Tuberculosis (TB), a chronic bacterial infection, causes more deaths worldwide than any
other infectious disease. TB is spread through the air and usually infects the lungs,
although other organs are sometimes involved. Some 2 billion people one-third of
the world's population are infected with the TB organism, Mycobacterium
Most people infected with M. tuberculosis never develop active TB. However, in
people with weakened immune systems, especially those infected with the human
immunodeficiency virus (HIV, the cause of AIDS), TB organisms may overcome the body's
defenses, multiply, and cause active disease. Each year, 8 million people worldwide
develop active TB and 3 million die.
TB Has Re-Emerged in the United States
In the United States, TB has re-emerged as a serious public health problem. In 1998, a
total of 18,371 active TB cases, in all 50 states and the District of Columbia, were
reported to the Centers for Disease Control and Prevention (CDC). Thanks largely to
improved public health control measures, this represents a 7 percent decrease in cases
from 1997 (19,851). In addition to those with active TB, however, an estimated 10 to 15
million people in the United States have latent TB infections and about 10 percent of
these will develop active TB at some time in their lives.
Minorities are affected disproportionately by TB: 54 percent of active TB cases in 1995
were among African-American and Hispanic people, with an additional 17.5 percent found in
Asians. In some sectors of U.S. society, TB rates now surpass those in the world's poorest
countries. Among African-American men in New York City aged 35 to 44, for example, 315 out
of 100,000 had active TB in 1993, many times the national average of 9.8 cases per 100,000
Drug Resistance a Concern
With appropriate antibiotic therapy, TB usually can be cured. In recent years, however,
drug-resistant cases of TB have increased dramatically.
Drug resistance results when patients fail to take their medicine consistently for the
six to 12 months necessary to destroy all vestiges of M. tuberculosis. In some U.S.
cities, more than 50 percent of patients often homeless people, drug addicts, and
others caught in poverty fail to complete their prescribed course of TB therapy.
One reason for this lack of compliance is that TB patients may feel better after only two
to four weeks of treatment and stop taking their TB drugs, some of which can have
unpleasant side effects.
Resistance also may develop when patients are treated with too few drugs or with
Particularly alarming is the increase in the number of people with multidrug-resistant
TB (MDR-TB), caused by M. tuberculosis strains resistant to two or more drugs. Even
with treatment, the death rate for MDR-TB patients is 40 to 60 percent, the same as for TB
patients who receive no treatment. For people coinfected with HIV and MDR-TB, the death
rate may be as high as 80 percent. The time from diagnosis to death for some patients with
MDR-TB and HIV may be only months as they are sometimes left with no treatment options.
From 1993 to 1997, 43 states and the District of Columbia reported cases of
multidrug-resistant TB. In addition, CDC received numerous reports of outbreaks of MDR-TB
in hospitals and prisons. During these outbreaks, MDR-TB has sometimes spread to hospital
patients, health care workers, prisoners, and prison guards.
What Caused TB's Resurgence?
During the 19th century, TB claimed more lives in the United States than any other
disease. Improvements in nutrition, housing, sanitation, and medical care in the first
half of the 20th century dramatically reduced the number of cases and deaths. TB's decline
hastened in the 1940s and 1950s with the introduction of the first effective antibiotic
therapies for TB. By 1985, the number of cases had fallen to 22,201 in the United States.
In 1985, however, the decline ended and the number of active TB cases in the United
States began to rise again. Several forces, often interrelated, were behind TB's
- The HIV/AIDS epidemic. People with HIV are particularly vulnerable to reactivation of
latent TB infections, as well as to disease caused by new TB infections. TB transmission
occurs most frequently in crowded environments such as hospitals, prisons, and shelters
where HIV-infected individuals make up a growing proportion of the population.
- Increased numbers of immigrants from countries with many cases of TB, many of whom live
in crowded housing. Because of language and economic difficulties, many immigrants have
limited access to health care and may not receive treatment. TB cases among immigrants
increased from 4,925 in 1986 to 7,640 in 1998, accounting for 42 percent of the national
- Increased poverty, injection drug use, and homelessness. TB transmission is rampant in
crowded shelters and prisons where people weakened by poor nutrition, drug addiction, and
alcoholism are exposed to M. tuberculosis. People in poor health, especially those
infected with HIV, also are prone to reactivation of latent TB infections.
- Poor compliance with treatment regimens, especially among disadvantaged groups. Some of
these people may remain contagious while others develop and pass on resistant strains of M.
tuberculosis that are difficult to treat.
- Increased numbers of residents in long-term care facilities such as nursing homes.
