Featured Image: A Colorado River cutthroat trout suffering from BKD. Photo by John Drennan

DENVER, Colo. – While many fish diseases have declined in recent years due to good management practices, cases of bacterial kidney disease (BKD) seem to be increasing in the western U.S. The disease is caused by the bacteria Renibacterium salmoninarum, which is common in cold water streams and lakes. The disease is characterized by the presence of grayish-white abscesses in the kidney and can cause death in both wild and hatchery trout.

After negative tests in the Colorado fish hatchery system for 18 years, in 2015 four state hatcheries, one federal hatchery, and a wild broodstock lake tested positive for the disease. An outbreak at one hatchery cost over $2.1 million and impacted fish management statewide with the loss of over 675,000 sport fish. In 2017, a statewide sampling effort led by CPW Research Scientist Dan Kowalski found the bacteria was common in trout habitat statewide, but generally occurred at low levels and only rarely caused outbreaks of disease in the wild. These recent detections of R. salmoninarum in hatcheries and wild fish populations in Colorado have generated additional questions about presence and infection intensity in trout and caused managers to revisit best management practices in hatcheries.

Colorado Parks and Wildlife has partnered with Colorado State University Cooperative Fish and Wildlife Research Unit and Ph.D. student Tawni Riepe to investigate important aspects of BKD in Colorado. Her work, while still ongoing, has already produced some interesting results. In one experiment, fish were caged in a water known to have fish infected with R. salmoninarumto look at direct bacterial transfer between infected and non-infected fish (a process known as horizontal transmission). The trial lasted for 90 days and involved 320 caged cutthroat trout. Only one fish tested positive for the bacteria that causes BKD, demonstrating that horizontal transmission was low under these conditions but occurred in a relatively short amount of time. A second experiment was designed to look at transmission from an infected fish to its offspring (known as vertical transmission). Early results confirm that eggs reared from fish infected with R. salmoninarum may have varying levels of the bacteria depending on the degree of infection within the parents.

“Understanding how the bacteria that causes BKD is transmitted from fish to fish or fish to egg to fish, is important to figuring out how to minimize the spread of the bacteria and the disease among hatchery and wild fish populations,” commented Riepe.

Another focus of Riepe’s research is to compare and improve testing methods to detect the bacteria that causes BKD. The goal is to determine the best way to test fish, what test to use, and if non-lethal tests can be developed to test fish without sacrificing them. Just like testing for human pathogens that cause disease, like COVID-19, there are several ways to detect bacteria in fish. A technique called quantitative polymerase chain reaction (qPCR) can be used to detect the DNA of the pathogen and to determine the intensity of infection in fish. Another approach is to test for antigens, proteins on the surface of the pathogen that a fish host uses to produce antibodies that attack the bacteria. Riepe, in collaboration with Dr. John Drennan, a Senior Fish Pathologist with CPW, is also working on methods to evaluate if currently healthy fish have been previously infected with the bacteria by testing for antibody production. All of these methods have their strengths and weaknesses, and this important research will help identify the best methods to test for the bacteria that causes BKD in Colorado trout populations. Current results indicate that using qPCR to test the mucus, kidney, and liver tissue of the fish produced the best results and, in several cases, the non-lethal test of a fish’s mucus produced similar results to more traditional organ tissue tests.

A final component of the research is to explore how the disease may impact wild trout populations. Brook trout are known to be particularly susceptible to R. salmoninarum infections that can lead to BKD, so Riepe and her colleagues are studying several brook trout populations in high elevation streams and lakes to determine if varying levels of the bacteria might affect age, growth and survival of the fish.

Riepe is also working closely with Dr. Eric Fetherman, an Aquatic Research Scientist from CPW, to conduct this important work and is being advised at CSU by Dr. Dana Winkelman. Together they hope to make some headway in the management of this disease to benefit fish populations and anglers of Colorado.

“Tawni’s work with R. salmoninarum represents some of the most comprehensive research conducted in inland trout populations and will not only benefit the State’s wild and hatchery-reared cutthroat trout populations and the anglers of Colorado, but also further contribute to our knowledge of bacterial kidney disease in the United States and worldwide,” said Fetherman.

“Collaborating with CPW has been one of the highlights of my Ph.D. experience,” Riepe said. “Not only has the expertise that lies within the agency’s biologists, hatchery managers, and aquatic researchers enhanced all the planning and executing of these research projects, but the support and advice I have received from everyone I am directly working with or behind the scenes has been unmeasurable and I am completely humbled.”

Additional photos: 

Dr. Eric Fetherman with blood serum collected from a cutthroat trout to test for antibodies produced in response to an infection by R. Salmoninarum

Tawni Riepi and CPW Technician Crosby Vail setting gill nets at Eagle Lake to collect brook trout to determine the effects of R. Salmoninarum bacteria in wild trout populations.

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