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Tuberculosis breaches borders, but not public health

November 01, 2007

Immigrants from countries with high rates of tuberculosis who move to countries of low TB incidence do not pose a public health threat to native citizens, according to researchers in Norway, who analyzed the incidence and genetic origins of all known cases of TB in the country between 1993 and 2005.

Their results were reported in the first issue for November of the American Thoracic Society's American Journal of Respiratory and Critical Care Medicine.

After gathering all available cultures from the identified cases and eliminating samples suspected of being contaminated in the lab, the researchers examined 2,173 cases of TB in the country over 12 years. They tracked outbreaks among native-born citizens and immigrants, and analyzed the genetic strain of each confirmed case using cultures obtained through patient samples at 14 laboratories that service the entire country.

They found little evidence to support the belief that immigrants from countries with high TB incidence present a public health threat to non-immigrant natives in low-incidence countries. Instead, the researchers documented an increase in number of strains in immigrants, but a decrease in the number and incidence of native infections, suggesting that while immigrants from high-TB regions do bring with them more strains of TB, they do not significantly contribute to the spread of TB within native-born populations of low-incidence countries.

"Immigrants have been accused of spreading TB. However, the current study demonstrated that the importation of M. tuberculosis, over 12 years, did not generate significant negative effects on the transmission of TB in a country that was considered to be in the elimination phase of this disease," wrote lead researcher, Ulf R. Dahle, Ph.D., of the Norwegian Institute of Public Health.

This finding is especially relevant in the current climate of increased public anxiety in the wake of Andrew Speaker, the American who traveled extensively while infected with multidrug-resistant TB disease, and Amado Isidro Armendariz Amaya, a Mexican businessman who is reported to have crossed the U.S. border 76 times, carrying with him construction materials and multidrug-resistant TB.

"Anyone involved in TB management or control needs no reminder of the key role played by human movement-across oceans, within rapidly industrializing countries, from war zones to refugee camps," wrote Kevin Schwartzman, M.D., M.P.H., of McGill University, a researcher unaffiliated with the study, in an accompanying editorial in the same issue of the journal.

The investigators found that the genetic diversity of the TB strains identified remained high throughout the 12 years at about 87 percent, indicating very limited transmission to both immigrants and non-immigrants within the country. "Had there been more extensive transmission on Norwegian soil, we would have seen a greater degree of similarity between infecting strains. The lack of similarity suggests that most patients acquired TB infection abroad," said Dr. Dahle.

Furthermore, they were unable to attribute any outbreak to the infamous Beijing family of TB, despite its presence in the country. Among all the imported strains during the 12 years, only 10 led to more than five cases of active TB within five years. "This indicates that the importation of M. tuberculosis isolates did not represent an immediate challenge to the national TB control program," wrote Dr. Dahle.

"The low number of clustered strains could not support the statement that public health in this recipient country was hampered by immigration from high-incidence countries," Dr. Dahle concluded. "It appeared that the TB control efforts were not overwhelmed by the challenge of imported TB- If these control strategies are well maintained, elimination of indigenous transmission of TB could be achievable, despite extensive import from high-incidence countries."

Ultimately, the strategies used to control TB are more important to public health than immigration. "The take-home message is not one of blame or stigmatization-quite the opposite," said Dr. Schwartzman. "By ensuring access to TB care and public health programs for all, Norwegian authorities are controlling TB and preventing transmission."

American Thoracic Society