In both the United States, and Minnesota in particular, the incidence of tuberculosis (TB) disease declined dramatically in 2009. The number of new cases of TB disease reported annually in the United States has decreased each year since 1993, albeit at a decelerating rate of decline in recent years. In 2009, however, the number of TB cases reported nationally (11,450) decreased by 11.4% from the number reported in 2008 (12,905). This was the largest single-year decrease recorded since national TB surveillance began in 1953. In Minnesota, the incidence of TB disease increased throughout much of the 1990s and fluctuated during the past decade, with peaks in 2001 (239 cases) and 2007 (238 cases). In 2009, 161 new cases of TB disease (3.1 per 100,000) were reported in Minnesota. This represents declines of 24% in the number (211 cases) and 23% in the rate (4.0 per 100,000) of TB disease reported statewide in 2008. In particular, from 2008 to 2009 in Minnesota, the number of TB cases reported among U.S.-born persons decreased 43%, while that among foreign-born persons decreased 23% In 2009, Minnesota’s TB incidence rate was below the national rate (3.8 per 100,000) but above both the median rate among 51 U.S. states and reporting areas (2.7 per 100,000) (Figure 7) and the U.S. Healthy People 2010 objective of 1.0 case per 100,000 population.
Reasons for the dramatic and unpredicted decrease in the incidence of TB disease nationwide during 2009 are unclear and are being investigated by CDC. While the decline may represent an actual reduction in the rate of disease due to improved TB control efforts or demographic changes among high-risk populations, other causes (e.g., under-diagnosis or reporting artifacts related to changes in the national TB case definition, TB case report form, and reporting software systems that occurred during 2009) also may have influenced the reported figures. The 24% decrease in the number of TB cases reported in Minnesota in 2009 follows an 11% decrease in 2008 cases (211) compared to 2007 (238). This likely is due to dramatic decreases in the number of primary refugees resettling in Minnesota in recent years. Notably, as the number of new refugees and immigrants arriving in Minnesota from sub-Saharan Africa has declined markedly since 2006, the percentage of foreign-born TB cases statewide who originate from that region also has decreased, from 66% in 2007 to 55% in 2009 (Figure 8).
The most distinguishing characteristic of the epidemiology of TB disease in Minnesota continues to be the large proportion of TB cases reported among foreign-born persons. Eighty percent of TB cases reported in Minnesota during 2009 occurred among persons born outside the United States. In contrast, only 60% of TB cases reported nationwide in 2009 were foreign-born. The 129 foreign-born TB cases reported in Minnesota during 2009 represented 30 different countries of birth; the most common region of birth among these patients was sub-Saharan Africa (55%), followed by South/Southeast Asia (26%). The ethnic diversity among foreign-born TB cases in Minnesota reflects the unique and constantly changing demographics of immigrant and other foreign-born populations arriving statewide. 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.
Twelve percent of the foreign-born TB cases reported in Minnesota in 2009 were diagnosed within 12 months after arriving in the United States. These cases likely represent persons who acquired TB infection outside the United States and began progressing to active TB disease prior to immigrating. Of 12 TB cases 15 years of age or older who were diagnosed during 2009 within 12 months of arriving in the United States and who arrived as immigrants or refugees, only 4 (33%) 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. Three-fourths of foreign-born TB cases reported in Minnesota during 2009 were diagnosed 2 or more years after arriving in the United States. These data suggest that more than 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 cases in 2009 were 15 to 44 years of age, whereas only 34% of U.S.-born TB cases occurred among persons in this age category. In contrast, 34% of U.S.-born TB cases were 45 years of age or older, while only 28% of foreign-born TB cases occurred in this age group. The proportion of pediatric patients <15 years of age was considerably larger among U.S.-born TB cases than among foreign-born cases (34% versus 7%, respectively), although most of these U.S.-born cases were children born in the United States 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.
The majority (81%) of TB cases reported during 2009 were identified as a result of presenting for medical care. Targeted public health interventions identified an additional 9% of TB cases in 2009. These methods of case identification included TB contact investigations (6%), follow-up evaluations subsequent to abnormal findings on pre-immigration exams performed overseas (2%), and domestic refugee health examinations (1%). The remaining 10% of new TB cases were identified through a variety of other means. In 2009, the percentage of TB cases identified through TB contact investigations returned to its usual level, after having increased from an annual average of 5% from 2004 through 2007 to 13% in 2008 due to three large TB outbreaks that occurred in specific populations during 2008.
