Sources of infection: Mycobacterium avium infections in pigs, humans and birds in Norway

Mycobacterium avium is the cause of tuberculosis in birds. In Norway, this disease no longer occurs in poultry, but is occasionally found in wild birds. This bacterium can also lead to infections in mammals, primarily in pigs and humans. Pigs infected with the bacterium seldom become diseased, but the infection can result in financial loss for farmers because meat from the infected animals has to be destroyed.

In humans, this bacterium can result in three different kinds of illness: pneumonia in patients already prone to lung disease, glandular fever in children or generalised infection in AIDS patients. In Norway, about 100 new cases of such infections occur each year.

The aim of the research study was to carry out a genetic comparison of bacteria isolates from birds, humans and pigs in Norway in order to chart possible paths of infection. Studies from other countries have revealed M. avium in the environment, particularly in conjunction with biofilm (bacteria coating) in water and piping systems. Johansen's study therefore examined the bacteria's ability to form a biofilm, since this is a characteristic that can contribute to the spread of infection.

The results of the study show that birds are not the source of infections in pigs caused by Mycobacterium avium since the isolates from birds were identified as M. avium subsp. avium, while the isolates from pigs, humans and the environment were identified as M. avium subsp. hominissuis.

Humans and pigs were found to be infected with related isolates of M. avium subsp. hominissuis, and related isolates were found in pigs in the same geographic area. Infection via M. avium subsp. hominissuis between pigs and humans cannot be completely ruled out, but it is more likely that the environment is the source of infection in both pigs and humans. The study revealed no connection between the ability to form a biofilm and other characteristics examined in the bacteria.

Tone Bjordal Johansen presented her doctoral thesis on 19th November 2009 at The Norwegian School of Veterinary Science. The thesis is entitled: "Characterisation of isolates of Mycobacterium avium with emphasis on IS elements and biofilm abilities."