxozoal chondritis

CAUSEMyxobolus cerebralis

CONDITION:  Whirling disease

CONDITION SYNONYM:  Black tail

GENERAL DISCUSSION:

LIFE CYCLE:

PATHOGENESIS:

TYPICAL CLINICAL FINDINGS:

TYPICAL GROSS FINDINGS:

TYPICAL LIGHT MICROSCOPIC FINDINGS:

ADDITIONAL DIAGNOSTIC TESTS:

DIFFERENTIAL DIAGNOSIS:

Spinal Deformities

Other Causes of Abnormal Swimming Behavior:

COMPARATIVE PATHOLOGY:

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, Innie CJ, Kinsel MJ, Kurobe T, Naples LM, Nyaoke A, Poll CP, Weber EP. 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. 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.
  6. Gilbert MA, Granath WO. Whirling disease of salmonid fish: Life cycle, biology, and disease. J Parasitol. 2003; 89(2): 658-667.
  7. Noga EJ, ed. Fish Disease: Diagnosis and Treatment. 2nd ed. Ames, IA: Wiley-Blackwell; 2010: 229-236.
  8. Nolan MW, Roberts HE, Zimmerman KL, Smith SA. Pathology in practice. 2010; 236(6): 631-633.


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