Aside from foreign-born persons, other high-risk population groups comprise much smaller proportions of the TB cases in Minnesota. Among cases reported in 2009, 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 14% of cases. Substance abuse (including alcohol abuse and/or illicit drug use) was the second most common of these other risk factors, with 6% of TB cases having a history of substance abuse during the 12 months prior to their TB diagnoses. Seven (4%) of the 161 TB cases reported in Minnesota during 2009 were infected with HIV; 6 (86%) of those HIV-infected TB cases were foreign-born, including one person each from Burma, Ethiopia, Liberia, Mexico, Tanzania, and Zambia. The percentage of new TB cases with HIV co-infection in Minnesota remains less than that among TB cases reported nationwide (10.2% in 2009). Other risk groups, such as homeless persons, correctional facility inmates, and residents of nursing homes, each represented only 1-2% of TB cases reported during 2009. Notably, after having increased to 5% in 2008 during a TB outbreak among homeless persons in the Twin Cities metropolitan area, the percentage of homeless TB cases declined to 2% in 2009, which was comparable to prior years during the past decade.
Twenty-one (24%) of the state’s 87 counties reported at least 1 case of TB disease in 2009, with the large majority (84%) of cases occurring in the metropolitan area, particularly in Hennepin (38%) and Ramsey (25%) counties, both of which have public TB clinics. From 2005 to 2009, however, the percentage of TB cases statewide that occurred in Hennepin County decreased from 50% to 38%, whereas the percentage reported in Ramsey County increased from 18% to 25%. Sixteen percent of TB cases reported statewide during 2009 occurred in the five suburban metropolitan counties (i.e., Anoka, Dakota, Carver, Scott, and Washington). Olmsted County represented 5% of cases reported statewide in 2009. The remaining 16% of cases occurred in primarily 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 2009, the highest TB incidence rate statewide was reported in Ramsey County (8.1 cases per 100,000 population), followed by Olmsted County (5.6 cases per 100,000 population) and Hennepin County (5.3 cases per 100,000 population). In 2009, the incidence rates in the five-county suburban metropolitan area (2.2 cases per 100,000), and Greater Minnesota (1.1 cases per 100,000) were considerably lower than that in the state overall. From 2008 to 2009, the TB incidence rates in Greater Minnesota, Hennepin County, and Ramsey County decreased 42%, 38%, and 8%, respectively. In contrast, the TB incidence rate in suburban metropolitan area increased 10% from 2008 to 2009.
Although identification of Mycobacterium tuberculosis in a clinical specimen remains the gold standard for the diagnosis of TB disease, the national TB case surveillance definition includes individuals with TB risk factors and signs or symptoms consistent with active TB whose acid-fast bacilli cultures are negative or were not obtained, but who improved clinically or radiologically while on TB therapy. The number of TB cases reported in Minnesota in 2009 included 40 (25%) such cases.
The prevalence of drug-resistant TB in Minnesota, particularly resistance to isoniazid (INH) and multi-drug resistance, exceeds comparable national figures for 2008 (the most recent year for which complete national data are available). In 2009, 20 (17%) of 120 culture-confirmed TB cases with drug susceptibility results available were resistant to at least one first-line anti-TB drug (i.e., isoniazid [INH], rifampin, pyrazinamide, or ethambutol). In particular, 12 (10%) cases were resistant to INH, and 2 (2%) cases were multidrug-resistant (i.e., resistant to at least INH and rifampin). Drug resistance is more common among foreign-born TB cases than it is among U.S.-born cases in Minnesota. Of particular concern, 3 (23%) of 13 MDR-TB cases reported from 2005 through 2009 were resistant to all four first-line drugs. These 3 pan-resistant MDR-TB cases represented three different countries of birth (China, Somalia, and the United States).
Another clinical characteristic of particular significance in Minnesota is the preponderance of extrapulmonary disease among foreign-born TB patients. Just over half (54%) of foreign-born TB cases reported from 2005 through 2009 had an extrapulmonary site of disease; in contrast, only approximately one-third (34%) of U.S.-born TB cases had extrapulmonary TB. The most common extrapulmonary sites of TB disease were lymphatic, bone/joint, peritoneal, and pleural. The unusually high incidence of extrapulmonary TB disease in Minnesota emphasizes the need for clinicians to be aware of the local epidemiology of TB 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.
- For up to date information see>> Tuberculosis (TB)
- Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2009