Disease resistance in aquaculture: 110 days of feeding
European seabass juveniles fed a mushroom-derived additive for 110 days survived a bacterial challenge better than controls. The 1 and 2 percent diets gave the clearest change in immune gene expression and plasma enzyme activity. Disease resistance in aquaculture works on a slow clock, and the 2 and 50 day samples showed nothing at all.
Why disease resistance in aquaculture is a feed decision now
Global aquaculture production reached 103 million tonnes in 2024, which works out at 53 percent of total aquatic animal production. Marine finfish account for a large share of that figure, and they are grown in pens where one bacterial outbreak can end a production cycle early. Bacterial disease, infection by Photobacterium damselae subsp. piscicida included, is still one of the leading constraints on the sector.
Vaccines and antibiotics handle part of the risk. Feed is the lever a farm pulls on its own, which is why natural bioactive ingredients have moved from specialty use into standard formulations. The commercial question is narrow. Does the ingredient change what happens after a pathogen arrives, and how long does that take?
Two different responses tend to get blended together in that discussion. One is whether the additive changes immune gene expression in a healthy fish. The other is whether the change matters once a pathogen is present.
How the feeding trial was built
The trial used European seabass (Dicentrarchus labrax) juveniles, 250 fish in total, and the additive was a commercial mushroom-derived feed additive made from Pleurotus ostreatus, Lentinula edodes and a fermented Hericium erinaceum mycelium ingredient. It was fed as a dietary inclusion for sustainable aquaculture, not applied as a treatment.
| Parameter | Detail |
|---|---|
| Species and stage | European seabass (Dicentrarchus labrax) juveniles |
| Fish used | 250 juveniles |
| Dietary treatments | Control plus four inclusion levels of a mushroom-derived feed additive: 0.2, 0.4, 1 and 2 percent, labeled HA0.2, HA0.4, HA1.0 and HA2.0 |
| Feeding period | 110 days |
| Sampling points | 2, 50 and 110 days of dietary treatment, plus 2 days after challenge |
| Tissue collected | Spleen and plasma, three juveniles per replicate tank |
| Replication | Gene expression n = 9 per treatment in spleen; plasma parameters n = 3 per tank |
| Gene panel | 10 genes related to the immune system, measured by RT-qPCR |
| Normalization | Two reference genes, 40S ribosomal protein and elongation factor 1 alpha, with the 2(-delta-delta-Ct) method |
| Challenge | Intraperitoneal injection of Photobacterium damselae subsp. piscicida at 2 x 10^7 CFU per fish, after 110 days of feeding |
| Survival monitoring | 9 days after challenge, 10 fish per dietary treatment in each of two survival tanks, n = 20 per treatment |
| Handling controls | Phosphate-buffered-saline injected groups, 100 percent survival throughout |
| Statistics | Two-way ANOVA for time by treatment interaction; one-way ANOVA with Tukey post hoc within sampling points at p < 0.05; pairwise Log-rank tests for survival |
The sampling schedule is what makes the trial readable. Spleen and plasma were taken three times inside the feeding period and once more 2 days after the challenge, which is how a response built up by feeding gets separated from a response triggered by infection.
The challenge was an intraperitoneal injection of Photobacterium damselae subsp. piscicida at 2 x 10^7 CFU per fish, given after 110 days of feeding, and survival was tracked for 9 days. Phosphate-buffered-saline injected groups were kept as handling controls and stayed at 100 percent survival, so the deaths recorded in the trial came from the pathogen rather than from the injection procedure.
Immune gene expression after 110 days
| Marker group | Direction at 110 days | Diets with a significant effect |
|---|---|---|
| Cytokine genes il1b, il10 and tnfa | Upregulated | HA1.0 and HA2.0 |
| Antimicrobial genes and iron-metabolism-related genes | Upregulated | HA1.0 and HA2.0 |
| Adaptive immunity markers mhcI and mhcII | Upregulated | HA0.4, HA1.0 and HA2.0 |
| Adaptive immunity markers cd4 and cd8 | Upregulated | HA0.4, HA1.0 and HA2.0 |
Nothing separated at 2 or 50 days of treatment. No analyzed gene differed significantly between the diets (one-way ANOVA p > 0.05). At 110 days, diets HA1.0 and HA2.0 significantly upregulated cytokine genes and antimicrobial- and iron-metabolism-related genes (p < 0.05). Adaptive immunity markers mhcI, mhcII, cd4 and cd8 were upregulated at the same point, particularly in HA0.4, HA1.0 and HA2.0 (p < 0.05). For il1b, il10, tnfa, mhcI, mhcII, cd4 and cd8, mRNA levels in HA1.0 and HA2.0 sat significantly above the control group at that time point.
