Streptococcus agalactiae is increasingly recognized as a significant threat to tilapia aquaculture worldwide. This bacterium, often referred to as the “silent killer,” can cause severe infections in tilapia, leading to substantial economic losses for the aquaculture industry.
Researchers from Assiut University, Minia University, and the University of Messina published a scientific review in the MDPI journal. Their study aimed to assess S. agalactiae infections in farmed Nile tilapia (Oreochromis niloticus), examining transmission, sources, risk factors, clinical signs, pathogenesis, virulence factors, and diagnostic, treatment, control, and prevention methods.
Streptococcosis: A Global Concern
The economic impact of streptococcosis on tilapia aquaculture is substantial, with outbreaks causing significant mortality and financial losses. The disease has been reported in at least 15 countries, with China, Egypt, Indonesia, and Thailand being among the most affected.
Streptococcosis, caused by the bacterium Streptococcus agalactiae (GBS), is a major threat to tilapia aquaculture. GBS is a Lancefield group B streptococcus characterized by spherical or ovoid cells that are facultative anaerobic, Gram-positive, and oxidase-negative. This bacterium is highly adaptable and can grow within a broad temperature range, from 25°C to 45°C.
The Impact of S. agalactiae
The effects of streptococcosis on tilapia aquaculture are alarming. Outbreaks have been reported on every continent, with infection rates reaching 50% and mortality rates exceeding 95%. The disease is especially prevalent in warm waters, where temperatures above 26°C and high stocking densities contribute to its spread.
S. agalactiae infections in tilapia can manifest in various forms, including:
- Meningoencephalitis: This condition affects the brain and spinal cord, causing symptoms like exophthalmia, corneal opacity, and abnormal swimming behavior.
- Septicemia: A systemic infection that spreads throughout the fish’s body, leading to multi-organ failure and eventual death.
The bacterium is highly contagious and can infect humans, terrestrial animals, and aquatic species, causing diseases. Besides tilapia, numerous freshwater and marine fish species are affected by S. agalactiae, including silver pomfret, giant grouper, gilthead sea bream, bighead carp, and others.
Contributing Factors to Outbreaks
The disease can spread through various routes, including direct contact with infected fish, contaminated water, and indirect contact with bacteria in the water. Several factors contribute to the spread of streptococcosis in tilapia.
Direct Transmission
S. agalactiae spreads through direct contact with infected fish, which release the bacterium via gills, mucus, and feces. This can occur through cannibalism of dead or dying fish or indirect contact with bacteria in water. The bacteria can also spread via the fecal-oral route, where infected fish excrete S. agalactiae in feces ingested by other fish.
Indirect Transmission
Indirect transmission occurs through contaminated water introduced by new fish batches or bacteria released from infected fish. The bacterium can survive in sterile freshwater for extended periods, allowing it to infect nearby fish populations.
Environmental Factors
Environmental factors play a crucial role in the spread of streptococcosis. High water temperatures, low dissolved oxygen levels, high stocking densities, and intensive farming practices promote bacterial growth and increase tilapia’s vulnerability to infection.
Risk Factors
Several risk factors influence the development of S. agalactiae infections in tilapia, including:
- High water temperatures (above 27°C)
- Low dissolved oxygen levels
- High stocking densities
- Intensive farming practices
- Poor environmental conditions, such as high ammonia levels
- Fish weight and/or age
Treatment of Streptococcosis in Tilapia
The most common treatment strategy during a confirmed bacterial disease outbreak in farmed fish populations is the administration of antibiotics. However, alternative treatments are emerging.
Antibiotic Susceptibility
Studies show that most S. agalactiae strains are susceptible to various antibiotics across different fish species. Isolates recovered from O. niloticus respond to several antimicrobial treatments. However, variations in antibiotic susceptibility and resistance within the same bacterial species can result from serotype differences and frequent or improper chemotherapy use.
In Vitro Susceptibility Testing
Oral antibiotics such as amoxicillin, enrofloxacin, oxytetracycline, and florfenicol are commonly used to treat S. agalactiae infections. In vitro testing methods include minimum inhibitory concentration (MIC) determination and disk diffusion assays.
In Vivo Trials
In vivo studies administering oral oxytetracycline and florfenicol to O. niloticus demonstrate these antibiotics can suppress infection processes. However, cumulative mortalities were observed 20 days post-treatment, indicating the medication could not entirely halt infection, allowing the disease to spread in cohabitation experiments.
Alternative Therapies
Medicinal herbs and other plants in aquaculture offer numerous advantages over chemical treatments, including improved growth performance, antioxidant activity, physiological conditions, and well-being, along with antimicrobial and hepatoprotective effects. Additionally, therapeutic plants are more cost-effective, readily available, and biodegradable compared to synthetic pharmaceutical substances.
Prevention and Control
Mitigating the impact of streptococcosis requires understanding the factors contributing to outbreaks. Environmental conditions like temperature, stocking density, and specific viral and fungal presence influence disease spread.
Although no definitive cure exists for streptococcosis, several preventive measures can minimize outbreak risks:
- Biosecurity: Strict biosecurity protocols, including disinfecting equipment and personnel, prevent bacteria from entering aquaculture facilities.
- Water Quality Management: Maintaining optimal water quality parameters reduces infection risks.
- Vaccination: Developing effective vaccines against S. agalactiae provides a valuable disease prevention tool.
- Antibiotic Treatment: In severe outbreaks, antibiotics may be necessary but should be used prudently to avoid antibiotic-resistant bacteria development.
Conclusion
As global demand for tilapia grows, addressing challenges posed by S. agalactiae is imperative. Implementing effective prevention and control strategies enables the aquaculture industry to mitigate the disease’s impact and ensure sustainable tilapia production.
Reference (open access)
Abdallah, E. S., Metwally, W. G., Mohamed, M. A., Albano, M., & Mahmoud, M. M. (2024). Streptococcus agalactiae Infection in Nile Tilapia (Oreochromis niloticus): A Review. Biology, 13(11), 914. https://doi.org/10.3390/biology13110914
