Mycobacterium avium infection can result in three different kinds of illness in humans

Through work carried out in connection with her PhD, Tone Bjordal Johansen has shown that Mycobacterium avium does not infect pigs via birds, as previously believed. M. avium can be divided into several subspecies, and the research shows that birds are infected by one particular subspecies, whereas pigs and humans are infected by another. Closely related bacteria were isolated in pigs and humans, which would indicate that pigs and humans are vulnerable to infection from the same sources in their surroundings.

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.