Tuberculosis, 2006

While the number of cases of tuberculosis (TB) disease reported nationally has decreased each year since 1993, the incidence of TB in Minnesota increased throughout much of the 1990s and peaked at 239 TB cases (4.8 cases per 100,000) in 2001. After 3 consecutive years (2002-2004) of decreasing incidence followed by a plateau at 199 new cases in 2005, the number of newly reported TB cases in Minnesota increased 9% to 217 cases in 2006. The increase in cases observed in 2006 was due primarily to a 62% increase in the number of U.S.-born cases, whereas the number of foreign-born cases counted in 2006 was essentially the same as the prior year. The 217 cases counted in 2006 represent an incidence rate of 4.2 cases per 100,000. Although the statewide incidence of TB disease is slightly less than the national rate (4.6 cases per 100,000 in 2006), the incidence rate in Minnesota exceeds the U.S. Healthy People 2010 objective of 1.0 cases per 100,000 (Figure 5).

The most distinguishing characteristic of the epidemiology of TB disease in Minnesota continues to be the very large proportion of TB cases reported among foreign-born persons. Although the percentage of foreign-born cases declined from 87% in 2005 to 81% in 2006, the data for 2006 are consistent with the average percentage of foreign-born cases (81%) reported from 2002 through 2006. In contrast, 57% of TB cases reported nationwide in 2006 were foreign-born.

The 175 foreign-born TB case-patients reported in Minnesota during 2006 represent 31 different countries of birth. The most common region of birth among foreign-born TB cases reported in 2006 was sub-Saharan Africa (59%), followed by South/Southeast Asia (22%) (Figure 6). The ethnic diversity among these foreign-born TB cases reflects the unique and constantly changing demographics of immigrant and other foreign-born populations arriving in Minnesota. This diversity also poses significant challenges in providing culturally and linguistically appropriate TB prevention and control services for populations most affected by and at risk for TB in Minnesota.

More than one-third (34%) of the 175 foreign-born TB case-patients reported in Minnesota during 2006 were diagnosed within 12 months after arriving in the United States. These cases likely represent persons who acquired latent TB infection outside the United States and began progressing to active TB disease prior to immigrating. Persons 15 years of age or older who arrive in the United States as immigrants or refugees receive a pre-immigration medical examination overseas that includes screening for pulmonary TB disease. Of 48 TB case-patients 15 years of age or older who were diagnosed in Minnesota during 2006 within 12 months of arriving in the United States and who arrived as immigrants or refugees, only nine (19%) had any TB-related condition noted in their pre-immigration medical exam results. These findings highlight the need for clinicians to have a high index of suspicion for TB among newly arrived foreign-born persons, regardless of the results of medical exams performed overseas. More than half (52%) of foreign-born TB case-patients reported in Minnesota during 2006 were diagnosed 2 or more years after arriving in the United States. These data suggest that at least half of foreign-born TB cases reported in Minnesota may be preventable by focusing on thorough domestic screening, evaluation, and treatment of latent TB infection among recently arrived refugees, immigrants, and other foreign-born persons.

The majority (65%) of foreign-born TB case-patients reported in Minnesota in 2006 were 15 to 44 years of age, whereas only 36% of U.S.-born TB cases occurred among persons in this age category. In contrast, 43% of U.S.-born TB case-patients were 45 years of age or older. The proportion of pediatric patients less than 5 years of age was considerably larger among U.S.-born TB cases than among foreign-born cases (12% versus 1%, respectively), although nearly all of these U.S.-born case-patients were children born in the U.S. to foreign-born parents. These first-generation U.S.-born children appear to experience an increased risk of TB disease that more closely resembles that of foreign-born persons. Presumably, these children may be exposed to TB as a result of travel to their parents’ country of origin and/or visiting or recently arrived family members who may be at increased risk for TB acquired overseas.

Aside from foreign-born persons, other high-risk population groups comprise much smaller proportions of the TB cases reported in Minnesota. Among TB cases reported in 2006, persons with certain medical conditions (excluding HIV infection) that increase the risk for progression from latent TB infection to active TB disease (e.g., silicosis, diabetes, prolonged corticosteroid therapy or other immunosuppressive therapy, end stage renal disease, etc.) were the most common of these other high-risk population groups, representing 15% of TB cases. Substance abuse (including alcohol abuse and/or illicit drug use) was the second most common of these other risk factors, with approximately 6% of TB case-patients having a history of substance abuse during the 12 months prior to their TB diagnosis. Eight (4%) of the 217 TB case-patients reported in Minnesota during 2006 were infected with HIV; six (75%) of those HIV-infected TB case-patients were foreign born, including two persons born in Kenya and one person each from El Salvador, Ethiopia, Mexico, and Somalia. The percentage of TB case-patients in Minnesota with HIV co-infection remains less than that among all TB cases reported nationwide. Other risk groups such as homeless persons, correctional facility inmates, and residents of nursing homes each represented only 1-2% of TB cases reported in Minnesota during 2006.

