Combatting lactococcosis in Australian farmed trout
To prevent a devastating finfish disease and reduce antibiotic use in Australian farmed trout, SAAFE partners are developing an effective vaccine and farm management strategies.
As Australia’s climate warms and water temperatures rise, trout farms across the country are facing a growing threat: the rising incidence of lactococcosis, a finfish disease caused by the bacterium Lactococcus garvieae. The disease, which is linked to warmer waters, causes severe mortality in farmed trout – affecting up to 40% of fish in affected ponds. In a bid to reduce the reliance on antibiotics and to safeguard the health of fish stocks, SAAFE partners are working together to develop and implement an effective vaccine against L. garvieae.
The economic toll of lactococcosis is significant. During the 2022–2023 summer season, for example, the Victorian trout farms most affected by the disease suffered an estimated $1.5 million in stock losses. Antibiotic treatment is costly, and frequent use raises the risk of antimicrobial resistance (AMR).
Some farms have used autogenous L. garvieae vaccines, which are custom-made from the bacterial strain – or strains – isolated from each farm. But these vaccines do not reliably offer long-term or very high levels of protection. “Australia’s general trend of warming is one that we see very clearly in water,” said Dr Matt Landos, Director of Future Fisheries Veterinary Services (FFVS), a consultancy involved in the project. Vaccine protection lasts only around 3–4 months before decreasing. “We’re aiming to get protection for closer to 6 months. That would get us through the worst of the current summers.”
Researchers from the University of Queensland are working on developing the vaccines, using inactivated forms of the bacteria that present antigens to elicit an immune response. Initially, they thought that the underperformance of the autogenous vaccines in Australia was due to genetic variability between bacterial isolates, as has been found internationally. This might occur if a farm’s vaccine did not include the right mix of bacterial strains found on that farm. But their genomic analyses of over one hundred samples of L. garvieae from across the country revealed that the bacterial strains are highly similar.
“Phase one trials of the vaccine in the lab delivered around 50% protection, which was a good start, but we hope to achieve better,” said Dr Landos. “We think other risk factors must be influencing how the fish respond to the vaccine and how the vaccinated fish respond to the bacteria.”
FFVS is working with trout farmers and University of Queensland researchers to learn more about these factors. For example, feed rate and feed type both appear to influence disease outbreaks. Using data collected from farms last summer, the researchers are now designing an upcoming trial to test modified vaccines presenting different antigens in combination with different diets.
“We've also identified some water quality parameters, such as the amount of gas dissolved in water, that warrant more surveillance in the coming summer,” said Dr Landos. “Excessive dissolved gas in water can stress the fish, and trout farms that are located below dams can experience elevations in total gas pressure.”
By searching for better antigen targets to modify the vaccine and learning more about other disease factors, the partners hope to identify viable management strategies to support improved vaccine prevention of disease.
This approach relies on close collaboration between trout farmers, veterinarians and University of Queensland researchers. “We’ve been fortunate to have almost all of the trout farms in Australia engaged in the project. This speaks to the importance of this disease for them,” Dr Landos said. “Collaborative projects like this one have tremendous synergy, allowing a much faster rate of development than we could achieve with lab insights alone.”