Spatio-temporal evaluation of Flavobacterium psychrophilum genotypes recovered from salmonid hatcheries in California, US

Abstract

Event Abstract Back to Event Spatio-temporal evaluation of Flavobacterium psychrophilum genotypes recovered from salmonid hatcheries in California, US Uzonna C. Uba1*, Beatriz Martínez-López2, Fernanda Sebastiao2, Thomas P. Loch3, Christopher Knupp3, Kaveramma Mukkatira4, Tresa Veek4, Christine Richey4, Mark Adkison4, Matt J. Griffin5 and Esteban Soto2* 1 University of California, Davis, United States 2 Department of Medicine & Epidemiology, School of Veterinary Medicine, University of California, Davis, United States 3 Department of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, United States 4 California Department of Fish and Wildlife, United States 5 Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, United States INTRODUCTION: Flavobacterium psychrophilum (1), is a gram-negative, filamentous, bacterium belonging to the family Flavobacteriaceae, and a major bacterial pathogen of cultured Salmonid aquaculture worldwide. It is the etiologic agent of Bacterial Cold-Water Disease (BCWD) and rainbow trout fry syndrome (RTFS) in salmonids, causing substantial economic losses (2). Juvenile fish are mostly affected, typically resulting high mortalities. Recent studies suggest that several genotypes of Flavobacterium psychrophilum can be found in California. However, the spatio-temporal distribution of this pathogen is unknown. This study investigated spatio-temporal distribution of different Flavobacterium psychrophilum genotypes recovered from outbreaks in cultured salmonids in 2015-2019. METHOD: First, we evaluated descriptively the spatio-temporal evolution of Flavobacterium psychrophilum isolates that were collected from Bacteria cold water disease outbreaks from 17 Salmon fish hatcheries spread across California from 2015-2019. Then, we used multiple correspondence analysis (MCA) to analyze the association between the different F. psychrophilum clonal complexes (CC) with water temperature as well as other spatio-temporal dependent features such as month and year, hatchery and fish species. RESULTS: Fifty-one Flavobacterium psychrophilum were recovered from BCWD outbreaks from 17 Salmon fish hatcheries in California from 2015-2019. The highest number of cases were concentrated in spring 2018, summer 2017 and summer 2018. When grouped by number of individual cases, CC 10 (which consists of sequence types 10,78, 85, 319) had the highest frequency of 30 (58.82%). When grouped by hatchery, CC10 had the highest number of occurrences of 21, CC296 and CC310 had only one occurrence. Sequence type 85 was present in 21 out of the 51 isolates. Clonal complex 10 occurred between temperature (5ºC to 18.8ºC) mean 12.79ºC, CC296 occurred at temperature 8ºC; CC310 occurred at 11.6ºC. The Clonal complex Singleton occurred at 8.8ºC-15.5 ºC mean 12.74ºC. CC296 was associated with a temperature lower than 10ºC, while CC10 had the highest temperature range inferring increased adaptation capability. ST85 had the widest temperature range 5ºC to 18.8ºC with a mean of 12.61ºC. ST 333 had a temperature range (8.8ºC to 15ºC). Darrah spring hatchery had the highest temperature range (5ºC -13.8ºC) with ST85 and CC10 present. While American River Hatchery had a temperature range of 8ºC - 16ºC but had ST296 and CC296 present in one of its cases at 8 ºC. CONCLUSION: The spatial analysis of BCWD in California confirms the genetic diversity of F. psychrophilum in North America. It further revealed that CC 10 and Sequence type 85 was the major driver of the outbreak. The results suggest that Flavobacterium psychrophilum outbreaks present different spatio-temporal patterns due to the different environmental conditions (e.g. water temperatures, hatchery management practices, etc.) and host species. Future studies need to further evaluate the different tropism to species and temperature variations. Spatio-temporal analysis of genetically diverse bacteria has been shown to be a valuable tool to understand disease dynamics that can be useful to develop better health management practices for this important disease in Salmonid aquaculture. Acknowledgements We acknowledge all who contributed to the success of this project.