Self-Study Modules on Tuberculosis
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2: Epidemiology of Tuberculosis
TB infection is one of the most common infections in the world. It
is estimated that 30% to 60% of adults in developing countries have
TB infection. Every year, about 8 million people develop TB disease
and 3 million people die of the disease. In fact, among people older
than 5 years of age, TB disease is the leading cause of death around
In the United States, physicians and other health care providers
are required by law to report TB cases to their state or local health
department. Reporting is very important for TB control. When the
health department learns about a new case of TB, it should take
steps to ensure that the person receives appropriate treatment.
The health department should also start a contact investigation.
This means interviewing a person who has TB disease to determine
who may have been exposed to TB. People who have been exposed to
TB are screened for TB infection and disease.
State and some big-city health departments report TB cases to the
federal Centers for Disease Control and Prevention (CDC) based on
certain criteria. These criteria are discussed in more detail in
Module 3, Diagnosis of Tuberculosis
Infection and Disease. CDC reports the number of TB cases that
occur each year in the United States.
In 1953, when nationwide TB reporting first began, there were more
than 84,000 TB cases in the United States. From 1953 through 1984,
the number of TB cases decreased by an average of 6% each year.
In 1985, the number of TB cases reached an all-time low of 22,201.
In 1986, however, there was an increase in TB cases, the first since
1953. Since 1985, the number of new cases has increased by 14% —
from 22,201 in 1985 to 25,313 in 1993 (Figure 2.1).
Figure 2.1 Reported TB cases, United States, 1953-1993. (*indicates
years when the reporting criteria were changed.) This is a figure
of the reported TB cases in the United States, 1953-1993.
We can attribute the recent increase in TB cases to at least four
- The HIV epidemic
- Immigration from countries where TB is common
- The spread of TB in certain settings (for example, correctional
facilities and homeless shelters)
- Inadequate funding for TB control and other public health efforts
Study Questions 2.1-2.3
2.1. What happened to the number of TB cases in the United
States between 1953 and 1984?
2.2. What happened to the number of TB cases in the United
States between 1985 and 1993?
2.3. Name four factors that have contributed to the recent
increase in the number of TB cases.
The number of TB cases at a certain place and time is often expressed
as a case rate. A case rate is the number of cases
that occur during a certain time period, divided by the size of
the population during that time period. (The case rate is often
expressed in terms of a population size of 100,000 persons.) For
example, in the United States in 1993, there were 25,287 TB cases
in a population of approximately 257,908,000 people. In other words,
the TB case rate was 9.8 TB cases per 100,000 persons. The TB case
rates for each state, the District of Columbia, and Puerto Rico
are shown in Figure 2.2.
Figure 2.2 TB case rates by state, United States, 1993. This
is a figure of the TB case rates by state, United States, 1993
Using case rates, health departments, CDC, and others can compare
the occurrence of TB cases in different places, time periods, and
groups of people. They have found that in certain groups, the rates
of TB are higher than in others. These high-risk groups can be divided
into two categories (Table 2.1):
People at Higher Risk for Exposure
Close contacts, or people who spend time with someone who
has infectious TB disease, are at high risk of being infected with
M. tuberculosis. Close contacts may include family members,
coworkers, or friends. Close contacts are discussed in more detail
in Module 3, Diagnosis of Tuberculosis
Infection and Disease.
In the United States, TB infection and disease occur often among
people born in areas of the world where TB is common, such as Asia,
Africa, and Latin America. In most cases, these foreign-born
persons become exposed to and infected with M. tuberculosis
in their country of birth. Of all TB cases reported to CDC in 1993,
29% were in foreign-born persons. This is an increase from 1986,
when 22% of cases were in foreign-born persons.
All people who apply for immigration and refugee status are screened
for TB disease before coming to the United States. Immigrants with
TB disease who are infectious at the time of screening are required
to receive treatment before they enter the United States. (Infectiousness
is discussed in Module 5, Infectiousness
and Infection Control.) However, some immigrants have TB disease
but are not infectious at the time of screening. Sometimes these
immigrants become infectious after they enter the United States.
Also, many immigrants have TB infection, but not TB disease, at
the time of screening. These immigrants may develop TB disease years
after they come to the United States. Finally, many people enter
the United States without being screened for TB disease, such as
students, tourists, and undocumented (illegal) aliens.
TB is also more common among the elderly. Many
elderly people were exposed to and infected with M. tuberculosis
when they were younger and TB was more common than it is today.
Because a larger proportion of elderly people have TB infection,
this group is at higher risk for TB disease. Of all TB cases reported
in 1993, 23% were in people 65 years of age and older, even though
this age group made up only 13% of the population. Elderly people
living in nursing homes are at an even higher risk for TB (see
Another risk group is low-income people. From
1985 to 1992, the average rate of TB cases was nearly eight times
higher in zip code areas with the lowest household income as in
areas with the highest household income. The reasons for this are
not entirely clear, but some possible reasons are crowding, inadequate
living conditions, malnutrition, and poor access to health care.
