Streptococcus agalactiae Sequence Type 283 in Farmed Fish, Brazil

Area of Science:

• Veterinary Microbiology
• Aquatic Animal Health
• Genomics

Background:

• Streptococcus agalactiae, particularly serotype III sequence type (ST) 283, has been associated with disease in various animal species and humans.
• Nile tilapia (Oreochromis niloticus) aquaculture is a significant industry in Brazil, making it vulnerable to bacterial disease outbreaks.
• Understanding the genetic relatedness of bacterial strains is crucial for managing disease spread and implementing control strategies.

Purpose of the Study:

• To characterize Streptococcus agalactiae serotype III sequence type (ST) 283 outbreaks in Nile tilapia farms in Brazil during 2016-2017.
• To determine the genetic relationship between tilapia isolates and strains from other animal hosts and humans.

Main Methods:

• Bacterial isolates from tilapia outbreaks were collected in Brazil.
• Whole-genome multilocus sequence typing (wgMLST) was performed on the isolates.
• Phylogenetic analysis was used to cluster the tilapia isolates with reference strains.

Main Results:

• The study identified outbreaks of Streptococcus agalactiae serotype III ST-283 in Brazilian Nile tilapia farms.
• Whole-genome multilocus sequence typing revealed that the fish isolates clustered closely with known zoonotic ST-283 strains.
• Genetic analysis also linked the tilapia strains to isolates from humans, frogs, dogs, cattle, and dolphins, suggesting a broader circulation of this clone.

Conclusions:

• The findings indicate that the zoonotic Streptococcus agalactiae serotype III ST-283 clone is circulating in Nile tilapia aquaculture in Brazil.
• The genetic relatedness between fish, human, and other animal isolates underscores the potential for interspecies transmission.
• This highlights the importance of surveillance and control measures in aquaculture to prevent disease spread and mitigate public health risks.