Tuberculosis, 2008

While the number of cases of tuberculosis (TB) disease reported in the United States has decreased each year since 1993, the incidence of TB in Minnesota increased throughout much of the 1990s and has fluctuated during the past decade. In 2008, 211 new cases of active TB disease were reported in Minnesota. The total number of TB cases reported in Minnesota during 2008 was similar to that in 1999. Between 1999 and 2008, however, the annual number of TB cases peaked in 2001 (239 cases) and again in 2007 (238 cases). Most notably, while the number of TB cases reported among foreign-born persons in Minnesota during 2008 was essentially the same (ie, 1 fewer case) as that in 1999, the number of cases reported among U.S.-born persons increased between those years. In particular, from 2007 to 2008, the number of TB cases reported among foreign-born persons statewide decreased 24%, while the number of cases reported among U.S.-born persons increased 60% (Figure 6). The 211 cases in 2008 represent an incidence rate of 4.0 cases per 100,000 population. In 2008, Minnesota’s TB incidence rate was slightly below the national rate (4.2 cases per 100,000 population) but above both the median rate among 51 U.S. states and reporting areas (3.0 cases per 100,000 population) and the U.S. Healthy People 2010 objective of 1.0 case per 100,000 population.

Three outbreaks of TB were identified and investigated during 2008. An outbreak among a foreign-born community in rural Minnesota involved a total of 14 cases that were linked either genotypically or through strong epidemiologic links. An especially challenging characteristic of this outbreak was the very high proportion of pediatric cases, primarily U.S.-born children of foreign-born parents. Among 14 cases with genotype or epidemiologic links, 10 were children under age 14, including 8 under age 5. A second outbreak of TB involving a total of 10 cases occurred among inmates and staff at a correctional facility. The third TB outbreak consisted of a total of 6 TB cases with matching genotypes among homeless persons in the metropolitan area. Investigations remain open for the homeless and correctional facility outbreaks. None of the outbreaks involved drug resistant strains of Mycobacterium tuberculosis.

The most distinguishing characteristic of the epidemiology of TB disease continues to be the large proportion of TB cases reported among foreign-born persons. Eighty-two percent of TB cases reported in Minnesota from 2004 through 2008 occurred among persons born outside the United States. In contrast, 59% of TB cases reported nationwide in 2008 were foreign-born. Notably, however, 73% of TB cases reported in Minnesota during 2008 were foreign-born, which represented a 14% decrease from the 85% of foreign-born TB cases reported in 2007. This decrease can be attributed to the TB outbreaks that occurred during 2008, an increase of pediatric cases, and a declining number of refugees entering Minnesota.

The 155 foreign-born TB case-patients reported in Minnesota during 2008 represented 24 different countries of birth. The most common region of birth among foreign-born TB cases reported in 2008 was sub-Saharan Africa (59%), followed by South/Southeast Asia (22%) (Figure 7). 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.

One-fifth of the foreign-born TB case-patients reported in 2008 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. 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 20 TB case-patients 15 years of age or older who were diagnosed in Minnesota during 2008 within 12 months of arriving in the United States and who arrived as immigrants or refugees, only four (20%) 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. Two-thirds of foreign-born TB case-patients reported in Minnesota during 2008 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 (70%) of foreign-born TB case-patients in 2008 were 15 to 44 years of age, whereas only 29% of U.S.-born TB cases occurred among persons in this age category. In contrast, 32% of U.S.-born TB case-patients were 45 years of age or older. The proportion of pediatric patients <5 years of age was considerably larger among U.S.-born TB cases than among foreign-born cases (39% versus 6%, respectively), although nearly all of these U.S.-born case-patients 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 (77%) of TB cases during 2008 were identified as a result of presenting for medical care. Other methods of case identification included TB contact investigations (13%), domestic refugee health examinations (3%), and follow-up evaluations subsequent to abnormal findings on pre-immigration exams performed overseas (2%). The remaining 5% of TB cases were identified through a variety of other means. Notably, the percentage of TB cases identified through TB contact investigations increased from an annual average of 5% from 2004 through 2007 to 13% in 2008. Again, this is reflective of the three major TB outbreaks that occurred in 2008.

