Black spot disease in shrimp is one of the serious issues affecting the aquaculture industry. This disease not only causes significant economic losses but also impacts the quality and productivity of shrimp populations.
Understanding the causes and symptoms of the disease is crucial for shrimp farmers in order to take proactive measures to protect their shrimp stocks.
Black Spot Disease in Shrimp
Main Causes
- Biological Causes
Pathogenic bacteria: Common types include Vibrio spp., Pseudomonas, and Aeromonas. These bacteria are capable of secreting substances that corrode the shrimp’s chitin shell, leading to ulcers and indentations that cause the shell to become wrinkled. If these shell wounds are left untreated for an extended period, they can darken and develop into black spots.
The primary cause of black spot disease in shrimp is bacterial infection.
Fungi: Fusarium solani was identified in 2022 as a causative agent of gill blackening and black spots in shrimp. This is a common fungus found in brackish water, freshwater, and soil. All shrimp species are at risk of fungal infection, with adult and near-adult shrimp being especially susceptible to severe infections.
Black Spot Disease Caused by Fusarium solani Infection (Histopathology and Electron Microscopy Observations)
Parasites:
Several parasitic species attach to the gills and cause damage. Typical examples include protozoa such as Lagenophrys, amoebae like Paramoeba sp., and filamentous bacteria like Leucothrix mucor. Among them, amoebic infections are associated with high mortality rates.
A study by Jee Eun Han et al. in 2019 identified Paramoeba sp. as the causative agent of gill blackening in whiteleg shrimp (Litopenaeus vannamei). The main contributing factors are believed to be environmental stress caused by increased water temperature or high salinity, combined with high stocking densities, which favor the proliferation of natural marine protozoa capable of causing disease.
- Environmental Causes
- Poor water quality:
Shrimp ponds with low water quality, high concentrations of toxic gases such as ammonia (NH₃) and nitrite (NO₂), and low dissolved oxygen levels can lead to gill damage and darkening. In cases where toxic gas concentrations exceed safe limits, severe gill blackening and high mortality rates may occur. - Low alkalinity in the pond:
Leads to a lack of algae, nutrients, and essential minerals needed for shrimp development. This can cause slow shell hardening after molting, making shrimp weak and more vulnerable to bacterial infections. - Low pH levels:
In low pH conditions, heavy metal salts such as aluminum and iron can accumulate on shrimp gills, causing the gills to darken in color.
These environmental factors weaken the shrimp’s immune system and create favorable conditions for biological pathogens to invade and cause disease.
- Identifying Symptoms
- Appearance of black spots on the body:
Black spots or dark patches appear on the shrimp’s shell, usually as small, scattered lesions. These spots can occur on the cephalothorax, body, tail, and legs.
The black spots often begin as small wounds or abrasions, which gradually expand and become more pronounced over time.
Symptoms of Black Spot Disease in Shrimp
Shell damage: The shrimp’s shell may show signs of lesions or cracking. Areas with black spots often become hardened and have a different texture compared to the rest of the body.
In severe cases, the shell can be completely destroyed, exposing the underlying muscle tissue.
- Reduced health and increased mortality:
Infected shrimp often swim sluggishly, show reduced activity, and respond weakly to stimuli. They may display signs of stress and are more prone to death when farming conditions change.
In severe cases, mass mortality can occur, especially under unfavorable environmental conditions such as sudden temperature changes or poor water quality. - Poor appetite and slow growth:
Diseased shrimp often eat less or stop feeding altogether. This leads to slow growth, reduced weight gain, and failure to reach the desired market size.
Decreased appetite and slow development also weaken the shrimp’s immune system, making them more susceptible to secondary infections.
3. Prevention and Control Measures
- Disease prevention:
- Water quality management: Regularly monitor and maintain key environmental parameters such as pH, dissolved oxygen, heavy metals, toxins, and harmful gases. Use probiotics periodically to break down excess organic matter and clean the pond bottom.
- Feed management: Provide feed in the correct dosage to avoid overfeeding and prevent water pollution.
- Nutritional supplementation: Add vitamins (especially vitamin C) and organic minerals to enhance shrimp immunity and resistance to disease.
- Stocking density: Maintain a proper stocking density to avoid overcrowding and reduce stress on shrimp.
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Disease Treatment:
- Identify the cause as soon as symptoms appear in order to address the root of the problem promptly.
- Once the disease becomes severe, treatment is often ineffective because shrimp significantly reduce their feed intake, lose resistance, and become harder to treat — especially in cases caused by biological agents.
Therefore, early prevention is the most effective and sustainable strategy.
