COVERT MORTALITY NODAVIRUS (CMNV)
By Dr.Wiphada Mitbumrung, Ph.D. Applied Marine Biosciences — Tue Aug 11 2026
Covert Mortality Nodavirus (CMNV), an RNA virus first associated with disease outbreaks in cultured shrimp in China.
How Does CMNV Spread?
One potential route is vertical transmission from parent to offspring. Viral particles were observed in reproductive cells, fertilized eggs and nauplii, demonstrating that vertical transmission can occur in shrimp species. CMNV can also spread horizontally between animals. Infected and healthy shrimp living together provide opportunities for pathogen transmission, while cannibalism of weak or dead infected shrimp may further spread viral material throughout the population. This is especially important in a shrimp pond because diseased shrimp often remain at the pond bottom. If these animals are consumed by healthy shrimp, infection may continue circulating without farmers observing obvious dead shrimp at the surface. Water is another potential pathway. Movement of contaminated pond water or effluent can expose other cultured or wild aquatic animals to CMNV. The virus has been detected in several aquatic species, including fish, indicating that CMNV ecology may extend beyond the cultured shrimp population itself. Feed and live or frozen aquatic organisms can also represent an introduction risk.
When Infection Meets Environmental Stress
The presence of a pathogen does not always mean that severe mortality will immediately occur. Shrimp may carry an infection while continuing to survive and grow, particularly when pond conditions remain favorable. However, deterioration in the environment can place additional physiological pressure on infected animals.
Shrimp rely primarily on innate immune mechanisms rather than the antibody-based adaptive immune response found in vertebrates. Maintaining immune defenses, repairing damaged tissues, osmoregulating and maintaining normal metabolism all require energy. When environmental conditions suddenly change, the shrimp must allocate more of its available energy to maintaining homeostasis. For example, sudden rainfall can simultaneously alter salinity, temperature, pH and alkalinity. A shrimp must quickly regulate the balance of water and ions between its body and the surrounding environment. If it is already fighting a viral infection, this additional physiological burden can reduce its capacity to cope with disease. Temperature is also an important epidemiological factor. Early observations of viral covert mortality disease reported severe mortality under warm culture conditions, with outbreaks particularly associated with water temperatures above approximately 28°C. However, temperature should be viewed together with the overall pond environment rather than as an isolated trigger. Another important stressor is nitrite (NO₂⁻-N). Elevated nitrite interferes with normal oxygen transport and creates additional physiological stress. For an already infected shrimp whose muscles, gills or other tissues may be compromised, poor water quality can further reduce its ability to maintain normal metabolism.
Poor water quality may accelerate the progression from infection to clinical disease and mortality.Conversely, good water quality does not eliminate CMNV. An infected population may still exhibit poorer growth or production performance even when massive mortality is not occurring.
CMNV + strong environmental stress → increased clinical disease and mortality
CMNV + relatively stable environment → infection may be less obvious, but growth and production can still be affected
What Should Farmers Look For?
Unlike shrimp affected by some acute diseases that may swim near the pond surface or edges, moribund CMNV-infected shrimp tend to sink and accumulate in deeper areas at the pond bottom. This behavior contributes directly to the “covert” nature of the disease because dead and weak shrimp can remain unseen.
Affected shrimp may show:
Pale body coloration
Soft shells
Reduced feeding and empty digestive tracts
Hepatopancreatic atrophy or discoloration
Slow and uneven growth
Whitish lesions in the abdominal muscles
Persistent unexplained mortality at the pond bottom
White Muscle: Similar Sign, Different Causes
White or opaque muscle is one of the clinical signs that can make CMNV difficult to recognize in the field. Muscle whitening is not specific to CMNV and can also occur with infectious myonecrosis virus (IMNV), bacterial disease such as vibriosis, mineral imbalance or deficiency, and severe environmental stress.
Field differentiation of white muscle
CMNV : Cloudy-white muscle distributed throughout the body (Nunes et al 2010)
IMNV : Muscle necrosis and whitening typically begins in the last abdominal segment; affected muscle may later become orange (Zhang et al, 2014)
Mineral deficiency : Muscle cloudiness, cramping or loss of normal transparency (Blue Aqua Breeding Center, 2025)
Bacteria infection : Focal or localized opaque/necrotic muscle. May occur with wounds, redness, melanization, shell lesions (Limsuwan & Chuchird, 2007)
Other environmental stress (Sudden DO, temperature, salinity or handling stress) : Muscle may become diffusely cloudy or opaque (Cischem Agricultural, 2025)Do Not Wait Until Shrimp Start Dying
The virus may already be present before the farmer recognizes a major problem. Transmission may occur through infected animals, biological feeds, water or other aquatic hosts. Once shrimp are infected, poor water quality and sudden environmental fluctuations can add physiological stress and increase the likelihood of production losses. Therefore, unexplained slow growth, falling survival, soft shells, white muscle or shrimp disappearing from expected biomass should not simply be considered a water-quality problem. For CMNV, the key is not simply treating mortality after it appears. Effective management starts earlier with biosecure seed and biological inputs, control of transmission routes, stable pond conditions, careful observation of growth and survival, and early pathogen detection.
Good pond conditions help shrimp tolerate infection but they cannot make the virus disappear.
References:
Nunes, A. J. P., Sabry-Neto, H., & Sá, M. V. C. (2010, September 1). Brazil study: Beta-glucans improve survival of IMNV-infected white shrimp. Responsible Seafood Advocate.
Zhang, Q., Liu, Q., Liu, S., Yang, H., Liu, S., Zhu, L., Yang, B., Jin, J., Ding, L., Wang, X., Liang, Y., Wang, Q., & Huang, J. (2014). A new nodavirus is associated with covert mortality disease of shrimp. Journal of General Virology, 95(12), 2700–2709.
Limsuwan, C., & Chuchird, N. (2007). Histopathological study of necrotizing hepatopancreatitis (NHP) infection in Litopenaeus vannamei in Thailand. In Proceedings of the 45th Kasetsart University Annual Conference: Fisheries (pp. 574–581). Kasetsart University.