Mycobacteriosis in fish

Mycobacteriosis in fish is a disease that is difficult to detect and therefore often underdiagnosed. For the same reason, information about the effects of this disease on the fish farming industry has been limited.

The development of two diagnostic tests has led to the discovery of a mycobacterium which causes disease in both cod and salmon and has never been detected in Norway before.

Reports submitted to the authorities of mycobacteriosis (tuberculosis) in fish have been sporadic and have only stemmed from aquarium fish or wild fish, not farmed fish. This is probably due to underdiagnosis. Adam Zerihun's doctoral research has led to the development of two methods of diagnosis based on real-time PCR and immunohistochemistry respectively. The tests are extremely sensitive and have been instrumental in detecting a mycobacterium that has never been found in Norway before. The bacterium was found both in farmed salmon and in burbot and experimental infection showed that Atlantic cod was very susceptible to the bacterium and became diseased. The isolated bacterium was identified as Mycobacterium salmoniphilum.

In cod and burbot infected with M. salmoniphilum, Zerihun discovered serious nodules, while only small or no such changes were found in infected Atlantic salmon. The formation of nodules in cod was shown to undergo several phases of development. The identification and characterization of these different stages are important for the appraisal of disease development and in order to estimate the time of infection. Zerihun's study indicates that the occurrence of the disease in farmed Atlantic salmon and in cod is more widespread than previously thought, both with regard to the range of host and climate variations.

Since the formation of nodules is not a typical symptom in salmon infected with M. salmoniphilum, it is highly probable that many cases of tuberculosis in salmon remain undiscovered. Fish that lose weight without there being any apparent disease or specific cause are classified in fish farming as "lost fish". A mycobacterial infection can be lurking behind various different symptoms, where weight loss is one characteristic.

For this reason, many salmon infected with tuberculosis can be the undetected reason for losses in the industry. The stress that arises when many fish are kept in closely packed surroundings weakens their immune system and allows mycobacteria to reproduce in many individuals. These factors are known to promote the occurrence of mycobacteriosis.

DVM Mulualem Adam Zerihun defended his doctoral research on 11th January 2013 at the Norwegian School of Veterinary Science with a thesis entitled: "Mycobacteriosis in marine and freshwater fishes: characterization of the disease and identification of the infectious agents".