Immune function declines with age, and as patients live longer, many suffer recurrences of
latent infections often acquired in early adulthood. Other elderly people, especially
those with weak immune systems, become newly infected with TB.
The TB Organism
TB is caused by repeated exposure to airborne droplets contaminated with M.
tuberculosis, a rod-shaped bacterium. The TB bacterium also is known as the tubercle
bacillus. (A small fraction of cases are caused by related bacteria, M. africanum
and M. bovis.)
M. tuberculosis, like other mycobacteria, has an unusual cell wall, a waxy coat
comprised of fatty molecules whose structure and function are not well known. This cell
wall appears to allow M. tuberculosis to survive in its preferred environment:
inside immune cells called macrophages, which ordinarily degrade pathogens with enzymes.
The coat of M. tuberculosis also renders it impermeable to many common drugs.
Biologists call M. tuberculosis and other mycobacteria "acid fast"
bacteria because their fatty cell walls prevent the cells from being decolorized by acid
solutions after staining during diagnostic tests.
Several factors make M. tuberculosis a difficult organism to study in the
laboratory, hampering TB research. The bacteria multiply very slowly, only once every 24
hours, and take a month to form a colony. By comparison, other bacteria such as E. coli
form colonies within eight hours. TB bacilli tend to form clumps, which makes working with
them and counting them difficult. Most daunting, M. tuberculosis, a dangerous,
airborne organism, can be studied only in laboratories that have specialized safety
TB is primarily an airborne disease. The disease is not likely to be transmitted
through personal items belonging to those with TB, such as clothing, bedding, or other
items they have touched. Adequate ventilation is the most important measure to prevent the
transmission of TB.
Because most infected people expel relatively few bacilli, transmission of TB usually
occurs only after prolonged exposure to someone with active TB. On average, people have a
50 percent chance of becoming infected with TB if they spend eight hours a day for six
months or 24 hours a day for two months working or living with someone with active TB,
researchers have estimated.
People are most likely to be contagious when their sputum contains bacilli, when they
cough frequently and when the extent of their lung disease, as revealed by a chest x-ray,
is great. TB is spread from person to person in microscopic droplets droplet nuclei
expelled from the lungs when a TB sufferer coughs, sneezes, speaks, sings, or
laughs. Only people with active disease are contagious.
Droplet nuclei are tiny and may remain in the air for prolonged periods, ready to be
inhaled. They are small enough to bypass the natural defenses of upper respiratory
passages, such as hairs in the nose or the hairlike cilia in the bronchial tubes.
Infection begins when the bacilli reach the tiny air sacs of the lungs known as alveoli,
where they multiply within macrophages.
People who have been treated with appropriate drugs for at least two weeks usually are
The site of initial infection is usually the alveoli the balloonlike sacs at the
ends of the small air passages in the lungs known as bronchioles. In the alveoli, white
blood cells called macrophages ingest the inhaled M. tuberculosis bacilli.
Some of the bacilli may be killed immediately; others may multiply within the
macrophages. Infrequently, but especially in HIV-infected people and in children, the
bacilli spread to other sites in the body. This dissemination sometimes results in
life-threatening meningitis and other problems.
During the two to eight weeks after initial infection in people with intact immune
systems, macrophages present pieces of the bacilli, displayed on their cell surfaces, to
another type of white blood cell the T cell. When stimulated, T cells release an
elaborate array of chemical signals. Once this response, called cell-mediated
hypersensitivity, is established, a person's T cells usually will respond to the
tuberculin skin test (PPD test) and produce a characteristic red welt.
Some of the T-cell signals produce inflammatory reactions; other signals recruit and
activate specialized cells to kill bacilli and wall-off infected macrophages in tiny, hard
grayish nodules known as tubercles.
From then on the body's immune system maintains a standoff with the infection,
sometimes for years. In the tubercles, TB bacilli may persist within macrophages, but
further multiplication and spread of M. tuberculosis are confined. Most people
undergo complete healing of their initial infection, and the tubercles calcify and lose
their viability. A positive TB skin test, and in some cases a chest x-ray, may provide the
only evidence of the infection.
If, however, the body's resistance is low because of aging, infections such as HIV,
malnutrition, or other factors, the bacilli may break out of the tubercles in the alveoli
and cause active disease.
On the average, people infected with M. tuberculosis have a 10 percent chance of
developing active TB at some time in their lives. The risk of developing active disease is
greatest in the first year after infection, but active disease often does not occur until
many years later.
Active TB usually results from the spread of bacilli from the alveoli through the
bloodstream or lymphatic system to other sites, usually elsewhere in the lungs or local
lymph nodes. In 15 percent of cases, the bacilli cause disease in other regions, such as
the skin, kidneys, bones, or reproductive and urinary systems.
