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Unmasking the White Spot: Dynamics of a Deadly Shrimp Disease

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By Milthon Lujan

Camarón con la enfermedad de la mancha blanca.
Shrimp with white spot disease.

Shrimp aquaculture, a booming industry that feeds millions of people, faces a threat: the White Spot Syndrome Virus (WSSV).

This lethal pathogen wreaks havoc on cultivated shrimp, causing massive mortalities and devastating economic losses. Understanding how WSSV spreads and how epidemics develop is crucial for designing effective control measures.

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Researchers from Hokkaido University, Takusui Co., Ltd., and Tokyo University of Marine Science and Technology conducted an epidemiological analysis using field-collected data during the initial phase of the WSSV outbreak to estimate the entire outbreak.

Additionally, they built a mathematical model describing the WSSV epidemic in aquaculture ponds and calibrated the model with data from a WSSV outbreak among kuruma shrimp (Penaeus japonicus) cultivated in a 40,000 m2 pond in Japan.

The Power of Epidemiology

Epidemiologists analyze disease transmission patterns, estimating key parameters such as recovery rate, infection mortality rate, and the transmission coefficient. Knowing these parameters allows scientists to build mathematical models that mimic the epidemic’s spread. Models like the SIR model enable predicting the disease’s course and the effectiveness of potential interventions.

The Shrimp Industry Challenge

Unlike traditional epidemiology, where infected individuals are easily identifiable, aquaculture presents a unique challenge. In shrimp ponds, White Spot Syndrome Virus (WSSV) infection progresses rapidly, making it nearly impossible to count infected individuals. Often, the only thing observed is the tragic aftermath of deaths.

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Modeling the Invisible

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The study introduces an innovative approach: extrapolating the entire epidemic trajectory based on the number of early deaths. By building a mathematical model of WSSV transmission and estimating its parameters through:

  • Infection experiments: Tracking disease progression after controlled exposure.
  • Real outbreak data: Analyzing mortality rates from real outbreaks in shrimp ponds.

The researchers attempted to estimate the “final size of the epidemic,” the total proportion of shrimp ultimately infected. This crucial information allows shrimp farmers and policymakers to implement specific control measures and prevent future devastations.

Unraveling the White Spot Mystery

By combining two powerful tools, the researchers painted a complete picture of WSSV impact:

  • Field data analysis: Leveraging data from the initial phase of outbreaks, they reconstructed the early stages of disease spread.
  • Mathematical modeling: Construct a model mimicking the real-world dynamics of WSSV in aquaculture ponds, capturing interactions between infected and susceptible shrimp.

This powerful approach allowed them to estimate a crucial parameter: the basic reproduction number (R0). R0 tells us how infectious a disease is, representing the average number of secondary infections caused by a single infected shrimp.

The Harsh Reality: a Highly Contagious Foe

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A crucial parameter is the basic reproduction number (R0). It tells us how infectious a disease is, representing the average number of secondary infections caused by a single infected individual. The estimated R0 values in this study were alarmingly high, ranging from 3.21 to 4.56 per pond. This translates to a sobering reality: without intervention, WSSV outbreaks are highly likely to spread rapidly, infecting an astonishing 98.0% to 99.7% of the entire shrimp population.

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Application in the Shrimp Industry

This study marks a significant breakthrough in understanding WSSV dynamics. By analyzing real-world outbreak data, it sheds light on the intricate interaction between pathogen, host, and environment within aquaculture ponds.

The study’s findings show a clear picture: early detection and swift action are crucial to contain WSSV outbreaks. Once WSSV infection is identified, immediate measures are essential:

  • Enhanced removal of dead shrimp: Prevents further virus spread.
  • Immediate harvest of the entire population: Minimizes losses and halts further transmission.

Additionally, the study’s results provide invaluable insights for:

  • Predicting the course of future outbreaks and issuing timely warnings.
  • Assessing the effectiveness of control strategies such as biosecurity measures and vaccination.
  • Developing new preventive and mitigation approaches to safeguard shrimp populations.
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Finally, the researchers note that the study has several limitations, including the assumption in the model that only cannibalism of dead and infected shrimp is a mode of transmission.

Conclusion

The researchers developed a mathematical model describing WSSV transmission in shrimp ponds and derived the relationship between R0 and observable epidemiological parameters.

The fight against WSSV continues, and this study provides a powerful tool. By solving the transmission puzzle, we can get closer to developing strategies for disease prevention or control.

The study has been funded by the Japan Society for the Promotion of Science (JSPS), KAKENHI, and the Science and Technology Research Partnership for Sustainable Development grant (SATREPS) from the Japan Science and Technology Agency (JST).

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Contact
Ryosuke Omori
Division of Bioinformatics, International Institute for Zoonosis Control, Hokkaido University
Kita-20 Nishi-10, Kita-Ku, Sapporo 001-0020, Japan.
Email: omori@czc.hokudai.ac.jp

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Reference (open access)
Omori, R., Hagino, T., Pattama, P., Ozaki, K., & Hirono, I. (2024). Estimating the basic reproduction number and final epidemic size of white spot syndrome virus outbreak in Penaeus japonicus in aquaculture ponds. Aquaculture, 740548.

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