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By Niecole Killawee

While humans are currently experiencing the COVID-19 pandemic, fish in the ocean have been experiencing infectious disease outbreaks of their own — ones that threaten the health of wild fisheries and the economics of the aquaculture industry.

Disease-ridden fish stocks have been increasing over the last few decades, due in part to an organism's reduced ability to fight off infection under ever-changing climate conditions. Luckily, just as it does with humans, the science of vaccinology can uncover a successful method for protecting both wild and farmed fish species from debilitating and often fatal diseases.

Image credit: Santander Lab, Memorial University

Javier Santander, an associate professor at Memorial University with a background in marine biology and microbiology, set out to discover the building blocks of a vaccine for fish with the help of an OFI seed fund. Now his team has a promising vaccine preparation and further funding to continue his innovative research.

"Without this fund, we wouldn't have this data and we wouldn't have been able to think about a vaccine that allows cross-protection and efficiency across all kinds of fish species," says Santander.

Using the seed fund, Santander pursued pioneering research in the field of fish vaccinology. His team's focus was not on fish, but on the genetics of the bacterial pathogens known to infect fish in the first place.

Before this work there was very little scientific understanding around how climate change affects host-pathogen interactions in the ocean — fundamental knowledge needed for vaccine discovery. But warmer ocean temperatures are known to drive bacterial evolution, so by studying how these disease-causing microbes are adapting and thriving in changing ocean conditions, Santander set out to help bridge the knowledge gap.

"Many people in our field are looking at the host, and not many are looking at the pathogen," says Santander. "Let's look at the pathogen and what we can use from it to get an immunoprotected response."

To do this, his team studied a bacterial pathogen called Vibrio anguillarum that causes vibriosis, one of the most frequent diseases in fish.

Image credit: Santander Lab, Memorial University

The project captured the complete DNA-sequence of a V. anguillarum specimen that was isolated from an infected lumpfish off the coast of Newfoundland, which is now available as open-source data.

After sequencing, the team grew the bacteria in warm water and low-iron settings that mimic climate change-driven ocean and host conditions to observe which genes were expressing under different conditions. This experiment allowed the team to capture what's called a transcriptome sequence. In other words, the information that details the bacteria's RNA.

Because Santander tackled the study of the pathogen from a genetic point of view, it was then possible for his team to employ a technique called reverse vaccinology to begin the process of vaccine discovery.

In reverse vaccinology, the genetic information of the pathogen is used to source potential antigens that can be used in a vaccine preparation (sound familiar? This technique is currently being used to explore vaccine candidates for COVID-19).

Developing a vaccine for Vibrio bacteria is challenging because they're "like the E. coli of the ocean," says Santander. Vibrio bacteria live everywhere in the ocean, but only a handful of species can cause illness in fish — as V. anguillarum does — while the rest are considered part of a healthy immune system and ecosystem.

With the incredible wealth of data that DNA and RNA sequences can provide, Santander's team was able to identify a specific protein entirely unique to V. anguillarum that allows the organism to adapt to rapidly changing conditions while maintaining its disease-causing nature. With this discovery, Santander and his team had found an antigen that proved key in the development of their most successful vaccine preparation.

Image credit: Santander Lab, Memorial University

"Our vaccine preparation conferred about 80 per cent protection," says Santander. "We evaluated commercial vaccines and they confer about 50-60 per cent protection."

After replicating their results three times in a laboratory setting, Santander's team is now collaborating with Cooke Aquaculture and the Canadian center for Fisheries and Innovation to do field safety trials using their vaccine preparation, with a goal to eventually commercialize the vaccine. Without the OFI seed fund, Santander is sure this work wouldn't have been possible.

"This grant allowed us to do unique things that nobody else would support... It's very evident that this type of knowledge is missing in aquaculture. This is fundamental vaccinology; we're filling in the gaps."