At the new sites, the body's immune defenses kill many bacilli, but immune cells and
local tissue die as well. The dead cells and tissue, along with live immune cells, form
granulomas whose centers have the consistency of soft cheese, where the bacilli survive
but do not flourish. The early symptoms of active TB can include weight loss, fever, night
sweats, and loss of appetite, or they may be vague and go unnoticed by the affected
As more lung tissue is destroyed and the granulomas expand, cavities in the lungs
develop, and sometimes break into larger airways called bronchi. This allows large numbers
of bacilli to spread when patients cough. As the disease progresses, the granulomas may
liquefy, perhaps as a result of enzymes secreted by the body's own immune cells. This
creates a rich medium in which the bacilli multiply rapidly and spread, creating further
lesions and the characteristic chest pain, cough, and, when a blood vessel is eroded,
Most patients do not suffer shortness of breath until the lungs are extensively damaged
by the formation of cavities. Symptoms of TB involving areas other than the lungs vary,
depending upon the organ affected.
The tuberculin skin test, also known as the Mantoux test, can identify most people
infected with tubercle bacilli six to eight weeks after initial exposure. A substance
called purified protein derivative (PPD) is injected under the skin of the forearm and
examined 48 to 72 hours later. If a red welt forms around the injection site, the person
may have been infected with M. tuberculosis, but doesn't necessarily have active
disease. Most people with previous exposure to TB will test positive on the tuberculin
test, as will some people exposed to related mycobacteria. An important exception is
people with severely weakened immune systems, such as those with HIV.
If a person has a significant reaction to the tuberculin skin test, additional methods
can determine if the individual has active TB. This is sometimes difficult because TB can
mimic other diseases, such as pneumonia, lung abscesses, tumors, and fungal infections, or
occur along with them. In making a diagnosis, doctors rely on symptoms and other physical
signs, a person's history of exposure to TB, and x-rays that may show evidence of TB
infection, usually in the form of cavities or lesions in the lungs.
The physician also will take sputum and other samples, because a positive bacteriologic
culture of M. tuberculosis is essential to confirm the diagnosis and determine
which drugs will work against the strain of TB the patient carries. Because M.
tuberculosis grows very slowly, the laboratory diagnosis requires approximately four
weeks. An additional two to three weeks usually are needed to determine the drug
susceptibility of the organism, making treatment decisions difficult.
Advances in Diagnosis
Recently, researchers supported by the National Institute of Allergy and Infectious
Diseases (NIAID) as well as other investigators developed tests that use nucleic acid
amplification to speed the diagnosis of TB from four weeks to two days. Another test in
development uses luminescent chemicals from the firefly to determine, in 24 to 48 hours,
which drugs can kill the TB strain a patient carries.
Treatment of Active Disease
The death rate for untreated TB patients is between 40 and 60 percent. With appropriate
antibiotics, however, people with drug-susceptible cases of TB can be cured more than 90
percent of the time.
Successful management of TB depends on close cooperation between the patient and
physicians and other health care workers. Patient education is essential, and many doctors
opt for supervised, directly observed therapy (DOT). Treatment usually combines the drugs
isoniazid (INH) and rifampin, which are given for at least six months, and pyrazinamide
and ethambutol (or streptomycin), which are used only in the first two months of
treatment. This treatment is referred to as short-course chemotherapy.
Therapy for MDR-TB
Treatment for MDR-TB often requires the use of a second line of TB drugs, all of which
can produce serious side effects. Therapy for 18 months to two years may be necessary, and
patients should receive at least three drugs to which the bacteria are susceptible.
TB is largely a preventable disease. In the United States, prevention has focused on
identifying infected individuals early especially those who run the highest risk of
developing active disease and treating them with drugs in a program of directly
INH prevents the disease in most people in close contact with infected people or who
are infected with the tubercle bacilli but who do not have active TB. The drug is given
daily for six to 12 months and strict patient compliance in taking medication is essential
to prevent drug-resistant strains from emerging. Adverse reactions to INH are rare,
although a small percentage of patients, especially those older than 35, suffer
INH-related hepatitis. Rifampin for one year is recommended for close contacts of patients
with INH-resistant TB organisms.
In the United States, people with any of the following risk factors should be
considered for preventive therapy, regardless of age, if they have not been previously
treated for TB:
- Close contacts of people with newly diagnosed infectious TB; (In addition, children and
adolescents who react negatively to the PPD test, but who have been in close contact with
infectious people within the past three months, should be considered for preventive
therapy. Therapy should continue until a second skin test is done 12 weeks after their
first contact with an infectious person.)
