On average, the isoniazid (INH) resistance rate is approximately 10% and the rifampin resistance rate is approximately 1%, with lower numbers in countries with good TB programs and higher numbers in the countries with poor TB programs.



Pathophysiology: Humans are the only known reservoir for Mycobacterium tuberculosis. TB is transmitted by airborne droplet nuclei, which may contain fewer than 10 bacilli. Exposure to TB occurs by sharing common airspace with a patient who is infectious. When inhaled, droplet nuclei are deposited within the terminal airspaces of the lung. Upon encountering the bacilli, macrophages ingest and transport the bacteria to regional lymph nodes.

The bacilli have 4 potential fates:

1. They may be killed by the immune system.

2. They may multiply and cause primary TB.

3. They may become dormant and remain asymptomatic, or

4. They may proliferate after a latency period (reactivation disease). Reactivation disease may

occur following either 2. or 3. above.

Frequency:

In The US: Beginning in 1985, a resurgence of TB was noted. The increase was observed primarily in ethnic minorities and especially in persons infected with HIV. TB control programs were revamped and strengthened across the United States. After peaking at 25,287 (1993), the number of reported cases began to fall again. In 2001, 15,989 cases of TB were reported to the US Centers for Disease Control and Prevention (CDC). An estimated 10-15 million people have latent infection. Among foreign immigrants, 74% of cases reported in 1998 were related to 7 countries:

1. Vietnam

2. Philippines

3. India

4. China

5. South Korea

6. Mexico

7. Haiti

Foreign born persons account for a steadily increasing proportion of all reported TB cases.

Internationally: An estimated 20-33% of the world's population is infected with M tuberculosis. Countries with the highest prevalence include Russia, India, Bangladesh, Pakistan, Pakistan, Indonesia, Philippines, Vietnam, Korea, China, Tibet, Hong Kong, Egypt, most Sub Saharan African countries, Brazil, Mexico, Bolivia, Peru, Colombia, Dominican Republic, Ecuador, Puerto Rico, El Salvador, Nicaragua, Haiti, Honduras, and areas undergoing civil war (e.g. Balkan Countries). Countries in Eastern Europe have an intermediate prevalence. Costa Rica, western and northern Europe, the United States, Canada, Israel, and most countries in the Caribbean have the lowest prevalence.

Mortality / Morbidity: The case fatality rate for TB was 50% for untreated patients before the advent of antibiotic therapy. Deaths worldwide are are estimated at 3 million per year. In United States, the mortality rate dropped from 12.4 deaths per 100,000 population (1953) to 0.6 deaths per 100,000 population (1993); this is approximately 7% per newly identified case.

Multidrug resistant during tuberculosis (MDR-TB) cases have a higher reported mortality rate. Patients with underlying diseases predisposing to active TB also have higher morality rates.

Morality of untreated congenital TB is 50%.

TB can mimic congenital syphilis or cytomegalovirus (CMV) infection.

Race: Based on 1990 CDC data, case rates were 10 times higher for Asians and Pacific Islanders; 8 times higher for non Hispanic blacks; and 5 times higher for Hispanics, Native Americans, and Native Alaskans, as compared to non Hispanic whites. However, race may not be an independent risk factor. Risk is best defined on the basis of social, economic, and medical factors.

Sex: Despite the fact that TB rates have declined in both sexes in the United States, certain differences exist. TB rates in women decline with age; in men, they increase with age. Men are more likely to have a positive tuberculin skin test. The reason for these differences may be social rather than biological in nature.

Age: In the 1997 CDC data for the United States, more than 60% of cases occurred in persons aged 25-64 years. The age specific risk was highest in persons older than 65 years. Infection in infants and young children (up to 5years) always indicates recent transmission. If left untreated, it may result in life threatening meningitis or disseminated disease, Elderly patients may not have typical signs and symptoms of infection because they may not mount a good immune response. In elderly patients, an active tuberculosis infection may present as an non resolving pneumonitis.