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Impact of Immigration on the Molecular Epidemiology of Mycobacterium tuberculosis in a Low-Incidence Country

¹ Norwegian Institute of Public Health, Oslo, Norway

Correspondence and requests for reprints should be addressed to Ulf R. Dahle, Ph.D., Division of Infectious Disease Control, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, N-0403 Oslo, Norway. E-mail: ulf.dahle{at}fhi.no

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.

Key Words: tuberculosis • epidemiology • fingerprinting • immigrants

AT A GLANCE COMMENTARY Scientific Knowledge on the Subject It is commonly expected that immigrants with tuberculosis (TB), imported to low-incidence countries, present a challenge to control programs of the receiving countries. What This Study Adds to the Field Immigration did not appear to result in increased transmission of TB. The TB situation among resident nonimmigrants and immigrants was not significantly affected by new arrivals. Importation of TB may not affect public health in the population of the recipient country.

 

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