While the prevalence of tuberculosis in recent years has declined globally, it has increased in Norway. The number of new cases of tuberculosis reported to the National Surveillance System for Communicable Diseases (MSIS) has increased from 201 in 1996 to 401 in 2013 (1). This notwithstanding, Norway still has one of the lowest prevalence figures for tuberculosis in the world, especially among the Norwegian-born.
It is assumed that approximately one-third of the world’s population are infected by Mycobacterium tuberculosis, but without any illness or ability to infect others (2). Of those who are infected, only a small minority, perhaps only 5 – 10 per cent, will develop active tuberculosis at some stage in life, and in approximately one-half of these it happens within the first two years after the initial infection (3, 4). In Norway, the IGRA test (Interferon-Gamma Release Assay) is used as an indicator of infection with tuberculosis, with or without a prior Mantoux skin test (5).
The risk of infected people falling ill with tuberculosis can be reduced through preventive treatment (3). A positive IGRA test provides no information on the time of infection, the test may remain positive even after treatment, and people with immune deficiency can obtain a false-negative result. Those who are found to be IGRA-positive by contact tracing are nevertheless considered newly infected in practice and are therefore commonly offered preventive treatment (5).
Of the 401 cases of tuberculosis that were reported in Norway in 2013, a total of 318 (79 %) were confirmed by culturing (1). Each strain of cultured mycobacteria is sent to the reference laboratory at the Norwegian Institute of Public Health, where it is screened for resistance and examined genetically. Approximately three-fourths of the patients had unique strains that had not been detected in Norway previously. The explanation is that most of those who fall ill with tuberculosis in this country have not contracted the infection here, but in a high-endemic country of origin (1, 6).
Tuberculosis is transmitted by droplet nuclei. In practice, only culture-positive, untreated pulmonary tuberculosis is infectious (7). For each case of pulmonary tuberculosis, the district medical officer shall consider initiation of contact tracing. The appropriate scope will depend on the contagiousness and contact network of the index patient and the vulnerability of the contacts (7).
Infectiousness is assessed through direct microscopy of respiratory secretions. If acid-fast bacilli (microscopy-positive) are detected, the patient is considered «definitely contagious», in case of culture-positive, microscopy-negative tests, the patient is «low-level contagious». A rough rule of thumb says that those who have been «within speaking distance» indoors (at a distance where they can converse comfortably) for more than eight hours in the company of someone defined as «definitely contagious» or for more than 40 hours with someone who is «low-level contagious» are to be considered as contacts exposed to infection (5, 7).
In April 2013, a case of tuberculosis was reported in a student at an educational institution in Eastern Norway. This education involves close physical contact and a large amount of physical activity over time. In the following months, eight of this student’s contacts, whereof six were students at the same educational institution, fell ill with tuberculosis. After subsequent analyses of bacterial DNA, another 13 tuberculosis patients were linked to the same outbreak, which consisted of 22 cases as of May 2014 (1).
The objective of this article is to describe this unusual outbreak of tuberculosis in Norway and how it was investigated, and to discuss various control measures that might have prevented it from assuming such proportions.
We wish to thank Ingunn Haakerud, Henriette Egebakken, Janne Oseberg Rønning, Camilla Rytterager Ingvaldsen, Hege S. Bjelkarøy and Irene Teslo for their contributions to the discussions, collection of data and analysis of samples at the initial stage, and Brita Winje, Bernardo Guzman, Einar Heldal and Karin Rønning for reviewing the manuscript and providing important comments.