TB infection and disease are also more common among homeless
people. CDC recently began collecting information on the
percentage of TB patients who are homeless. In 1993 about 5% of
TB patients in 36 U.S. reporting areas were homeless. In addition,
according to studies published in 1986, from 18% to 51% of homeless
people have TB infection. Homeless people may be at higher risk
of developing TB disease once infected because of malnutrition and
poor access to health care. Moreover, in some areas they may be
more likely than the general population to be infected with HIV.
People who inject illicit drugs are more likely
to be exposed to or infected with M. tuberculosis. This
may be because a large proportion of people in this risk group have
other risk factors for exposure to TB, such as being in a low-income
group and having poor access to health care. People who inject illicit
drugs are also at high risk of developing TB disease once infected,
perhaps because they are more likely to be HIV infected. Also, it
is possible that injecting illicit drugs weakens the immune system.
In certain settings, such as nursing homes and correctional facilities,
the risk of being exposed to TB is higher than in other places.
This is because many people in these facilities are at risk for
TB. The risk of exposure to TB is even higher if the facility is
For example, TB is a problem in nursing homes.
In a 29-state survey conducted in 1984 and 1985, CDC found that
the rate of TB disease was twice as high for elderly people living
in nursing homes as for elderly people not living in nursing homes.
TB is also a problem in correctional facilities.
A CDC study conducted in 1984 and 1985 showed that there were four
times as many TB cases in people living in correctional facilities
as there were in people of the same age who did not live in correctional
facilities. There are several reasons why rates of TB disease are
higher in correctional facilities. First, many inmates already have
TB infection and therefore are at higher risk of developing TB disease.
Second, an increasing number of inmates are infected with HIV, which
means that they are more likely to develop TB disease if they become
infected with M. tuberculosis. Finally, some correctional
facilities are crowded, which promotes the spread of TB.
Other settings where people at risk for TB are grouped together
are homeless shelters and drug treatment
centers. People who live or work in these settings are
at higher risk of being exposed to TB.
People who work in health care facilities, such
as clinics and hospitals, may be exposed to TB on the job. The risk
of exposure depends on the number of TB patients in the facility,
the employee's duties, and the effectiveness of the infection control
procedures in the facility. Infection control procedures, or measures
to prevent the spread of TB, are discussed in more detail in
Module 5, Infectiousness and Infection Control.
|Study Questions 2.4-2.6
2.4. Name eight
groups of people who are more likely to be exposed to or
infected with M. tuberculosis.
2.5. Why is the risk of being exposed to TB higher in certain
settings, such as nursing homes or correctional facilities?
2.6. What are some reasons why rates of TB disease are
higher in correctional facilities?
|Case Study 2.1
For each of the following
people, circle the factor or factors known to increase the
risk of being exposed to or infected with M. tuberculosis.
Each person may have more than one risk factor.
- Mr. Davidson:
rides the subway every day
is 80 years old
- Mr. LeFevre:
works at a nursing home
immigrated from Europe
- Ms. Montoya:
was born in Latin America
has a father who had pulmonary TB disease
- Ms. Parker:
volunteers in the emergency room of an inner-city
works in a day care center
- Mr. Dudley:
released from prison last year
People at Higher Risk for TB Disease
Anyone who has TB infection can develop TB disease, but some people
are at higher risk than others (see Module
1, Transmission and Pathogenesis of Tuberculosis). HIV-infected
people are at highest risk. High-risk groups include
- People with HIV infection
- People with other medical conditions that appear to increase
the risk for TB (see Module 1,
Transmission and Pathogenesis of Tuberculosis)
- People recently infected with M. tuberculosis (within
the past 2 years)
- People with chest x-ray findings suggestive of previous TB
- People who inject illicit drugs
The Connection Between TB and HIV
What evidence shows that the HIV epidemic has contributed to the
increase in the number of TB cases? First, the areas that
have been the most affected by the HIV epidemic have also reported
the largest increases in TB cases. Between 1985 and 1992,
New York, California, Florida, Texas, and New Jersey reported the
most AIDS cases and the biggest increase in TB cases.
Second, from 1985 to 1992, the largest increase in TB cases
occurred among people aged 25 to 44, the age group most affected
by AIDS (in 1991, 74% of all reported AIDS cases fell into
this age group). From 1985 to 1992, reported TB cases increased
by 55% in this age group.
Third, TB is common among AIDS patients. TB cases
and AIDS cases are reported to state and local health departments,
and the case reports are compiled into registries. At CDC, the TB
and AIDS registries were matched for the years 1981 through 1990.
Researchers found that approximately 5% of the persons reported
to have AIDS also had TB.
Fourth, HIV infection is common among TB patients.
This was shown in a 1991 survey of patients recently diagnosed with
or suspected of having TB disease. In this survey, blood samples
were taken from patients in 35 TB clinics in 19 cities. About 3000
samples were tested for HIV infection. In some areas, none of the
samples tested positive for HIV; in New York City, 61% of the samples
tested positive. Overall, about 8% of the samples tested positive
The risk that people who are infected with both M. tuberculosis
and HIV will develop TB disease was studied by Dr. Peter Selwyn
in New York City. For 2 years Dr. Selwyn observed a group of people
who injected drugs and who were infected with M. tuberculosis.
