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


OBJECTIVE
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.


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.
Division of Infectious Disease Control, Norwegian Institute of Public Health, Oslo, Norway. ulf.dahle@fhi.no
Ulf R
UR
Dahle
Vegard
V
Eldholm
Brita A
BA
Winje
Turid
T
Mannsåker
Einar
E
Heldal
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D016000:Cluster Analysis; D004641:Emigration and Immigration; D006801:Humans; D015994:Incidence; D007223:Infant; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000235:genetics*; D009664:Norway / Q000453:epidemiology; D012150:Polymorphism, Restriction Fragment Length / Q000235:genetics; D012307:Risk Factors; D014376:Tuberculosis / Q000453:epidemiology* / Q000635:transmission