During 2010, 135 new cases of tuberculosis (TB) disease (2.6 cases per 100,000 population) were reported in Minnesota, which represents a decline of 16% in both the number of cases and incidence rate since 2009 and the lowest number of cases recorded statewide since 1996 (131 cases). In particular, from 2009 to 2010, the number of TB cases reported in Minnesota among U.S.-born persons decreased 22%, while cases among foreign-born persons decreased 15%. In 2010, Minnesota’s TB incidence rate was below the national rate (3.6 cases per 100,000 population) but slightly higher than the median rate among 51 U.S. states and reporting areas (2.5 cases per 100,000 population) and well above the U.S. Healthy People 2010 objective of 1.0 case per 100,000 population (Figure 6).
Despite a significant and ongoing decline in the number of TB cases reported among foreign-born persons in Minnesota in recent years, 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-one percent of TB cases reported in Minnesota during 2010 occurred among foreign-born persons. In contrast, only 61% of TB cases reported nationwide in 2010 were foreign-born. The 110 foreign-born TB cases reported in Minnesota during 2010 represented 24 different countries of birth; the most common region of birth among these patients was sub-Saharan Africa (52%), followed by South/Southeast Asia (30%), and Latin America (including the Caribbean) (12%) (Figure 7). 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.
Among foreign-born TB cases reported in Minnesota during 2010, 12% were diagnosed with TB disease less than 12 months after arriving in the United States, and an additional 9% 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 8 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 (13%) 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. Seventy-nine percent of foreign-born TB cases reported in Minnesota during 2010 were diagnosed more than 2 years after arriving in the United States. These data suggest that more than three-fourths of foreign-born TB cases reported in Minnesota may be preventable by focusing on thorough domestic screening and treatment of latent TB infection (LTBI) among recently arrived refugees, immigrants, and other foreign-born persons. Recent changes in the technical instructions for the pre-immigration medical evaluation required for immigrants and refugees, which were initiated in specific regions in 2007 and then gradually expanded worldwide, appear to have significantly reduced the number of TB cases reported in Minnesota among newly arrived immigrants and refugees. In particular, from 2008 to 2010, the percentage of foreign-born TB cases reported in Minnesota that were diagnosed with TB disease less than 12 months or 1 to 2 years after arrival in the United States declined 40% (20% to 12%) and 61% (23% to 9%), respectively.
The age distribution of TB cases reported in Minnesota differs markedly between U.S.-born and foreign-born patients, reflecting differing predominant risks of exposure to TB among these populations. The majority (66%) of foreign-born TB cases reported in 2010 were 15 to 44 years of age, whereas the majority of U.S.-born TB cases occurred among persons at younger and older ends of the age spectrum, with only 44% of U.S.-born TB cases occurring among persons 15 to 44 years of age. In contrast, 40% of U.S.-born TB cases were 45 years of age or older, while only 31% of foreign-born TB cases occurred in this age group. Among U.S.-born persons, older adults were alive when TB was much more prevalent than in recent decades and, therefore, are more likely than younger persons to have been infected with TB. The proportion of pediatric patients less than 15 years of age also was considerably larger among U.S.-born TB cases than among foreign-born cases (16% versus 3%, 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 recently arrived family members who may be at increased risk for TB acquired overseas.
The majority (84%) of TB cases reported in Minnesota during 2010 were identified as a result of presenting with symptoms for medical care. Various targeted public health interventions identified the remaining 16% of cases. Such methods of case identification traditionally are considered high priority, core TB prevention and control activities; they include TB contact investigations (1%), follow-up evaluations subsequent to abnormal findings on pre-immigration exams performed overseas (1%), and domestic refugee health examinations (1%). Notably, however, an additional 14% of TB cases were identified through a variety of other means (e.g., occupational screening) that typically are considered lower priority activities. In 2010, the percentage of TB cases identified through TB contact investigations (1%), in particular, declined markedly, from an annual average of 5% for 2004 through 2007, to 13% in 2008 (which reflects two large TB outbreaks that occurred in specific populations during that year), and 6% in 2009. Overall, the 16% of TB cases identified in 2010 through targeted screening and other similar activities represents the smallest percentage of TB cases identified through active case-finding and screening activities since 2001, which likely reflects recently enhanced medical protocols used by overseas panel physicians screening refugees and immigrants for TB prior to travel to the United States. It also suggests potential missed opportunities for public health TB control activities.
Aside from foreign-born persons, other high-risk population groups comprise much smaller proportions of the TB cases in Minnesota. Among cases reported in 2010, persons with certain medical conditions (excluding HIV infection) that increase the risk for progression from LTBI to active TB disease (e.g., 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 17% (23) of cases. Notably, these patients represent the largest annual proportion of TB cases reported with such medical conditions since at least 1993, when MDH 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 2010 having a history of substance abuse during the 12 months prior to their TB diagnoses. Six (4%) of the 135 TB cases reported in Minnesota during 2010 were infected with HIV; 4 of those HIV-infected TB cases were foreign-born, including 1 case each from Honduras, Mexico, Somalia, and Vietnam. The percentage of new TB cases with HIV co-infection in Minnesota remains less than that among TB cases reported nationwide (8.6% of those with an HIV test result). Other risk groups, such as correctional facility inmates, homeless persons, and residents of nursing homes, each represented less than 5% of TB cases reported during 2010. The percentages of TB cases that occurred among correctional facility inmates and nursing home residents both increased, from 5-year averages of 2% and 1%, respectively, for 2005 through 2009, to 3% (4 cases) and 2% (3 cases), respectively, in 2010.