- People with positive tuberculin skin tests and abnormal chest x-rays compatible with
inactive TB (lesions caused by prior disease);
- People whose skin test results have recently converted from negative to positive;
- People with positive skin test reactions who also have special medical conditions known
to increase the risk of TB (e.g., HIV infection, diabetes mellitus) or who are on
- HIV-positive people or those suspected to be HIV-infected who now have, or had at any
time in the past, positive skin test reactions, but who do not have active infection; and
- Injection drug users who have positive skin test reactions.
In addition, people younger than 35 in the following groups should be considered for
preventive therapy if they have positive skin test reactions:
- Foreign-born people from countries where TB is common;
- People in medically underserved, low-income groups, especially African Americans,
Hispanics, and Native Americans; and
- Residents of long-term care facilities such as prisons, nursing homes, and mental
Health care workers in frequent contact with TB patients or involved with high-risk
procedures such as those that induce coughing should have a skin test every six months.
Hospitals and clinics caring for high-risk populations can take precautions to prevent
the spread of TB. All patients should be taught to cover their mouths and noses when
coughing or sneezing. Ultraviolet light can be used to sterilize the air, and negative
pressure rooms and special filters are available, as are special respirators and masks,
that filter out the droplet nuclei. Until they are no longer infectious, hospitalized TB
patients should be isolated in rooms with controlled ventilation and air flow.
More Effective Vaccines are Needed
In those parts of the world where the disease is common, a vaccine composed of live,
attenuated (weakened) mycobacteria from cows (M. bovis, called bacillus
Calmette-Guerin [BCG]) is given to infants as part of the immunization program recommended
by the World Health Organization (WHO). In infants, BCG prevents the spread of M.
tuberculosis within the body, but does not prevent initial infection.
In adults, the effectiveness of BCG has varied widely in large-scale studies. In
addition, positive skin test reactions occur in people who have received BCG vaccine, thus
limiting the effectiveness of the PPD skin test to identify new infections. As a result,
BCG is not recommended for general use in the United States. Because of BCG's limitations,
more effective vaccines are needed.
TB and HIV Infection
WHO estimates that 4.4 million people worldwide are coinfected with TB and HIV. By the
year 2000, TB will claim 1 million lives annually among the HIV-infected, WHO projects,
making TB the leading cause of death in HIV-infected individuals. In the United States, an
estimated 100,000 HIV-infected people also carry M. tuberculosis, according to CDC.
TB frequently occurs early in the course of HIV infection, often months to years before
other opportunistic infections such as Pneumocystis carinii pneumonia. TB may be
the first indication that a person is HIV-infected, and often occurs in areas outside the
lungs, particularly in the later stages of HIV disease.
In the United States, people coinfected with TB and HIV develop active TB at a rate of
about 8 percent each year. By comparison, otherwise healthy individuals infected
with M. tuberculosis have a 10 percent lifetime risk of developing active
TB. People with HIV also are at greater risk of having a new infection progress directly
to active disease.
MDR-TB in people coinfected with HIV appears to have a more rapid and deadly disease
course than seen in patients with MDR-TB who are otherwise healthy.
Diagnosing TB in HIV-infected people is often difficult. These patients frequently have
conditions that produce symptoms similar to those of TB, and may not react to the standard
tuberculin skin test because their immune systems are suppressed. Although investigators
have hypothesized that a two-stage TB skin test might be more reliable than a single-stage
test in HIV-infected individuals, a recently completed NIAID study found this not to be
X-rays, sputum smears, and physical exams may also fail to provide an indication of TB
infection in HIV-infected individuals. As a consequence, doctors must often decide to
begin anti-TB therapy in HIV-infected people suspected of having active TB while waiting
for the results of cultures of sputum or other specimens.
NIAID Research Agenda for Tuberculosis
NIAID, the lead institute for TB research at the National Institutes of Health,
supports more than 100 research projects related to TB. In fiscal year 1999, NIAID will
devote an estimated $40 million to TB research.
NIAID has a comprehensive TB research agenda that supports the following:
- Studies of the epidemiology and natural history of TB.
- Basic research into the biology of TB and the host immune response to M. tuberculosis.
- The development of new tools to diagnose TB.
- The development of new drugs or new ways to deliver standard drugs.
- Clinical trials of anti-TB therapies.
- The development of new vaccines to prevent TB.
- Training to increase the number of TB researchers.
- New ways to educate health care workers and the public about TB prevention.
This multi-disciplinary program draws on the Institute's expertise in immunology and
microbiology, as well as its capabilities in drug and vaccine development honed as part of
the research effort in AIDS and other infectious diseases.