Impact of immigration on the molecular epidemiology of Mycobacterium tuberculosis in a low-incidence country

Am J Respir Crit Care Med. 2007 Nov 1;176(9):930-5. doi: 10.1164/rccm.200702-187OC. Epub 2007 Aug 2.


Rationale: Programs to prevent the incidence rate of tuberculosis (TB) from increasing in many low-incidence countries are challenged by international travel and immigration from high-burden countries.

Objectives: The current study aimed to determine the effect of such immigration on the genetic diversity of Mycobacterium tuberculosis isolates in an entire nation's population during 1994-2005.

Methods: A total of 3,131 patients were notified with TB during the 12-year period. Of these, 2,284 (73%) had TB verified by culture, and isolates from 2,173 (96%) of these were analyzed by IS6110 restriction fragment length polymorphism.

Measurements and main results: Only 31% of the included strains were isolated from nonimmigrants, the remaining 69% were isolated from immigrants. Although the incidence increased throughout the period, the genetic diversity remained high. A total of 135 clusters were identified; the percentage of recent disease was reduced among nonimmigrants, and remained stable among the immigrants during the study period. Although 69% of the isolates originated from immigrants from high-incidence countries, the established TB control program in the receiving country was adequate for the prevention of disease transmission. On average per year, only 2 nonimmigrants and 13 immigrants developed disease as a result of infection within the country by imported M. tuberculosis.

Conclusions: Twelve years of M. tuberculosis importation as a result of immigration from high-incidence countries had little influence on the transmission of this pathogen in the receiving low-incidence country. To prevent future increase of transmission of TB, the current control strategies of low-incidence countries are adequate but must be maintained.

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Aged, 80 and over
  • Child
  • Child, Preschool
  • Cluster Analysis
  • Emigration and Immigration*
  • Humans
  • Incidence
  • Infant
  • Middle Aged
  • Mycobacterium tuberculosis / genetics*
  • Norway / epidemiology
  • Polymorphism, Restriction Fragment Length / genetics
  • Risk Factors
  • Tuberculosis / epidemiology*
  • Tuberculosis / transmission