Pulmonary and Critical Care Update Update in Tuberculosis 2007

The Centers for Disease Control and Prevention recently published the trends in tuberculosis (TB) incidence in the United States (1). In 2006, a total of 13,767 TB cases were reported in the United States, at a rate of 4.6 per 100,000, representing a 3.2% decline from 2005. The incidence of TB in 2006 was the lowest recorded since 1953, but the rate of decline has slowed since 2000. Foreign-born persons and racial/ethnic minority populations continued to be affected disproportionately. The TB rate among the former was 9.5 times that of United States– born persons. The slowing of decline in the overall national TB rate and the inability to effectively address persistent disparities in TB rates between United States–born and foreign-born persons, and between whites and racial/ethnic minority populations, may hamper progress toward the goal of TB elimination in the United States. Current guidelines only recommend targeted tuberculin testing and treatment of latent TB infection (LTBI) for foreignborn subjects who have entered the United States within the previous 5 years. Cain and coworkers assessed the epidemiologic basis for this guideline (2). Although the reported cases of TB declined among United States–born individuals by 62% in the period 1993–2004, those among foreign-born persons increased by 5%. Over half of all reported cases of TB occurred among the foreign born. Among 14,517 cases of TB reported in 2004, 3,444 (24%) of these were among foreign-born persons who had entered the United States more than 5 years previously. The rate of TB disease was 21.5 of 100,000 among foreign-born individuals overall, and 11.9 of 100,000 among those having stayed for more than 5 years, as compared with 2.7 of 100,000 for United States–born persons. Thus, almost onequarter of all TB cases occur among foreign-born persons who have resided in the United States for longer than 5 years, and the case rates for such persons from selected regions of origin remain substantially elevated. Issues with this high-risk group should be specifically addressed to enable elimination of TB in the United States. Because immigration from areas with high TB burden would theoretically threaten TB control in countries with low disease incidence, Dahle and coworkers undertook a study in Norway to evaluate the effect of immigration on the genetic diversity of Mycobacterium tuberculosis isolates on a nationwide basis over a 12-year period (1994–2005) (3). Of the 1,865 M. tuberculosis isolates with more than four copies of IS6110 in restriction fragment length polymorphism (RFLP) analysis, 385 isolates (20.6%) were assigned to 135 clusters, and the remaining 1,480 isolates (79.4%) showed unique patterns. The percentage of clustered isolates, suggesting recent transmission, was reduced among nonimmigrants and remained stable among immigrants during the study period. Thus, import of M. tuberculosis as a result of immigration from high-incidence countries might not necessarily affect the transmission dynamics of M. tuberculosis in lowincidence countries. Although 69% of the isolates originated from immigrants from high-incidence countries, and the incidence of TB was increasing among such immigrants during the study period, the established TB control program appeared adequate in preventing disease transmission. It is clearly important that these well-functioning control strategies are maintained. HIV infection has a major but unquantified impact on the risk of TB. DeRiemer and coworkers studied all patients with TB in San Francisco from 1991 to 2002 (4). The initial isolates of M. tuberculosis were genotyped using IS6110 RFLP as the primary method, resulting in delineation of clusters. Genotypic clusters with at least one HIV-positive person were larger, lasted longer, and had a shorter time between successive cases relative to clusters with only HIV-uninfected persons (all P , 0.05). Overall, 13.7% of TB cases were attributable to HIV infection, and an estimated 405 excess TB cases occurred. Thus, coinfection with HIV has amplified the local TB epidemic during this period. This relationship should lead to corresponding enhancement of control strategies in appropriate populations. In countries with low prevalence of TB, the disease can be found at high rates among certain risk groups, including illicit drug users and homeless persons. These populations would likely benefit from targeted interventions for early case detection and enhanced treatment supervision. In response to the rising notification rates of TB among illicit drug users and homeless persons in Rotterdam, The Netherlands, after the discontinuation of a chest radiograph screening program in 1996, a mobile digital X-ray screening program was reinstituted in 2002 among these at-risk subjects. de Vries and coworkers studied trends and characteristics of TB among these risk groups and assessed the impact of the screening program on disease transmission using molecular epidemiology (5). During the study period, 206 TB cases among illicit drug users and homeless persons were found, representing 11.4% of the total caseload in the city. The annual number of TB cases declined from 24 at the start of the screening program to 11 in 2005. In 1997–2002, more than 80% of the illicit drug users or homeless persons were infected with one of the M. tuberculosis strains prevalent among these risk groups. After nearly 4 years of systematic chest radiograph screening, this proportion fell to 45% in 2005. Thus, the investigators concluded that radiographic screening among these risk groups decreased incident cases and reduced disease transmission. From the results of this study, such screening should be continued to prevent resurgence of TB. Because there has not been a consensus on whether tobacco smoking increases the risk of TB infection, disease, or mortality, (Received in original form October 23, 2007; accepted in final form October 23, 2007)