Unfortunately, Nile tilapia is susceptible to bacterial infections, with Aeromonas hydrophila being one of the main culprits. This unpleasant bacterium can cause severe diseases and death in tilapia, resulting in significant economic losses for fish farmers.
However, there is a solution.
A recently published study by scientists from UNESP/FCAV, Universidade de Araraquara, Instituto Butantan, and Universidade Brazil has identified a promising weapon in the fight against A. hydrophila: doxycycline.
They evaluated the immunopharmacological activity of doxycycline, orally administered to Nile tilapia (Oreochromis niloticus) during experimental infection with Aeromonas hydrophila.
Unleashing the power of doxycycline
Doxycycline is an antibiotic, a semi-synthetic derivative of tetracycline, used to treat various infections caused by certain types of bacteria. It is taken orally for the treatment of certain sexually transmitted infections (STIs).
Various studies have demonstrated the potential of doxycycline to treat diseases in aquaculture species such as channel catfish (Ictalurus punctatus), grass carp (Ctenopharyngodon idella), and rainbow trout (Oncorhynchus mykiss). Researchers report that the pharmacokinetic properties of doxycycline are superior compared to older tetracyclines, as it has a higher relative lipophilicity.
Aeromonas hydrophila has been highlighted as the most prevalent waterborne gram-negative pathogen and is responsible for hemorrhagic septicemia due to aeromonosis, causing significant economic losses in aquaculture worldwide due to its high morbidity and mortality rates.
Researchers evaluated the effectiveness of orally administered doxycycline in tilapia infected with A. hydrophila. They observed a clear dose-dependent response:
- Low dose (20 mg/kg): Infected fish showed classical signs of aeromonosis, including exophthalmos (pop-eye), fin erosion, and hemorrhagic septicemia.
- Medium dose (40 mg/kg): Symptoms were milder, and survival rates improved.
- High dose (80 mg/kg): No apparent disease, with a 93.3% survival rate!
Enhancement of tilapia’s defense system
Doxycycline not only killed bacteria but also bolstered the immune response of tilapia. Here’s how:
- Increased antibody production: Fish treated with higher doses produced more antibodies against A. hydrophila, essentially learning to fight off the invader.
- Reduction of inflammation: Doxycycline reduced monocytosis (an inflammation marker) and the production of reactive oxygen species (ROS), preventing cell damage caused by infection.
A practical solution for tilapia farmers
These findings offer concrete benefits for tilapia aquaculture:
- Effective treatment: Doxycycline, especially at high doses, proved to be effective in fighting A. hydrophila infections.
- Improved fish health: By reducing mortality and protecting the immune system of tilapia, doxycycline promotes healthier fish populations.
Risks of doxycycline use
While doxycycline is promising, it is crucial to use it responsibly and cautiously:
- Antibiotic resistance: Excessive use of antibiotics can lead to resistance in bacteria, rendering them ineffective. Use doxycycline only under veterinary supervision and follow responsible antibiotic use practices.
- Environmental impact: Antibiotics can have unintended consequences on the environment. Proper disposal and responsible use are essential.
- Residues in tilapia meat: Morshdy et al., (2022) reported concentrations of doxycyclines in harvested tilapia in Egypt that exceeded permissible maximum limits (MPL), emphasizing that heat treatment, particularly grilling, can reduce doxycycline residues.
“The results confirm the hypothesis that the antimicrobial action of doxycycline favors the defense mechanisms of treated tilapia, mainly at a dose of 80 mg/kg,” concluded the researchers.
The success of doxycycline against A. hydrophila is a significant step forward in tilapia protection. Further research is needed to fully understand how it stimulates the fish’s immune response and explore other sustainable solutions to prevent and manage infectious diseases in aquaculture.
By combining responsible antibiotic use with ongoing research, we can ensure a healthier future for tilapia and a more sustainable fishing industry. Additionally, there are currently some vaccines available to control A. hydrophila.
The study was funded by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico).
Marco Antonio de Andrade Belo
Laboratório de Farmacologia e Toxicologia Animal
Universidade Brazil, Descalvado, SP, Brazil
de Oliveira, S. L., da Costa, C. C., Aracati, M. F., Rodrigues, L. F., Montassier, H. J., Conde, G., … & de Andrade Belo, M. A. (2023). Doxycycline treatment modulates the immune response of tilapia and controls Aeromonas hydrophila infection. Aquaculture, 740504.