Vaccination reduces shedding of salmonid alphavirus subtype 3, but bacterial co-infection influences the effect

This study demonstrates that while commercial vaccines (Clynav and AlphaJect Micro 1-PD) effectively reduce Salmonid alphavirus subtype 3 shedding in individually housed Atlantic salmon, concurrent bacterial co-infection with *Tenacibaculum dicentrarchi* alters the vaccine's impact on shedding duration and cumulative release.

The Gist

Imagine a fish farm as a bustling city where the residents are Atlantic salmon. In this city, a dangerous virus called SAV3 (Salmonid Alphavirus) is spreading like a cold in a crowded office. The virus doesn't just make fish sick; it gets passed around through the water, like a sneeze in a shared ventilation system.

The big question for the farmers is: Can vaccines stop the virus from spreading, or do they just make the fish feel better while they keep sneezing?

This paper is like a detective story where scientists tested two different "medicine" strategies to see which one was better at stopping the virus from leaking into the water.

The Two "Medicines" (Vaccines)

The scientists tested two commercial vaccines:

1. Clynav: Think of this as a digital blueprint. It's a DNA vaccine that gives the fish's immune system a "Wanted Poster" of the virus so it can learn to recognize and fight it.
2. AlphaJect Micro 1-PD: Think of this as a training dummy. It's a whole virus that has been "killed" (inactivated) and put in an oil-based booster. It's like showing the immune system a fake enemy so it can practice its defense moves.

The Experiment: Two Different Neighborhoods

To test these vaccines, the scientists set up two different scenarios:

Scenario A: The Private Apartments (Individual Tanks)
Imagine each fish living in its own luxury studio apartment with no neighbors. They were all exposed to the virus, but because they were alone, they couldn't catch it from each other.

• The Result: Both vaccines worked great! The vaccinated fish were much better at keeping the virus inside their bodies. They didn't "sneeze" (shed virus) into the water as much as the unvaccinated fish.
• The Winner: The AlphaJect (the training dummy) was slightly better at keeping the virus completely contained.

Scenario B: The Apartment Complex (Cohort Tanks)
Now, imagine 60 fish living together in a single, crowded room. This is how real fish farms operate. But here's the twist: in this crowded room, a bacterial infection (a different kind of germ called Tenacibaculum) also broke out, causing skin sores and fin rot.

• The Result: This is where things got messy.
  • The Clynav (blueprint) vaccine still worked reasonably well, keeping the virus shedding low and short.
  • The AlphaJect (training dummy) vaccine had a weird reaction. While it reduced the total amount of virus in the water, the fish started "sneezing" for a much longer time.
  • Why? The scientists suspect that the stress of the crowded room and the bacterial skin infection confused the fish's immune system. It was like a soldier who was well-trained but got distracted by a fire drill (the bacteria), causing them to keep fighting the virus longer than necessary, even though they were vaccinated.

The "Leaky" Vaccine Problem

The paper warns about something called a "Leaky Vaccine."
Imagine a house with a leaky roof. If a vaccine is "leaky," it stops the rain (disease) from soaking the furniture (killing the fish), but the roof still drips (the fish still sheds virus).

• If a vaccine stops the fish from dying but lets them keep shedding virus, the virus can spread to other fish that aren't vaccinated, or even evolve to become stronger.
• The good news: Both vaccines in this study were not leaky in the private apartments. They stopped the virus from getting out.
• The bad news: In the crowded, stressful apartment complex with bacteria, the "training dummy" vaccine (AlphaJect) became a bit leaky, letting the virus out for a longer time.

The Big Takeaway

1. Vaccines work, but context matters: A vaccine that works perfectly in a quiet lab might act differently in a noisy, crowded, and stressful real-world farm.
2. Co-infections are trouble: If fish are fighting a virus and a bacteria at the same time, the vaccine might not work as well as expected. The stress of the bacteria seems to mess up the vaccine's ability to stop the virus from spreading.
3. Water sampling is a superpower: The scientists developed a cool new way to test the water (like checking the air for smoke) to see if fish are shedding virus without having to kill them. This is a huge step forward for monitoring fish health.

In short: Vaccines are great tools to stop fish viruses, but if the fish are stressed by other infections or crowded conditions, the vaccine might need a little help to keep the virus from spreading. It's not just about giving the fish a shot; it's about keeping their whole environment healthy too.

Technical Summary

1. Problem Statement

Aquaculture faces significant challenges from waterborne viral diseases, specifically Pancreas Disease (PD) caused by Salmonid alphavirus subtype 3 (SAV3) in Atlantic salmon. The primary transmission route is horizontal via water, driven by the shedding of infectious virus particles from infected fish. While commercial vaccines (specifically the DNA vaccine Clynav and the inactivated whole-virus vaccine AlphaJect Micro 1-PD) are effective at reducing individual mortality and disease severity, their capacity to limit viral shedding and thus reduce transmission pressure (herd immunity) is not fully understood. Furthermore, real-world aquaculture environments often involve co-infections (e.g., bacterial pathogens) and stressors that may modulate vaccine efficacy. This study aims to quantify the impact of these two vaccines on SAV3 shedding kinetics and investigate how bacterial co-infection alters these dynamics.

2. Methodology

The study utilized a high-throughput experimental design involving Atlantic salmon post-smolts, comparing individually housed fish against cohort-housed fish to isolate environmental variables.

