Spotlight on: Nervous Mortality Syndrome (NMS)

Picture above: Presporogonic stages of the NMS agent showing as circular/oval basophilic structures in the brain tissue of Atlantic salmon © Mar Marcos Lopez, PHARMAQ Analytiq

Etiology and Life Cycle

Nervous Mortality Syndrome (NMS) is a uncommon disease affecting Atlantic salmon in the early marine stage, caused by an unclassified myxosporean parasite thought to belong to the genus Myxobolus^1,4. A similar disease is well-known in rainbow trout, known as ‘Whirling Disease’- however while Whirling Disease is caused by Myxobolus cerebralis and primarily targets skeletal cartilage in freshwater, the NMS agent is a distinct marine parasite that is associated with parasitic stages in CNS tissue. Because NMS typically impacts post-smolts within six to eight weeks of sea transfer, its life cycle is believed to involve a marine or brackish-water invertebrate intermediate host (such as an oligochaete or polychaete). However, the exact transmission dynamics and definitive hosts remain unknown.

The disease is primarily known from case descriptions from Ireland in the 1990s², although our diagnostic service is aware of a single historical detection in Scotland in the early 2000s. While rare, NMS has also been identified in Ireland in 2026.

For those unaware, genetic sequencing has shown that myxosporean parasites such as the NMS and Whirling Disease agents are actually highly evolved and simplified cnidarians- meaning they’re essentially micro-jellyfish which have adapted into a parasitic lifestyle.

Clinical Presentation

The syndrome primarily manifests during a cohort's first summer at sea.^2,1,3 In some cases, a rise in unmarked mortality may be the principle observation. Early clinical signs include a notable drop in appetite and generalized lethargy, with affected fish clustering near the net surface. As the disease progresses, salmon exhibit striking neurological symptoms: erratic circling, sudden frantic bursts of surface swimming, disorientation, and a temporary loss of equilibrium. In acute epizootics, cumulative mortality can escalate rapidly, historically exceeding 90% within a month, though survivors typically develop strong immunity and resume normal growth.²

Histopathological Findings

Definitive diagnosis relies on histopathological evaluation of the central nervous system (brain tissue). Under light microscopy, intercellular presporogonic (developmental) stages of the parasite appear as distinct strings or clusters of basophilic, circular-to-ovoid cells measuring 5 to 9 µm in diameter, often enclosed within a visible capsule.² These structures are distributed throughout the brain—particularly the cerebellum, optic tectum, and mid-brain—as well as the spinal cord. The accompanying pathology is characterized by non-purulent encephalitis and encephalomyelitis, featuring multifocal areas of gliosis, microglial activation, perivascular cuffing, and focal necrosis in severe cases.¹

Control and Mitigation

Currently, there are no licensed therapeutic treatments or vaccines available for NMS. Management relies entirely on preventative biosecurity and stress mitigation. Because outbreaks closely align with the stressful window of seawater adaptation, optimizing smolt quality and avoiding handling or transport stressors during high-temperature periods is critical. Fallow periods, strict biosecurity protocols, and rapid mortality removal to eliminate potential spore reservoirs represent the primary containment strategies for marine farms.

References

¹ Frasca, S., Poynton, S. L., West, A. B., & Van Kruiningen, H. J. (1998). Epizootiology, pathology, and ultrastructure of the myxosporean associated with parasitic encephalitis of farmed Atlantic salmon Salmo salar in Ireland. Diseases of Aquatic Organisms, 32(3), 211–225. https://doi.org/10.3354/dao032211 Cited by: 15

² Rodger, H. D., Turnbull, T., Scullion, F. T., Sparrow, D., & Richards, R. H. (1995). Nervous mortality syndrome in farmed Atlantic salmon. Veterinary Record, 137(24), 616–617.

³ Scullion, F. T., Scullion, M. G., Sparrow, D., Rodger, H. D., & Sheahan, B. J. (1996). Encephalitis and mass mortality of farmed salmon smolts in an isolated sea bay in Ireland. Veterinary Record, 138(7), 161-162.

⁴ Frasca, S., Linfert, D. R., Tsongalis, G. J., Gorton, T. S., Garmendia, A. E., Hedrick, R. P., West, A. B., & Van Kruiningen, H. J. (1999). Molecular characterization of the myxosporean associated with parasitic encephalitis of farmed Atlantic salmon Salmo salar in Ireland. Diseases of Aquatic Organisms, 35(3), 221–233. https://doi.org/10.3354/dao035221