Twenty-nine (33%) of the state’s 87 counties reported at least one case of TB disease in 2006, with the majority (76%) of cases occurring in the metropolitan area, particularly in Hennepin (44%) and Ramsey (20%) Counties, both of which have public TB clinics. Twelve percent of TB cases occurred in the five suburban Twin Cities metropolitan counties (i.e., Anoka, Dakota, Carver, Scott, and Washington). Olmsted County, which maintains a public TB clinic staffed jointly by the Olmsted County Health Department and Mayo Clinic, represented 7% of TB cases reported statewide in 2006. The remaining 17% of cases occurred primarily in rural areas of Greater Minnesota. MDH calculates county-specific annual TB incidence rates for Hennepin, Ramsey, and Olmsted Counties, as well as for the five-county suburban metropolitan area and collectively for the remaining 79 counties in Greater Minnesota. In 2006, the highest TB incidence rate (10.8 cases per 100,000) was reported in Olmsted County, followed by Ramsey and Hennepin Counties, respectively. The TB incidence rate in Ramsey County increased from 7.0 cases per 100,000 in 2005 to 8.5 cases per 100,000 in 2006, exceeding the steadily declining incidence rate in Hennepin County (8.2 cases per 100,000 in 2006) for the first time since Minnesota began reporting county-specific TB incidence rates in 1992.

Drug-resistant TB is a critical concern in the prevention and control of TB in Minnesota, as well as nationally and globally. The prevalence of drug-resistant TB in Minnesota, particularly resistance to isoniazid (INH) and multi-drug resistance, exceeds comparable national figures. In 2006, 25 (14%) of 177 culture-confirmed TB cases were resistant to at least one first-line anti-TB drug (i.e., INH, rifampin, pyrazinamide, or ethambutol). In particular, 18 (10%) cases were resistant to INH, and two (1%) cases were multidrug-resistant (i.e., resistant to at least INH and rifampin). One of the multidrug-resistant TB (MDR-TB) cases also met the definition of extensively drug-resistant TB (XDR-TB); this is the only case of XDR-TB reported to date in Minnesota since MDH began maintaining data on drug susceptibility results for isolates of Mycobacterium tuberculosis in 1993. These data represent a decrease in the prevalence of MDR-TB during 2006, which averaged approximately 3% of cases annually from 2002 through 2005. Drug resistance is approximately twice as common among foreign-born TB cases as it is among U.S.-born cases in Minnesota. Of particular concern, 10 (45%) of 22 MDR-TB cases reported from 2002 through 2006 were resistant to all four first-line drugs. These 10 pan-resistant MDR-TB case-patients represented eight different countries of birth (i.e., one each from China, Ethiopia, Laos, Moldova, South Korea, and Thailand, and two each from Somalia and the United States). One of the two U.S.-born pan-resistant patients had resided in Africa for several years; the other was a young child infected by a foreign-born family member.

Another clinical characteristic of significance among TB cases in Minnesota is the preponderance of extrapulmonary disease among foreign-born TB patients. Half (50%) of foreign-born TB case-patients counted from 2002 through 2006 had an extrapulmonary site of disease; in contrast, only 35% of U.S.-born TB case-patients had extrapulmonary TB (Figure 7). The unusually high incidence of extrapulmonary TB disease in Minnesota exemplifies the need for clinicians to be aware of the epidemiology of TB in Minnesota and to have a high index of suspicion for TB, particularly among foreign-born patients and even when the patient does not present with a cough or other common symptoms of pulmonary TB.

The epidemiology of TB in Minnesota highlights the need to support global TB elimination strategies, as well as local TB prevention and control activities targeted to foreign-born persons. TB in Minnesota occurs primarily, although not exclusively, among foreign-born persons, with TB case-patients representing many countries of origin and varied cultural backgrounds. Although the incidence of TB in Minnesota is less than the national rate, the prevalence of drug-resistant TB in Minnesota is high, and extrapulmonary sites of disease are common, especially among foreign-born cases. The proportion of TB cases occurring in persons under 5 years of age in Minnesota exceeds the comparable figure nationally, with many of these children being born to foreign-born parents. These trends suggest that the incidence of TB in Minnesota is not likely to decrease in the foreseeable future.

• For up to date information see>> Tuberculosis (TB)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2006