Bacterial Kidney Disease (BKD): A Disease Guide

Bacterial kidney disease, caused by Renibacterium salmoninarum, is a chronic granulomatous infection of salmonids. Vertical transmission via eggs is the key control challenge.

Overview

Bacterial kidney disease (BKD) is a chronic, systemic infection of salmonid fish caused by Renibacterium salmoninarum, a slow-growing, gram-positive bacterium that survives inside host macrophages as a facultative intracellular pathogen. It is a cold-water disease, with the bacterium most active at water temperatures of about 15 degrees C or below. The infection progresses slowly and produces characteristic granulomatous lesions in internal organs, especially the kidney.

Affected species

BKD affects members of the family Salmonidae, including salmon and trout. Among Pacific salmon, pink, sockeye, and Chinook salmon are noted as particularly susceptible, while Atlantic salmon (Salmo salar) is comparatively less so. Fish that survive an outbreak commonly remain carriers, which is important for how the disease persists and spreads.

Clinical signs

• Swollen kidney, and often heart, spleen, and liver, with creamy-white to greyish granulomatous nodules
• Lethargy, darkening of the body, and anaemia
• Exophthalmia (bulging eyes)
• Abdominal distension and ascites
• Skin blebs or blisters and haemorrhages around the fins
• Pale gills

Transmission

Renibacterium salmoninarum is transmitted both horizontally and vertically. Horizontal entry occurs through the gastrointestinal tract, skin lesions, and the eyes. Critically, the bacterium can enter the egg yolk during fertilization, allowing true vertical transmission from broodstock to offspring that surface disinfection of eggs cannot eliminate. This intra-ovum transmission is the central challenge for control.

Diagnosis

Because clinical signs are variable and infections can be subclinical, laboratory confirmation is essential. Methods include bacterial culture on specialized cysteine-enriched media, ELISA, fluorescent antibody testing (IFAT), and real-time quantitative PCR. Combining multiple tests improves reliability, particularly for screening apparently healthy broodstock.

Treatment, control and prevention

There is no reliably curative treatment and no consistently effective vaccine. Antibiotics such as erythromycin can reduce infection but act slowly, require prolonged courses, and antibiotic-resistant strains have been reported. Control therefore centers on biosecurity and on preventing vertical transmission.

• Screen broodstock by ELISA and PCR and cull eggs from high-positive females to reduce intra-ovum transmission
• Injectable erythromycin given to broodstock before spawning has been used to lower vertical transmission, under veterinary direction and where legally permitted
• Source eggs and stock from tested, low-prevalence populations
• Avoid stress, overcrowding, and poor water quality that trigger clinical disease
• Disinfect equipment, separate year classes, and remove and dispose of mortalities