Of those who were also infected with HIV, 14% developed
TB disease. Of those who were not infected with
HIV, none developed TB disease. This study suggests that the risk
of developing TB disease is about 7% to 10% each year
for people who are infected with both M. tuberculosis and
HIV. In contrast, the risk of developing TB disease is 10% over
a lifetime for people infected only with M. tuberculosis.
The results of this study and other studies indicate that
HIV infection is the strongest known risk factor for the development
of TB disease in people with TB infection (see
Table 1.2 in Module 1, Transmission and Pathogenesis of Tuberculosis).
|Study Questions 2.7-2.10
2.7. Name five
groups of people who are more likely to develop TB disease
2.8. What evidence shows that the HIV epidemic has played
a part in the recent increase in the number of TB cases?
Name four pieces of evidence.
2.9. If a person is infected with both M. tuberculosis
and HIV, what are his or her chances of developing TB disease?
How does this compare to the risk for people who are infected
only with M. tuberculosis?
2.10. What is the strongest known risk factor for the development
of TB disease?
|Case Study 2.2
For each of the following
people, circle the factor or factors known to increase the
risk of developing TB disease once infected. Each person
may have more than one risk factor.
- Mr. Sims:
is HIV infected
- Mr. Allen:
has high blood pressure
- Ms. Li:
has chest x-ray findings suggestive of previous
has heart problems
- Mr. Vinson:
became infected with M. tuberculosis 6 months
Race and Ethnicity
Information about the race and ethnicity of people who are reported
to have TB shows that TB affects certain racial and ethnic minorities
disproportionately. Of all the TB cases reported in the United States
in 1993, more than 70% were in racial and ethnic minorities. This
includes non-Hispanic blacks, Hispanics, Asians and Pacific Islanders,
and American Indians and Alaskan Natives. (Hispanic is an ethnicity,
not a race. People of Hispanic origin may be of any race.)
In 1993, 35% of the reported TB cases in the United States were
in non-Hispanic blacks, even though this group made up only 12%
of the total U.S. population. Similarly, 20% of the TB cases were
in Hispanics, a group which made up only 10% of the U.S. population;
and 15% were in Asians and Pacific Islanders, who made up 3% of
the U.S. population. In other words, the percentage of U.S. TB cases
that occur in blacks, Hispanics, and Asians is higher than expected
based on the percentage of these minorities in the U.S. population
(Figures 2.3 and 2.4).
Figure 2.3 Reported TB cases by race and ethnicity, United States,
1993. (Note: Percentages do not add to 100 because of rounding.)
This is a figure of the reported TB cases by race and ethnicity,
United States, 1993.
Figure 2.4 Racial and ethnic groups by percentage of U.S. population,
1993. Population estimates are based on 1990 census data. This is
a figure of racial and ethnic groups by percentage of U.S. population,
The idea that certain racial and ethnic minorities are disproportionately
affected by TB can also be shown in terms of case rates. In 1993,
there were 3.6 TB cases in non-Hispanic whites for every 100,000
non-Hispanic whites in the general population. In other words, the
TB case rate for non-Hispanic whites was 3.6 cases per 100,000 persons.
The case rate for Asians and Pacific Islanders was 44.5 cases per
100,000 persons, about 12 times higher. This means that Asians and
Pacific Islanders were 12 times more likely than non-Hispanic whites
to have TB. Similarly, the case rate for non-Hispanic blacks was
about 8 times higher than the case rate for non-Hispanic whites;
for Hispanics, about 6 times higher; and for American Indians and
Alaskan Natives, about 4 times higher (Table 2.2).
* The relative risk is a comparison of case rates between two groups.
In this table, all case rates are compared to the case rate for
non-Hispanic whites because non-Hispanic whites have the lowest
case rate. For example, the relative risk for Asians and Pacific
Islanders is 12, because the case rate for this group is 12 times
higher than the case rate for non-Hispanic whites.
A major reason why rates of TB disease are higher for some racial
and ethnic groups may be that a greater proportion of people in
these groups have other risk factors for TB. These risk factors
include birth in a country where TB is common, HIV infection, low
socioeconomic status (for example, low level of employment or income),
and exposure to TB in a high-risk setting (see Special
TB in Children
In 1993 about 7% of all reported TB cases were in children younger
than 15 years old. TB is becoming more common in children; in fact,
from 1985 to 1993, the number of TB cases in children increased
The occurrence of TB infection and disease in children provides
important information about the spread of TB in homes and communities.
For example, when a child has TB infection or disease, we learn
- TB was transmitted relatively recently
- The person who transmitted TB to the child may still be infectious
- Other adults and children in the household or community have
probably been exposed to TB; if they are infected, they may develop
TB disease in the future
|Study Questions 2.11-2.12
racial and ethnic groups are disproportionately affected
2.12. When a child has TB infection or disease, what may
be true about the spread of TB in the child's home or community?
Name three things.