The reach of the response depends on the marker. Innate genes in the panel answered only to the two highest doses, while the adaptive immunity markers also moved at 0.4 percent. An underdosed feed can therefore produce a signal in a gene expression assay even when the outcome measure later separates only the higher doses.
Plasma enzyme activity in supplemented fish
Plasma lysozyme, peroxidase and bactericidal activities all increased as inclusion rose. Bactericidal activity was significantly higher in every supplemented diet than in the control at 110 days (one-way ANOVA p < 0.05). That reach is wider than the gene panel, because bactericidal activity separated all four inclusion levels from the control while the cytokine genes separated only HA1.0 and HA2.0. During the feeding phase, plasma enzymes read as the more sensitive indicator and gene expression as the more selective one.
Survival after the bacterial challenge
| Group | Survival after the bacterial challenge |
|---|---|
| Control diet | Reference group for the pairwise comparisons |
| HA1.0 | Significantly higher than control (p < 0.05) |
| HA2.0 | Significantly higher than control (p < 0.05) |
| Phosphate-buffered-saline injected | 100 percent survival throughout |
Survival decides whether an ingredient stays in a formulation, and here it split the treatments. Fish fed HA1.0 and HA2.0 had significantly higher survival than controls after the challenge (p < 0.05). Relative percent survival was reported for the supplemented groups, and the highest inclusion levels performed best.
The advantage came with a matching plasma profile at 2 days post challenge, higher lysozyme, peroxidase and bactericidal activities in the same groups. The measured inflammatory cytokine transcripts did not go up any further at that point. These fish were not reacting more loudly to the infection. They were carrying more active antimicrobial enzymes into it.
The counterintuitive part: 50 days tells you nothing
Feed additive reviews are usually short. A farm runs a trial for a month or two, checks growth or a blood marker, and decides. This trial shows what that habit costs. None of the 10 genes measured differed significantly at 2 or 50 days, and every measurable immunological effect arrived at 110 days, which is about four months of continuous feeding.
Timing is the part of disease resistance in aquaculture that buying decisions tend to skip. A product can look inactive at 50 days and still be useful at 110 days. The reverse mistake is just as easy, because a short trial can pick up a transient signal that has nothing to do with survival after a real infection. Fifty days is not a cheaper version of the same answer.
The challenge result carries a second surprise. Better survival came without a larger cytokine response at 2 days post challenge. Anyone expecting a louder immune reaction at the moment of infection will not find one here. The gain sits in antimicrobial enzyme activity instead.
What this means for farm managers
- Give the additive 110 days of continuous feeding before you judge it. A 50 day check reads as a null result even when the product works.
- Judge on survival after challenge, or on field survival during an outbreak, rather than on a blood marker alone.
- Use the 1 and 2 percent inclusion levels if you want the survival effect seen in this trial.
- Keep a control group and a phosphate-buffered-saline injected reference group in any challenge you run in house.
- Request both the challenge survival data and the gene expression data from the supplier, since the two do not always agree.
- Test plasma lysozyme, peroxidase and bactericidal activity if your laboratory can run them, because those markers tracked the outcome more closely than cytokine transcripts.
- Handle residue, withdrawal and cost-of-production questions as separate work. This trial does not answer them.
Limits of the evidence
- One species and one life stage, European seabass juveniles. The timeline does not transfer to another species without new work.
- A laboratory challenge with a controlled injection dose, not the uneven and repeated exposure of a working farm.
- A small survival end point, 20 fish per dietary treatment, which limits how precisely the size of the benefit can be estimated.
- No residue, withdrawal or cost-of-production data in the source, so the additive cannot be priced from this evidence.
- Spleen gene expression and plasma enzymes only. Mucosal and gut level responses were not part of the design.
