During 2011, 137 cases of tuberculosis (TB) disease (2.6 cases per 100,000 population) were reported, compared to 135 cases in 2010. Although this represents an increase of 1% in the number of cases and a 4% increase in the incidence rate compared to 2010, the number of cases reported annually has decreased 42% since 2007, when 238 cases (the highest number in the past decade) were reported. From 2010 to 2011, the number of TB cases reported among U.S.-born persons in Minnesota decreased 16%, while cases among foreign-born persons increased 5%. In 2011, Minnesota’s TB incidence rate was below the national rate (3.4 cases per 100,000 population) but slightly higher than the median rate among 51 U.S. states and reporting areas (2.4 cases per 100,000 population) and well above the U.S. Healthy People 2020 objective of 1.0 case per 100,000 population (Figure 6). Seven (5%) of the TB cases reported in Minnesota in 2011 died due to TB or TB-related causes.
The incidence of TB disease is disproportionately high in racial minorities in the United States and in Minnesota. In 2011, 13 TB cases occurred among whites (0.3/100,000 population). In contrast, 66 TB cases occurred among blacks (22.1/100,000), 40 among Asians (17.5/100,000), and 3 among American Indians (4.7/100,000). The majority (62/66, or 94%) of black TB cases reported in Minnesota in 2011 were foreign-born. There were no TB cases reported among individuals who self-reported being multi-racial.
The most distinguishing characteristic of the epidemiology of TB disease in this state continues to be the large proportion of cases that occur among persons born outside the United States. Eighty-five percent of cases reported in 2011 occurred among foreign-born persons. In contrast, 63% of TB cases reported nationwide in 2011 were foreign-born. The 116 foreign-born TB cases reported in Minnesota during 2011 represented 25 different countries of birth; the most common region of birth among these patients was sub- Saharan Africa (53%), followed by South/Southeast Asia (28%), and Latin America (including the Caribbean) (13%) (Figure 7). Among U.S.-born pediatric TB cases, 80% (8 of 10) have at least one foreign-born parent. 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.
Among foreign-born TB cases reported in Minnesota during 2011, 16% were diagnosed with TB disease less than 12 months after arriving in the United States, and an additional 13% were diagnosed 1 to 2 years after their arrival in this country. Many of these cases, particularly those diagnosed during their first year in the United States, likely represent persons who acquired TB infection prior to immigrating and began progressing to active TB disease shortly after arriving in the United States. Of 17 TB cases 15 years of age or older who were diagnosed in Minnesota within 12 months of arriving in the United States and who arrived as immigrants or refugees, only 1 had any TB-related condition noted in their pre-immigration medical examination reports. 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.
The majority (80%) of TB cases reported in Minnesota during 2011 were identified as a result of presenting with symptoms for medical care. Various targeted public health interventions identified the remaining 20% of cases. Such methods of case identification traditionally are considered high priority, core TB prevention and control activities; they include TB contact investigations (4%), follow-up evaluations subsequent to abnormal findings on pre-immigration exams performed overseas (1%), and domestic refugee health assessments (1%). Notably, however, an additional 13% of TB cases were identified through a variety of other means (e.g., occupational screening) that typically are considered lower priority activities.
Aside from foreign-born persons, individuals with other risk factors comprise much smaller proportions of the TB cases in Minnesota. Among cases reported in 2011, 20% (27) of TB cases occurred among persons with certain medical conditions (excluding HIV infection) that increase the risk for progression from latent TB infection (LTBI) to active TB disease (e.g., diabetes, prolonged corticosteroid or other immunosuppressive therapy, end stage renal disease, etc.). Notably, these patients represent the largest annual proportion of TB cases reported with such medical conditions since at least 1993, when we initiated an electronic surveillance database that included data on TB-related risk factors among reported cases. This observation of a trend toward a growing risk category among TB cases reported in Minnesota in recent years illustrates the importance of TB screening and, if indicated, treatment for LTBI among patients with underlying medical conditions that increase the risk for progression from LTBI to active TB disease. Following these underlying medical conditions, the next most common risk factor among TB cases was substance abuse (including alcohol abuse and/or illicit drug use), with 5% of TB cases reported in 2011 having a history of substance abuse during the 12 months prior to their TB diagnoses. Three (2%) of the 137 TB cases reported in Minnesota during 2011 were infected with HIV. The percentage of new TB cases with HIV co-infection in Minnesota remains less than that among TB cases reported nationwide (7.9% of those with an HIV test result). Other risk groups, such as correctional facility inmates, homeless persons, and residents of nursing homes, each represented 1% of TB cases reported during 2011.
Twenty-five (29%) of the state’s 87 counties had at least 1 case of TB disease in 2011. The large majority (74%) of cases occurred in the metropolitan area, particularly in Hennepin (43%) and Ramsey (23%) counties, both of which have public TB clinics. Nine percent of TB cases reported statewide during 2011 occurred in the five suburban metropolitan counties (i.e., Anoka, Dakota, Carver, Scott, and Washington). Olmsted County, which also maintains a public TB clinic, represented 7% of cases reported in 2011. The remaining 19% of cases occurred in primarily rural areas of Greater Minnesota. We calculate 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 2011, the highest TB incidence rate statewide was reported in Olmsted County (6.2 cases per 100,000 population), followed by Ramsey County (6.1 cases per 100,000 population) and Hennepin County (5.1 cases per 100,000 population). In 2011, the incidence rates in the five-county suburban metropolitan area (1.0 cases per 100,000 population) and Greater Minnesota (1.1 cases per 100,000 population) were considerably lower than that in the state overall (2.6 cases per 100,000 population).
The prevalence of drug-resistant TB in Minnesota, particularly resistance to isoniazid (INH) and multi-drug resistance (i.e., resistance to at least INH and rifampin), historically has exceeded comparable national figures. In 2011, of 109 culture-confirmed TB cases with drug susceptibility results available, 22 (22%) were resistant to at least one first-line anti-TB drug (i.e., INH, rifampin, pyrazinamide, or ethambutol), including 12 (12%) cases that were resistant to INH. Three (3%) cases of multidrug-resistant (MDR) TB were reported in 2011. One case of extensively drug resistant (XDR) TB occurred in Minnesota in 2006.
Another clinical characteristic of particular significance is the high proportion of extrapulmonary TB disease in Minnesota. Over half (53%) of foreign-born TB cases and 43% of U.S.-born TB cases reported in 2011 had an extrapulmonary site of disease. Among extrapulmonary TB cases, by far the most common sites of TB disease were lymphatic (51%), followed by bone/joint (15%), pleural (13%), and various other sites that each represented less than 10% of such cases.
The national goal of TB elimination by 2010, which was established in 1989 by the Advisory Council for the Elimination of Tuberculosis in partnership with the CDC, remains unmet, both nationally and in Minnesota. The incidence 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 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). From 2008 through 2010, the statewide TB incidence rate decreased an average of 17% per year. The significant and largely sustained annual decreases in Minnesota’s TB incidence rate since 2007 appear to be optimistic indicators of a real and substantial reduction in the occurrence of TB in Minnesota. This decline likely is attributable to several factors, including dramatic decreases in the number of primary refugees resettling in Minnesota in recent years (particularly a marked decline since 2006 in the number of those arriving in Minnesota from sub-Saharan Africa) and changes initiated in 2007 in the technical instructions for the overseas medical examinations required for refugees and some new immigrants.
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