Piscine lactococcosis is a significant emerging threat to wild and cultured fish in the United States. A wide range of hosts are susceptible, including fish from cold, temperate, and warm environments in freshwater or marine systems, as well as humans and other terrestrial animals. The disease in an established issue in European and Asian aquaculture but is rapidly encroaching on ecologically and economically important fish populations in the Americas. Lactococcosis typically presents as a hemorrhagic septicemia causing high mortality rates. Antimicrobial treatment often fails, and development of carrier fish or biofilm formation may lead to recurrent outbreaks. Historically, the disease was attributed to the gram-positive pathogen Lactococcus garvieae. However, recent work revealed there are three distinct agents of piscine lactococcosis – L. garvieae, L. petauri and L. formosensis – which are phenotypically and genetically similar, leading to widespread misdiagnosis. While the three species overlap in host and geographic range, there are relevant differences in host-specific virulence, regional prevalence and antimicrobial susceptibility that impact the success of management interventions. Practical and reliable methods for diagnosis of the specific lactococcosis agents is therefore crucial for implementing appropriate preventative and treatment strategies.
To this end, we compared currently available and recently developed diagnostic methods for their ability to distinguish isolates to the species level, including single and multi-locus gene sequencing, end-point and quantitative PCR (qPCR) assays, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), fatty acid methyl ester analysis (FAME), and API® and Biolog™ systems. Sequencing of the gyrB gene was the most accurate and discriminatory single-gene analysis method (Fig. 1). A qPCR assay based on a putative glycosyltransferase gene was also able to specifically distinguish L. petauri. Biochemical tests and MALDI-TOF MS showed some species-specific patterns in sugar, fatty acid metabolism or protein profiles, but should be complimented by additional analysis. This work provides direct recommendations for specific diagnosis of piscine lactococcosis for improved disease management.