Bottom line
A mushroom-derived additive at 1 or 2 percent, fed for 110 days without a break, changed immune gene expression and plasma enzyme activity in European seabass juveniles and improved survival after a Photobacterium damselae subsp. piscicida challenge. The 2 and 50 day samples found nothing. Disease resistance in aquaculture from an ingredient like this is a four month proposition, and buyers who want a fast answer will keep getting the wrong one.
FAQ
What is disease resistance in aquaculture
It describes how well farmed fish limit the damage when a pathogen reaches them. Two separate things can be measured: immune gene expression in tissues such as spleen, and survival after an experimental challenge. A feed additive can shift the first without shifting the second, so survival is the end point that decides whether an effect is real.
How long does a supplemented feed need before immunity changes
In this trial the first measurable immune changes appeared at 110 days of continuous feeding. The 2 and 50 day sampling points found no significant differences in any of the 10 immune genes measured. That timeline is about four months of continuous feeding, which is much longer than the review window most farm managers use when they judge a new additive.
Which inclusion level performed best
The 1 and 2 percent diets performed best. Both produced significantly higher survival than the control after the bacterial challenge, and both showed the strongest change in cytokine and adaptive immunity markers at 110 days. The lower inclusion levels were not among the best performers for survival.
Why did the early sampling points show no immune response
Because the additive needs time to build an effect rather than acting on contact. At 2 and 50 days of treatment no analyzed gene differed significantly between the diets, while at 110 days the two highest inclusion levels had upregulated cytokine genes. A short trial is therefore likely to report a null result for a product that does work.
What pathogen was used in the challenge test
The challenge used Photobacterium damselae subsp. piscicida, a bacterium that causes disease in marine finfish. Fish received an intraperitoneal injection of 2 x 10^7 CFU per fish after 110 days of feeding. Survival was then monitored for 9 days, and the phosphate-buffered-saline injected control groups stayed at 100 percent survival throughout.
Does a survival gain in a laboratory challenge prove a field benefit
Not on its own. A challenge model gives every fish the same dose at the same moment, which a farm cannot reproduce, and this survival end point rested on 20 fish per treatment. A survival gain is good evidence that the ingredient changes the outcome, but the size of the gain on a farm still has to be confirmed under commercial conditions.
Which immune genes were upregulated after 110 days
Diets HA1.0 and HA2.0 significantly upregulated cytokine genes and antimicrobial- and iron-metabolism-related genes at 110 days. The adaptive immunity markers mhcI, mhcII, cd4 and cd8 also rose, particularly in HA0.4, HA1.0 and HA2.0. For il1b, il10, tnfa, mhcI, mhcII, cd4 and cd8, mRNA levels in the two highest diets were significantly higher than in controls.
What plasma markers changed in supplemented fish
Plasma lysozyme, peroxidase and bactericidal activities all increased with supplementation. At 110 days, bactericidal activity was significantly higher in every supplemented diet than in the control. At 2 days after the challenge the same three activities were higher in the groups that survived best, with no extra change in the measured inflammatory cytokine transcripts.
Can a lower inclusion level still deliver a benefit
It can move part of the response. The 0.4 percent diet was among the diets that raised the adaptive immunity markers at 110 days, and bactericidal activity was higher than control in all supplemented diets. Survival after challenge separated only the 1 and 2 percent levels, so a low dose buys a partial response rather than the full outcome.
How should a farm manager evaluate a feed additive like this
Set the review window before the trial starts, and make it at least 110 days of continuous feeding. Choose survival after a challenge as the primary end point and keep a control group alongside the supplemented fish. Ask for the challenge data as well as the gene expression data, because the two do not always move together.
What are the main limits of this trial
It used a single species and a single life stage, European seabass juveniles. It was a laboratory challenge rather than a farm trial, and the survival end point rested on a small number of fish per treatment. The source also provides no residue, withdrawal or cost-of-production data, so the additive cannot be priced from this evidence alone.
Is survival after challenge the right end point
For a product sold as an immunomodulator, yes, because survival is what the farm is buying. Gene expression and plasma enzyme activity explain how an effect happens, but they can change without changing the outcome. The practical test is whether more fish are alive at the end of a challenge or an outbreak.