Fifteen (17%) of the state’s 87 counties reported at least 1 case of TB disease in 2010. This is a marked decrease from recent years, during which the number of counties where TB was reported ranged from 23 (26%) in 2009 to 29 (33%) in 2006. This likely reflects continuing decreases in the number of TB cases reported statewide and the number of primary refugee arrivals in Minnesota. The large majority (87%) of cases occurred in the metropolitan area, particularly in Hennepin (50%) and Ramsey (25%) counties, both of which have public TB clinics.
Twelve percent of TB cases reported statewide during 2010 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 4% of cases reported in 2010. The remaining 9% of cases occurred in primarily rural areas of Greater Minnesota. The most notable changes in the geographic distribution of TB cases reported in Minnesota during 2010 were a decrease in the number of counties reporting any TB cases, an increase in the proportion of cases that occurred in Hennepin County, a decrease in the proportion of cases in Greater Minnesota (excluding Olmsted County). In particular, while the percentage of TB cases statewide that occurred in Hennepin County decreased markedly from 50% in 2005 to 38% in 2009, the county’s proportion of cases rebounded to 50% in 2010. Also, the percentage of TB cases reported in Greater Minnesota (excluding Olmsted County) decreased from a 5-year average of 15% for 2005 through 2009 to 9% in 2010. 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 2010, the highest TB incidence rate statewide was reported in Ramsey County (6.7 cases per 100,000 population), followed by Hennepin County (5.8 cases per 100,000 population) and Olmsted County (4.2 cases per 100,000 population). In 2010, the incidence rates in the five-county suburban metropolitan area (1.3 cases per 100,000 population) and Greater Minnesota (0.5 cases per 100,000 population) were considerably lower than that in the state overall (2.6 cases per 100,000 population). From 2009 to 2010, the TB incidence rate declined statewide and in each of these county-specific categories, except Hennepin County, where the TB rate increased 9%, from 5.3 cases per 100,000 population in 2009 to 5.8 cases per 100,000 population in 2010.
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, including in 2009 (the most recent year for which complete national data are available). In 2010, however, all forms of drug resistance routinely monitored through epidemiologic data collected by MDH declined markedly. Of 109 culture-confirmed TB cases with drug susceptibility results available, 12 (11%) were resistant to at least one first-line anti-TB drug (i.e., INH, rifampin, pyrazinamide, or ethambutol), including 5 (5%) cases that were resistant to INH. No cases of multidrug-resistant (MDR) TB were reported in 2010. In comparison, the prevalence of any first-line drug resistance, INH resistance, and MDR-TB among 120 culture-confirmed TB cases reported in Minnesota during 2009 were 17%, 10%, and 2%, respectively. Drug resistance is more common among foreign-born TB cases than it is among U.S.-born cases in Minnesota. Of particular concern, 2 (22%) of 9 MDR-TB cases reported from 2006 through 2009 were resistant to all four first-line drugs. These 2 cases were born in China and the United States.
Another clinical characteristic of particular significance in Minnesota is the preponderance of extrapulmonary disease among foreign-born TB patients. Over half (55%) of foreign-born TB cases reported from 2006 through 2010 had an extrapulmonary site of disease; in contrast, less than one-third (32%) of U.S.-born TB cases had extrapulmonary TB. Among extrapulmonary TB cases, by far the most common sites of TB disease were lymphatic (56%), followed by pleural (8%), bone/joint (8%), peritoneal (7%), and various other sites that each represented less than 5% of such cases. 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.
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 (averaging 3.8% per year from 2000 through 2008), with a notable exception in 2009, when an unprecedented 11.4% decrease in the national TB incidence rate was recorded. From 2009 to 2010, however, the national TB incidence rate decreased by a less substantial and more typical 3.9%. 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 (including an atypically large decrease of 23% in 2009, which paralleled the unusual and unexpected decrease in the national TB rate reported for that year). While preliminary national data for 2010 suggest that the remarkable decrease reported in the TB incidence rate in the United States (and Minnesota) for 2009 likely was an aberration, the significant and 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 new immigrants and refugees. Continued progress toward meeting the national goal of TB elimination will require numerous advances in various TB prevention and control strategies and tools, including better diagnostic tests and screening strategies to identify persons with LTBI and TB disease, shorter and more easily tolerated treatment regimens for LTBI and active TB disease, an effective vaccine, and improvements in both global TB control and TB prevention and control strategies targeted to disproportionately affected populations in the United States. Maintaining and disseminating current, comprehensive and detailed TB surveillance data, such as those presented here, will continue to be critical for identifying trends in the epidemiology of TB, which should inform and shape local TB prevention and control strategies.
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