Chapter 2 - The TB Epidemic
TB is a contagious bacterial disease caused by Mycobacterium tuberculosis. Like the common cold, TB is spread through the air. The main source of infection is a person with TB of the lungs (pulmonary TB) who coughs, sneezes or spits, and spreads infectious droplets containing the bacteria in the air.
TB infection, Disease, and Treatment
Once infected with M. tuberculosis, a person stays infected for many years, and often for life. The vast majority (90 percent) of people infected with M. tuberculosis do not develop the disease of tuberculosis. Active disease occurs in an average of 10 percent of those who are infected. Various physical or emotional stresses trigger progression from infection to disease. Any weakening of the immune system--for example, by malnutrition or HIV infection--increases the chances for disease to develop.
Left untreated a person with active TB will infect on average 10 to 15 persons a year. The most effective approach to TB control is the identification and cure of these infectious cases. Proper treatment of infectious cases makes them very quickly non-infectious so that they can no longer spread TB to others. Because effective treatment breaks the cycle of tarnsmission, cure is the best prevention.
This is even more important because of the emergence of multidrug-resistant TB. MDR-TB is a human-made phenomenon caused by inconsistent or partial treatment when TB bacilli become resistant to the most common anti-TB drugs. For instance, doctors and health workers may prescribe the wrong drugs or the wrong combination of drugs, the drug supply is unreliable, or patients do not take all their medicines regularly for the required period of time. Once the bacilli become resistant to one or more anti-TB drugs, the infected person can go on to infect others with the same drug-resistant strain. Drug-resistant TB is more difiicult and more expensive to treat, and more likely to be fatal.
The Global Burden of TB
About one-third of the world's population is infected by M. tuberculosis. In 1997, there were about 8 million new cases of TB and 2-3 million deaths worldwide. TB kills more youth and adults than any other single infectious agent in the world today. The developing world is the worst affected with 95 percent of all TB cases and 98 percent of TB deaths. And 75 percent of TB cases in developing countries are among those in their most economically productive years (15-45).
Today, HIV is the most powerful factor known to increase the risk of progression from TB infection to disease. Currently, 9.4 million people are dually infected with TB and HIV. These people have a 50 percent chance of developing active TB during their lifetime.
By 1997, approximately 800,000 TB cases were attributed to HIV--10 percent of the global incidence of TB. In Africa, about 30 percent of all TB cases are now due to HIV. In some of the worst affected countries in Sub-Saharan Africa, more than 60 percent of TB patients are HIV-positive. The worsening impact of HIV on the incidence of TB is threatening to overwhelm not only otherwise effective TB control programmes, but also primary health care services in general.
TB in the Western World
With the advent of drugs to cure TB, there was an annual decline in the incidence of TB infections of approximately 12 percent because of widespread case-finding and high cure rates (often obtained through use of prolonged hospitalization) in addition to socio-economic development. Political commitment and funding for TB control led to the widespread and effective application of chemotherapy, resulting in further decline in the annual incidence of TB infections.
TB in the Developing World
Many developing countries lack the systems and funding necessary to ensure the widespread effective application of anti-TB chemotherapy. The strong managerial approach of the DOTS strategy enables the effective use of available technologies (sputum smear microscopy and anti-TB drugs) for TB control. Implementation of the DOTS strategy can accelerate the decline in the annual rate of TB infections in these countries.
Developing countries such as Algeria, Chile, Cuba, and Uruguay--which since the 1970s have instituted efficient case-finding with high cure rates--have demonstrated the same effect. The consensus is that high cure rates supplemented by efficient case-finding have an impact in decreasing the transmission of TB infection and the incidence of TB disease.