• Experimental Groups:
  • Vaccination: Three groups: Non-vaccinated (NV), Clynav-vaccinated, and AlphaJect Micro 1-PD-vaccinated.
  • Challenge: A bath challenge was performed using water from a tank containing 140 "shedder" fish previously injected with SAV3.
  • Housing: After challenge, fish were distributed into:
    • Individual Tanks (IND): 1 fish per 20L tank (n=8 per group).
    • Cohort Tanks (COH): 60 fish per 250L tank (4 replicates per group).
    • Controls: Non-challenged groups for all vaccination types.
• Co-infection Factor: During the experiment, cohort tanks developed clinical signs of a bacterial infection, later identified as Tenacibaculum dicentrarchi. Individual tanks remained free of these clinical signs.
• Shedding Monitoring (VIRADEL):
  • Water samples (100 mL) were collected daily from tanks.
  • Filtration: Samples were filtered through 1.0 µm pre-filters and 0.45 µm cellulose nitrate filters.
  • Detection: Nucleic acids were extracted from filters and analyzed via RT-qPCR targeting the SAV3 nsp1 region to quantify viral RNA.
  • Bacterial Detection: qPCR was also used to detect T. dicentrarchi, T. maritimum, and T. finnmarkense in water samples.
• Tissue Analysis:
  • Fish were sampled at 32–35 days post-challenge (dpc).
  • Viral Load: RT-qPCR on heart, pancreas, and spleen tissues.
  • Histopathology: Lesion scoring (0–3) for inflammation and degeneration in heart, pancreas, and skeletal muscle.
  • Bacterial Confirmation: Immunohistochemistry (IHC) and modified Gram staining confirmed T. dicentrarchi in skin lesions of moribund cohort fish.

3. Key Contributions

• High-Throughput Shedding Assay: The study validates a robust, non-lethal method using VIRADEL (Virus Adsorption and Elution) combined with RT-qPCR to quantify viral shedding in water, allowing for continuous monitoring of transmission potential without sacrificing fish.
• Vaccine Efficacy on Transmission: It provides empirical data comparing two distinct vaccine platforms (DNA vs. Inactivated) specifically regarding their ability to reduce viral shedding, a critical parameter for epidemiological modeling.
• Context-Dependent Efficacy: The study highlights that vaccine performance is not static; it is significantly influenced by environmental stressors and bacterial co-infections (T. dicentrarchi), which altered shedding kinetics in cohort settings.

4. Key Results

A. Individually Housed Fish (No Co-infection)

• Shedding Reduction: Both vaccines significantly reduced shedding compared to non-vaccinated (NV) controls.
  • AlphaJect Micro 1-PD: Showed the strongest effect. 0/8 fish shed virus (vs. 8/8 NV). It significantly reduced shedding duration and cumulative shedding (AUC).
  • Clynav: 4/8 fish shed virus. It significantly reduced shedding duration and cumulative shedding compared to NV, though less effectively than Micro 1-PD.
• Correlation: There was a strong positive correlation ($r = 0.83$) between cumulative shedding and viral load in the heart tissue, suggesting heart viral load is a proxy for shedding potential.
• Pathology: Vaccinated fish showed significantly lower lesion scores in pancreas and muscle compared to NV fish.

B. Cohort Housed Fish (With T. dicentrarchi Co-infection)

• Altered Kinetics: The presence of bacterial co-infection and higher stocking density changed the shedding profile.
  • AlphaJect Micro 1-PD: While it still significantly reduced cumulative shedding (AUC) compared to NV, it resulted in a significantly longer duration of shedding compared to both NV and Clynav groups.
  • Clynav: Showed a trend toward reduced cumulative shedding and shorter duration, but differences were not statistically significant compared to NV.
• Viral Load: In cohort tanks, Micro 1-PD vaccinated fish had higher viral loads in the heart, pancreas, and spleen compared to Clynav and NV groups at 32 dpc, suggesting the co-infection may have compromised the vaccine's ability to control viral replication or maintain neutralizing antibody levels.
• Mortality: Cohort tanks experienced higher mortality and severe clinical signs (fin erosion, ulcers) due to the bacterial co-infection, whereas individual tanks had no mortality.

5. Significance and Implications

• "Leaky" Vaccine Risks: The study demonstrates that while vaccines protect individuals, they can sometimes alter transmission dynamics. In the cohort setting, the Micro 1-PD vaccine led to prolonged shedding periods, potentially creating a "healthy shedder" scenario that could sustain transmission chains in a farm environment.
• Importance of Co-infections: The findings underscore that vaccine efficacy is context-dependent. Bacterial co-infections (common in aquaculture) can negate the benefits of vaccination regarding transmission control, even if the vaccine protects against mortality.
• Epidemiological Modeling: The strong correlation between tissue viral load and water shedding supports the use of tissue sampling as a proxy for transmission risk in future studies.
• Future Directions: The authors recommend incorporating functional infectivity assays (to distinguish between infectious virions and non-infectious RNA) and evaluating vaccine performance under realistic co-infection scenarios to ensure true herd immunity.

In conclusion, while both vaccines reduce SAV3 shedding in controlled individual settings, the AlphaJect Micro 1-PD vaccine demonstrated superior reduction in shedding in the absence of co-infection but exhibited compromised efficacy (prolonged shedding) in the presence of Tenacibaculum co-infection. This highlights the necessity of evaluating vaccines under complex, multi-pathogen conditions relevant to commercial aquaculture.