Microbiology

Renibacterium salmoninarum is the cause of bacterial kidney disease (BKD), a chronic, debilitating disease with high mortality, that is characterized by granulomatous lesions primarily on the kidney and other organs. The disease occurs worldwide in cultured and wild salmon. BKD affects the health, fitness and survival of salmonids at all life stages during both fresh and saltwater phases. In the Pacific Northwest Columbia River drainage and estuary, it is a serious problem in cultured broodstocks of endangered chinook and sockeye salmon and hatchery raised fish. Currently there are no completely effective vaccines available to prevent R. salmoninarum infections, and antibiotic treatment has also proven less than optimal.

Typical BKD lesion on the kidney of a juvenile   chinook salmon

Measures to control BKD have had limited success and presently there are no satisfactory treatments or an effective vaccine. One of the biggest factors in preventing disease is the fact that R. salmoninarum is passed vertically from the female to her eggs, as well as horizontally from fish to fish. The use of chemotherapeutics including antibiotics has been hampered because of many factors, including the intracellular life cycle of the bacterium in the infected host, and the general problem in delivery of these compounds into the early life stages of salmon when bacterial populations are lowest. The approved antibiotic for treatment of R. salmoninarum infections is erythromycin, a member of the macrolide class. Unfortunately, this antibiotic does not penetrate cells well, bacteria easily develop resistance to it, and continued prophylactic use of it to treat endangered captive broodstock populations can sometimes lead to toxic reactions and lowered reproductive success.

We have approached the problem of treating BKD in endangered captive populations from two points. One is to use the alternative macrolide antibiotic, azithromycin, to treat the infection. Azithromycin has several pharmacological features that may be advantageous in treating BKD, including the ability to cross eukaryotic cell membranes, and a longer active half-life. The second approach is to test various vaccine candidates to assist the fish immune system in preventing or alleviating infections. Both approaches are showing some promise, and it is hoped that applying these strategies to salmon at their earliest life stage may result in populations free of the pathogen.