Infectious Diseases - Tuberculosis (tb)

Tuberculosis (TB), a communicable disease caused by the bacterium Mycobacterium tuberculosis, is spread from person to person through the inhalation of airborne particles containing M. tuberculosis. The particles, called droplet nuclei, are produced when a person with infectious TB of the lungs or larynx forcefully exhales, such as when coughing, sneezing, speaking, or singing. These infectious particles can remain suspended in the air and can be inhaled by someone sharing the same air.

Most TB (approximately 85 percent) occurs in the lungs (pulmonary TB). Risk of transmission is increased where ventilation is poor and when susceptible people share air for prolonged periods with a person who has untreated pulmonary TB. The disease, however, may occur at any site of the body, such as the larynx, the lymph nodes, the brain, the kidneys, or the bones. This type of TB infection, which occurs outside the lungs, is referred to as extrapulmonary. With the exception of laryngeal TB, people with extrapulmonary TB are usually not considered infectious to others.

TB does not develop in everyone infected with the bacteria. In the United States, about 90 percent of infected people never show symptoms of TB. Nevertheless, 5 percent of people infected develop the disease in the first or second year after infection. Another 5 percent show symptoms later in life. For people with compromised immune systems, the risk of developing TB is much higher. For example, 8 percent of those infected with both TB and HIV (the virus that causes AIDS) develop full-blown TB symptoms within a year.

Ancient Enemy and Continuing Threat

Each year, two million people worldwide die from TB, according to the WHO, and more than eight million people become sick with TB annually. Overall, one-third of the world's population is infected with the TB bacillus. This has increased dramatically since the HIV/AIDS epidemic swept through many countries. In 1992 the WHO estimated that at least four million adults worldwide—primarily in sub-Saharan Africa, Latin America, and Asia—had been infected with both AIDS and M. tuberculosis. TB accounts for 11 percent of deaths from AIDS worldwide.

After several decades of decline, TB made a comeback in the United States in the late 1980s and early 1990s. From 1985 to 1993, more than 64,000 new TB cases were reported. In 1992 the CDC reported 26,673 cases of TB, up from 22,201 in 1985. Since 1992 the number of cases has declined steadily, and by 2001 it had decreased to 15,989. (See Table 7.3.)

During 2003, a total of 14,871 TB cases were reported to the CDC, which was a 1.4 percent decrease in cases and a 1.9 percent decline in the rate from 2002. This decline is attributed to new public health programs that monitor the complicated drug-treatment protocols for patients with TB. The success of prevention and treatment programs varies depending on the location and population. Despite these overall national declines in TB incidence, substantial disparities exist between rates in the majority of U.S. residents and rates in two U.S. populations—foreign-born people and U.S.-born non-Hispanic blacks, both of which experience higher rates of TB.

Treatment has become increasingly difficult because new strains of multidrug-resistant (MDR) TB have developed. If the disease is not properly treated or if treatment is not completed, some TB can become resistant to drugs, making it much harder to cure. In 2003 MDR TB was more common in foreign-born people (1.2 percent) than in U.S.-born residents (0.6 percent). According to the CDC, in 2000, 80.8 percent of patients with TB completed therapy in one year or less, and 92.2 completed therapy overall.

The increase in TB cases in the 1980s and the emergence of antibiotic-resistant strains of TB prompted Congress to increase the NIH budget for TB research. The research budget of the National Institute of Allergy and Infectious Diseases (NIAID) soared from $3.1 million in 1991 to $35 million in 1996. In 2001 NIAID budgeted nearly $10 million for TB vaccine research and also funded basic research, epidemiology research, drug development and treatment efforts, and training to improve the number and skills of researchers and health care practitioners.

March 24 of each year has been designated "World TB Day" by NIAID to recognize the global threat to health posed by the disease. If the disease is not controlled and treatment is not improved, it is estimated that between 2002 and 2020, approximately one billion people will be newly infected with TB, more than 150 million people will get sick from TB, and thirty-six million will die of TB, according to the WHO.