Veterinary professor finds harmful bacterial pathogen in Virginia farm-raised rainbow trout

An emerging strain of harmful bacteria has been found for the first time in farm-raised rainbow trout in Virginia.

“Other people that are culturing trout in the state need to know about it, and individuals in states around our state need to know about it,” said Stephen A. Smith, professor in the Department of Biomedical Sciences and Pathology and director of the Aquatic Medicine Program at the Virginia-Maryland College of Veterinary Medicine. “Because they're going to be seeing it probably in North Carolina soon and probably some other places with the transportation of fingerlings between states. I wouldn't be surprised if it's already spread to other states.”

The discovery was published in the peer-reviewed Journal of Aquatic Animal Health, entitled “First isolation of Carnobacterium maltamaticum from farmed Rainbow Trout in Virginia.”

“This was one of my diagnostic cases that was submitted to our college,” Smith said. “The fish came in with skin lesions that were not typical of more common trout diseases we have in the United States.

"We were able to isolate this bacterium that had never been reported by our lab, and I’ve been doing diagnostics here for 30 years. I’ve never seen that particular genus and species of bacteria isolated from those fish.”

Smith said colleagues at Michigan State were able to point him toward cases they had found previously, and analyses determined it the same strain of bacteria they had seen in the Great Lakes region.

Courtney E. Harrison and Thomas Loch of Michigan State University are co-authors with Smith, as is Shelley J. Newman of Newman Specialty VetPath in Hicksville, New York.

Smith said C. maltamaticum can cause skin lesions and kidney and liver problems in trout. If left untreated, large-scale fish die-offs are possible. Treatment with antibacterial agents have proven effective, he said.

“This is an emerging disease, we've got it in Michigan, we've got it in a number of different places around the Great Lakes, we have it now in Virginia,” Smith said. “It’s an emerging disease that people should know about. That's why we did such a thorough workup of the phenotypic characterization and then we also did some molecular sequencing of it to make sure it was exactly the same.”

While some previous studies found female fish more susceptible to the C. maltamaticum, the Virginia discovery of the bacterium was in an “all-male rainbow trout broodstock population, suggesting that pseudo-kidney disease can ensue if conditions are favorable, regardless of sex,” Smith’s paper states.

The site of the Virginia discovery is described in the paper as “a commercial aquaculture facility with spring-fed, flow-through raceways” that “experienced morbidity and mortality of production-sized rainbow trout over a two-week period in January 2018.”

Water quality and oxygen levels were found within acceptable ranges, ruling those out as factors. Affected fish displayed anorexia, wasting, skin lesions, and abnormal floating.

Smith has been on the veterinary college’s faculty since 1991, performing diagnostics and research on aquatic animals and teaching courses on wildlife and exotic animals.