FRANCISELLOSIS IN TILAPIA – A NEW CHALLENGE FOR FARMERS

• The global tilapia market is growing strongly, with production expected to exceed 7 million tonnes in 2025.
• According to the Vietnam Association of Seafood Exporters and Producers (VASEP), Vietnam's export value of tilapia and red tilapia in 2024 was over USD 41 million, up 137% on the previous year. Of this, tilapia exports were over USD 27.8 million and red tilapia USD 13.2 million.

Francisella noatunensis is an emerging pathogen that is especially dangerous for intensively farmed tilapia. It is an intracellular bacterium that lives inside macrophages, so it is very hard to clear with conventional antibiotics.

1. Characteristics and signs

• Fish eat less, swim sluggishly and float near the surface; many swim tilted to one side.
• Some fish stay still and hardly react when someone approaches.
• Internal examination: many small white nodules (granulomas) are clearly visible in the liver, spleen and kidney. This is the most characteristic sign.
• Organs affected: enlarged spleen, white spots on the liver, swollen kidney, and signs of inflammation in the intestine.
• Fish become weak and emaciated, grow slowly, and easily catch other opportunistic diseases such as Aeromonas, Edwardsiella, Pseudomonas or parasites.

Figure in the original: clinical signs suggestive of francisellosis seen in diseased fish during sampling at farm 2: (a) melanosis and skin ulceration at the fin base (arrows); (b) ascites; (c) melanosis and exophthalmia (arrow); (d) melanosis, skin ulceration at the fin base and pale areas on the gills (arrow).⁽¹⁾

2. A real outbreak (Guangdong, China, 2023)

• Tilapia weighing from 50 g to 2,000 g can all be affected.
• The outbreak occurred after the water temperature rose from 20 °C to 25 °C, together with heavy rain of more than 10 mm.
• Mortality was over 40% within a short time, causing heavy losses for farmers.
• Diseased fish often float at the surface, swim tilted or stay still.
• Necropsy: multifocal white nodules in the liver, spleen and kidney – the typical lesion.

Figure in the original: geographical distribution of piscine francisellosis (black dots: Francisella noatunensis subsp. orientalis; red dots: F. noatunensis subsp. noatunensis).⁽²⁾ The places and years marked on the map are:

3. How to examine and identify the cause

1. Observe behaviour: fish gasping at the surface, swimming off-balance, eating less.
2. Gross necropsy: look for white nodules in the liver, spleen and kidney.
3. Histopathology: detects granulomas containing many macrophages.
4. PCR: the most accurate method for identifying Francisella noatunensis.
5. Rule out other diseases: Mycobacterium, Nocardia and some other bacteria also cause white nodules and must be excluded.

Key point: do not rely only on external signs. The disease can only be confirmed by dissection plus laboratory testing.

Figure in the original: red tilapia infected with Francisella noatunensis subsp. orientalis, showing white nodules in the internal organs (arrows) of naturally infected fish (a, b) and experimentally infected fish (c). Haematoxylin and eosin-stained sections of the spleen show granulomatous lesions (d) and a typical granuloma with central necrosis surrounded by epithelioid macrophages (e).⁽³⁾

Figure in the original: Giemsa-stained smear of the anterior kidney of red tilapia showing intracellular and extracellular Francisella bacteria (arrows); scale bar 10 µm.⁽⁴⁾

4. Extent of losses

• Mortality: 30–50% (over 40% after only a few weeks in some places) (China, Brazil, Thailand).
• Fish are weak and grow slowly, and the harvest is smaller.
• Secondary outbreaks happen easily, raising disease-management costs.
• Direct economic losses, plus damage to reputation and the supply chain.

5. Limitations in control

• Antibiotics (tetracycline, oxytetracycline, florfenicol) are still used, but their effect does not last because:
  • sick fish often stop eating, so it is hard to deliver medication through feed;
  • there is a risk of antibiotic resistance.
• Vaccines currently available (inactivated whole-cell, autogenous) are still mostly at the trial stage and are not very practical, because small fish are hard to inject.

Practical recommendations

• Early detection = lower losses.
• Combining correct antibiotic use + a stable environment + stronger disease resistance is a sustainable strategy.
• Always watch the fish closely after heavy rain and changes in temperature.

References

• ¹ Assis, G. B. N., de Oliveira, T. F., Gardner, I. A., Figueiredo, H. C. P., & Leal, C. A. G. (2016). Sensitivity and specificity of real-time PCR and bacteriological culture for francisellosis in farm-raised Nile tilapia (Oreochromis niloticus L.). Journal of Fish Diseases, 39(12), 1–9. https://doi.org/10.1111/jfd.12559
• ^2,3,4 Nguyen, V. V., Dong, H. T., Senapin, S., & Pirarat, N. (2015). Francisella noatunensis subsp. orientalis, an emerging bacterial pathogen affecting cultured red tilapia (Oreochromis sp.) in Thailand. Aquaculture Research, 46(11), 1–9. Wiley. https://doi.org/10.1111/are.12802
• ⁵ Xu, M., Li, F., Chen, B., Deng, Y., Chen, D., Geng, Y., Ouyang, P., & Huang, X. (2024). Isolation, identification and histopathological observation of Francisella noatunensis subsp. orientalis from Nile tilapia (Oreochromis niloticus). Aquaculture, 741, 741532. https://doi.org/10.1016/j.aquaculture.2024.741532

Solutions from Than Vuong

1. Targeted antibiotics (use as prescribed by a veterinarian, with antibiotic sensitivity testing)

• FLOR-MAX (florfenicol) – treatment of intestinal infection caused by Edwardsiella ictaluri in pangasius (tra and basa catfish).
• OXYTETRA – treatment of intestinal haemorrhage caused by Aeromonas and Pseudomonas in pangasius (tra catfish).

2. Stabilising the environment and reducing stress

• SOILMAX New – helps improve the pond bottom, helps reduce NH₃ and H₂S, and helps stabilise the environment.
• YUCCA GRO – helps absorb toxic gases; emergency support when fish gasp at the surface.
• VIBRIO CLEAR – helps reduce bacteria and limit pathogens in the water.

3. Building resistance and recovery

• LIVER GRO – supports the liver and detoxification, and helps recovery after disease.
• BETAGLUCAN GRO – helps stimulate immunity, supporting fish against intracellular bacteria.
• GRO C30 – supplies vitamin C, helps reduce stress and supports disease resistance.

English text adapted from Than Vuong's original Vietnamese infographic.