Aside from foreign-born persons, other high-risk population groups comprise much smaller proportions of the TB cases. Among cases reported in 2008, persons with certain medical conditions (excluding HIV infection) that increase the risk for progression from latent TB infection to active TB disease (eg, 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 11% of cases. Substance abuse (including alcohol abuse and/or illicit drug use) was the second most common of these other risk factors, with 10% of TB case-patients having a history of substance abuse during the 12 months prior to their TB diagnoses. Eleven (5%) of the 211 TB case-patients reported in Minnesota during 2008 were infected with HIV; nine of those HIV-infected TB case-patients were foreign-born, including three persons born in Mexico, two persons from Sudan, and one person each from Cameroon, Ethiopia, Nigeria, and South Africa. The percentage of new TB case-patients with HIV co-infection remains less than that among TB cases reported nationwide. Five percent of TB case-patients reported in Minnesota during 2008 were homeless. Other risk groups, such as correctional facility inmates and residents of nursing homes, each represented only 1-2% of TB cases reported during 2008.

Twenty-six (30%) of the state’s 87 counties reported at least one case of TB disease in 2008, with the majority (78%) of cases occurring in the metropolitan area, particularly in Hennepin (46%) and Ramsey (21%) Counties, both of which have public TB clinics. Eleven percent of TB cases occurred in the five suburban metropolitan counties (ie, 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 2% of cases reported statewide in 2008. The remaining 20% 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 2008, the highest TB incidence rate statewide was reported in Ramsey County (8.8 cases per 100,000 population), followed closely by Hennepin County (8.5 cases per 100,000 population). Both Ramsey and Hennepin counties’ TB incidence rates were more than twice the statewide rate. In 2008, the incidence rates in Olmsted County (2.8 cases per 100,000), the five-county suburban metropolitan area (2.0 cases per 100,000), and Greater Minnesota (1.9 cases per 100,000) were considerably lower than that in the state overall. From 2007 to 2008, the TB incidence rates in Hennepin County, the suburban metropolitan area, and Ramsey County decreased 15%, 9%, and 6%, respectively. In contrast, the TB incidence rate in Greater Minnesota increased 36% from 2007 to 2008. Most notably, from 2007 to 2008, Olmsted County’s TB incidence rate declined 80%.
 
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 for 2007 (the most recent year for which complete national data are available). In 2008, 23 (15%) of 149 culture-confirmed TB cases reported in Minnesota were resistant to at least one first-line anti-TB drug (ie, isoniazid [INH], rifampin, pyrazinamide, or ethambutol). In particular, 17 (11%) cases were resistant to INH, and 2 (1%) cases were multidrug-resistant (ie, resistant to at least INH and rifampin). The prevalence of MDR-TB in Minnesota has declined during the past 5 years, from 4% in 2004 to 1% in 2008. Drug resistance is more common among foreign-born TB cases than it is among U.S.-born cases in Minnesota. Of particular concern, two (13%) of 16 MDR-TB cases reported from 2004 through 2008 were resistant to all four first-line drugs. These two pan-resistant MDR-TB case-patients represented two different countries of birth (China and Somalia).

Another clinical characteristic of significance is the preponderance of extrapulmonary disease among foreign-born TB patients. Just over half (53%) of foreign-born TB case-patients from 2004 through 2008 had an extrapulmonary site of disease; in contrast, only 37% of U.S.-born TB case-patients had extrapulmonary TB (Figure 8). The most common extrapulmonary sites of TB disease are lymphatic, pleural, peritoneal, and bone/joint. The unusually high incidence of extrapulmonary TB disease 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. The prevalence of drug-resistant TB in Minnesota is higher than that nationally, and extrapulmonary sites of disease are common, especially among foreign-born cases. The proportion of TB cases occurring among persons under 5 years of age in Minnesota exceeds the comparable figure nationally, with many of these children having foreign-born parents. These trends suggest that the incidence of TB in Minnesota is not likely to significantly decrease in the foreseeable future.

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Updated Wednesday, December 08, 2010 at 03:29PM