Columnaris disease bacteria detected in Brazil for first time, threatening farmed fish

A bacterium that kills farmed fish has arrived in Brazil for the first time, and researchers are racing to understand what it means for the country's aquaculture industry. Scientists identified Flavobacterium bacteria—the pathogen behind columnaris disease—in fish farm samples across São Paulo, Minas Gerais, and Paraná. Until now, this disease had only been documented in Asia and the United States. The discovery, published in the journal Microbial Pathogenesis, signals an urgent need for surveillance systems and new vaccines to protect the fish that feed millions of Brazilians.

Columnaris disease attacks the skin and fins of fish, leaving whitish lesions and causing necrosis in the gills. The bacteria consume epithelial cells and can kill fish within days, particularly young larvae and fry. The disease primarily threatens Nile tilapia—the most widely farmed fish globally and a cornerstone of Brazil's aquaculture sector, sold commercially as Saint Peter. But the threat extends far beyond tilapia. Researchers found the pathogen infecting native Brazilian species raised for food: tambaqui, pacu, lambari, and Amazon spotted catfish. Among eleven bacterial strains isolated in the study, six belonged to Flavobacterium oreochromis, a species previously known to infect only tilapia. The discovery that it now colonizes native fish represents a significant expansion of the disease's reach.

What makes this outbreak particularly concerning is how well these pathogens have adapted to Brazil's environment. The bacteria thrive at 28 degrees Celsius—the average temperature of Brazil's inland waters. At this temperature, they produce robust biofilms, protective matrices that allow the bacteria to survive dormant periods and resume multiplication when conditions improve. This adaptation means the pathogen can persist on farm equipment and in water systems, making it far harder to eliminate through standard cleaning. Daniel de Abreu Reis Ferreira, who led the research during his doctoral studies at São Paulo State University's Aquaculture Center, emphasizes the implications: "This underscores the importance of robust hygiene and disinfection protocols to prevent the colonization of equipment used for fish handling."

One strain, Flavobacterium davisii, presented an especially troubling finding. It was detected in Amazon spotted catfish—a different order of fish from those it typically infects. This suggests the bacterium's host range is broader than previously understood, expanding the number of species at risk. The same strain also showed an unusual adaptation: while most Flavobacterium species reduce biofilm production at higher temperatures, F. davisii maintains significant biofilm production even at 35 degrees Celsius. Ferreira describes this as a metabolic trade-off—the bacterium sacrifices mobility at higher temperatures but gains survival advantage through biofilm formation.

Identifying these bacteria in the first place required meticulous work. The organisms glide through culture media, sometimes becoming nearly invisible under the microscope depending on which medium is used. Ferreira and his supervisor, Fabiana Pilarski, a professor at the Aquaculture Center and associate researcher at the Science Center for the Development of Aquaculture Health, had to exercise exceptional care during visual examination to avoid missing infections entirely.

The research team is pursuing multiple strategies to contain the threat. Studies by other groups have shown that Flavobacterium bacteria struggle in salty water, suggesting that adding salt to farm systems could reduce the pathogen's ability to establish itself. However, determining the optimal salinity levels for each fish species requires further investigation. More promising is vaccine development. Researchers are conducting genomic studies on the isolated bacteria to identify potential vaccine targets, with the goal of creating autogenous vaccines—customized formulations tailored to the specific strains present at each farm. Pilarski envisions bath treatments using weakened bacteria, particularly beneficial for young fish whose immune systems are still developing. Such an approach would allow farmers to vaccinate large numbers of fingerlings simultaneously, protecting them during their most vulnerable stage.

The arrival of columnaris disease in Brazil represents a convergence of ecological and economic pressures. As global trade in aquaculture products continues, pathogens move with it. The disease now threatens not only the livelihoods of fish farmers across three major Brazilian states but also the food security of communities dependent on farmed fish. The race is on to establish surveillance systems that can detect outbreaks early, develop vaccines that work against local strains, and implement farm protocols that prevent the bacteria from taking hold. What happens in Brazil's fish farms over the next months will shape how the global aquaculture industry responds to this newly arrived threat.