New generation of francisellosis-resistant tilapia

The expansion of tilapia farming faces a growing threat: francisellosis, a bacterial disease caused by Francisella orientalis that can lead to mortality rates exceeding 60% in commercial production. This pathogen primarily affects fry and juveniles during the colder months, becoming one of the main threats to Brazilian fish farming.

Although treatments with antibiotics and some experimental vaccines exist, these solutions present practical challenges and are not always effective in the long term. Faced with this outlook, a recent study published in the journal Aquaculture confirms the viability of a more sustainable and promising alternative: genetic improvement to breed tilapia that are naturally resistant to the disease.

The work, developed at the Aquaculture Center of the Universidade Estadual Paulista (Caunesp) in Jaboticabal, in partnership with the Fisheries Institute (IP-APTA), indicates that genetic improvement is an effective tool to reduce the impacts of the disease on the sector, initiating the development of an improved line.

The challenge of Francisellosis in tilapia production

Francisellosis is a stealthy disease. The clinical signs are often non-specific, such as lethargy, loss of appetite, and ascites (fluid accumulation in the abdominal cavity), which makes early diagnosis difficult. Internally, the bacterium causes the formation of granulomas in vital organs like the spleen, liver, and kidneys, compromising the fish’s health.

Current control strategies, such as the use of antibiotics mixed into feed, often fail because sick fish stop eating. Meanwhile, injectable vaccines are impractical for small fish, which are the most vulnerable. This has driven the search for more robust preventive methods, and genetics is emerging as the primary tool.

How can resistance to a disease be “measured”?

The central objective of a selective breeding program is to identify individuals that possess desirable traits and use them as breeders for the next generations. To achieve this, researchers conducted a meticulous experiment that spanned two complete selection cycles and evaluated 112 tilapia families.

The methodology can be summarized in the following steps:

• Family creation: 66 families were produced in the base generation (G0) and 112 in the next (G1), maintaining a genealogical record (pedigree) to track the kinship of each individual.
• Controlled challenge: A portion of the fish from each family was exposed to the F. orientalis bacterium in a controlled manner via an intraperitoneal injection. This allows for standardizing the dose and evaluating the immune response of each animal under controlled conditions.
• Resistance measurement: Two key traits were recorded: binary survival (whether the fish lived or died) and time to death (TD), measured in hours. A longer survival time indicates greater resistance.
• Selection and second generation: Using statistical models, the “Estimated Breeding Value” (EBV) for resistance was calculated for each family. Individuals from the most resistant families were selected as parents to form the next generation (G1). For comparison, six “control” families were also created using individuals with the worst EBVs (most susceptible).

Genetics works, and it works fast

The study’s findings are conclusive and offer an optimistic outlook. “We applied a rigorous bacterial challenge protocol over two generations, and the data showed that resistance to francisellosis has a consistent genetic basis,” explains Baltasar Garcia, a postdoctoral researcher at Unesp and the first author of the article.

• Resistance is heritable: The study estimated the heritability (h²) of resistance. For time to death, heritability was moderate, with values of 0.399 in G0 and 0.282 in G1. “The observed heritability was moderate, which means it is possible to achieve rapid genetic gains in disease resistance by selecting the best individuals,” Garcia adds.
• Selection improves survival: The clearest proof of success was the performance comparison. While the control families showed the shortest survival times, the selected animals resisted for significantly longer periods. According to Garcia, the mathematical models indicate that the genetic values of the animals from the generation selected for resistance were 80% higher than the average of the initial population in of survival. “This represents a significant change for producers, especially during the winter, when francisellosis outbreaks tend to be more severe,” he highlights.
• Genetic gain visible in a single generation: “We observed real genetic gain in the very first generation of selection,” states Diogo Hashimoto, a researcher at Caunesp and co-author of the article. The study found that the average genetic value (EBV) for survival was 108.5% higher when comparing the resistant G1 group with the G1 control group.
• Consistent resistance across different temperatures: A crucial point was to evaluate whether the selected fish maintained their resistance in both warm (30°C) and cold (21°C) waters, as the disease worsens in the cold. “The analysis showed that there was no relevant genotype-by-environment interaction, which means that the animals maintain their performance in both higher and lower temperatures,” Hashimoto completes. This high genetic correlation (0.92 for TD and 0.86 for survival) ensures that selection is effective under different farming conditions.

A sustainable solution for aquaculture

The adoption of genetic improvement programs for disease resistance is still uncommon in Brazilian fish farming, but the data show that it is an urgent strategy. “Today, the control of francisellosis depends mostly on the use of antibiotics, which have limited efficacy and raise environmental concerns. There are vaccines in development, but genetic selection is a real, available, and long-term solution,” Hashimoto adds.

For Garcia, the results open a new perspective for fish farming. “We are talking about a sustainable solution that reduces dependence on antibiotics and contributes to the biosecurity of the sector. This is particularly important in a context of growing aquaculture and increasing environmental and sanitary demands in domestic and foreign markets,” he adds.

An integrated vision for aquaculture health

This study is part of a larger initiative coordinated by researcher Maria José Tavares Ranzani de Paiva of the Fisheries Institute (IP-APTA). “Our objective is to develop integrated solutions for the major health challenges in Brazilian fish farming. This includes rapid diagnostic tools, effective vaccines, and, as this work shows, genetic improvement programs that make fish naturally more resistant,” states Paiva.

She emphasizes that the project addresses not only francisellosis but also other emerging diseases like those caused by the ISKNV virus and bacteria such as Streptococcus and Flavobacterium. The final goal, according to Paiva, is “to deliver a technological package to the production chain that strengthens Brazilian aquaculture, promoting food security, health, genetic quality, and value addition to products.”

The researchers are now working to expand the program, accumulating resistance over new generations and integrating genomic analyses to further accelerate the gains. “Our goal is that, in the future, producers can acquire fry that are already certified not only for good zootechnical performance but also for resistance to diseases like francisellosis,” Hashimoto concludes.

Diogo T. Hashimoto
São Paulo State University (Unesp), Aquaculture Center of UNESP
14884-900 Jaboticabal, SP, Brazil
Email: [email protected]

Reference
Garcia, B. F., Restrepo-Arango, J. A., Silva Filho, M. S., Sousa, E. L., Agudelo, J. F., Mastrochirico-Filho, V. A., Manso, S. C., Pereira, C. S., Dias, D. C., Leonardo, A. F., Fonseca, F. S., Tachibana, L., Ranzani-Paiva, M. J., Pilarski, F., & Hashimoto, D. T. (2025). Genetic selection for resistance to Francisella orientalis shows significant selection response in Nile tilapia. Aquaculture, 606, 742584. https://doi.org/10.1016/j.aquaculture.2025.742584