Comparative efficacy of Bacillus probiotics and formalin-killed bacterin against Vibrio anguillarum in European eel elvers

Protecting a Valuable Fish

European eels are an important food fish in Europe and North Africa, but their farming is threatened by a serious bacterial disease called vibriosis, which can wipe out young eels in a matter of days. This study asks a practical question with big implications for fish farmers and consumers: is it better to protect young eels using a vaccine delivered through the water, or by adding beneficial “good bacteria” (probiotics) to their feed? The answer could reduce antibiotic use, improve animal welfare, and make eel farming more sustainable.

The Problem: Deadly Germs in Eel Farms

One of the main culprits behind vibriosis in eels is a bacterium called Vibrio anguillarum, sometimes known historically as the cause of the “red pest of eels.” It attacks the skin and internal organs, leading to bleeding, ulcers, and high death rates. In the past, antibiotics were widely used in fish farms to fight such infections, but overuse has contributed to drug-resistant bacteria and residues in food and the environment. As a result, many countries now restrict antibiotic use, pushing aquaculture to find cleaner, safer ways to keep fish healthy.

Two Safer Strategies: Vaccines and Good Bacteria

The researchers compared two antibiotic-free approaches in European eel elvers—young eels weighing about 25–30 grams. One approach was an immersion vaccine: the fish were briefly placed in water containing killed Vibrio cells, then given a booster two weeks later. Their diets remained otherwise standard. The other approach used a commercial probiotic made of three Bacillus species, mixed into the feed at either a lower or higher dose. A fourth group of eels served as an untreated control and received only the basic diet. All groups were reared for 28 days under the same tank and water conditions.

Measuring the Eels’ Front-Line Defenses

To see how well each treatment prepared the eels to fight disease, the team examined both blood and skin mucus—the first barriers that invading germs encounter. They measured lysozyme, an enzyme that can break down bacterial cell walls, and tested how well these fluids could kill Vibrio in the lab. They also quantified antibody levels using a plate test that reveals how strongly the eel’s immune system recognizes and clumps the bacteria. After these measurements, all eels were injected with a standard, disease-causing dose of Vibrio anguillarum, and the scientists tracked survival for 10 days, calculating a “relative percent survival” to compare protection among groups.

What Worked Best for the Young Eels

Both the vaccine and the probiotic diets clearly strengthened the eels’ defenses compared to the untreated control group. Eels that were vaccinated or fed probiotics showed higher lysozyme activity, stronger bactericidal power in their blood and mucus, and higher antibody levels against Vibrio. When the real infection test came, all treated groups suffered fewer deaths than the control fish. The relative survival was highest in the vaccinated eels, while the two probiotic doses offered intermediate protection. In simple terms, the vaccine produced the strongest, broadest immune boost, while the Bacillus probiotics added meaningful but somewhat smaller benefits.

What This Means for Fish Farmers

For eel farmers, the study’s bottom line is practical: both immersion vaccination and probiotic feed can reduce losses from vibriosis without relying on antibiotics, but vaccination provides the most reliable shield for young eels. Probiotics remain valuable as a supportive measure that improves general health and resistance, yet they do not fully match the protection seen with the vaccine. The authors recommend immersion vaccination with the tested bacterin as a core biosecurity tool in eel aquaculture, and suggest that future work should explore combining vaccines and probiotics, along with economic analyses, to design the most cost-effective and sustainable disease control strategies.

Citation: Abdel-Latif, H.M., Abdel-Razek, N. & Khalil, R.H. Comparative efficacy of Bacillus probiotics and formalin-killed bacterin against Vibrio anguillarum in European eel elvers. Sci Rep 16, 4367 (2026). https://doi.org/10.1038/s41598-026-35298-8

Keywords: eel aquaculture, fish vaccination, probiotics, vibriosis, Vibrio anguillarum