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Education Materials > Publications > Self-Study Modules on TB > Module 2 > Reading Material

Self-Study Modules on Tuberculosis

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Module 2: Epidemiology of Tuberculosis

Reading Material

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 the world.

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 factors:

  • 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.

Answers 

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 who are more likely to be exposed to or infected with M. tuberculosis

    This category includes people who live or work in certain settings (see Special Settings).

  • People who are more likely to develop TB disease once infected


 

Table 2.1
Groups at High Risk for TB
People at Higher Risk for Exposure or Infection People at Higher Risk for TB Disease
  • Close contacts of people with infectious TB
  • People born in areas of the world where TB is common (for example, Asia, Africa, or Latin America)
  • Elderly people
  • Low-income groups with poor access to health care, including homeless people
  • People who inject illicit drugs
  • People who live or work in residential facilities (for example, nursing homes or correctional facilities)
  • Other people who may be exposed to TB on the job (for example, some health care workers)
  • People in other groups as identified by local public health officials
  • 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 disease
  • People who inject illicit drugs

People at Higher Risk for Exposure or Infection

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 Special Settings).

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. 

Special Settings

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 crowded.

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?

Answers


 
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.

  1. Mr. Davidson:
    rides the subway every day
    is 80 years old
  2. Mr. LeFevre:
    works at a nursing home
    immigrated from Europe
  3. Ms. Montoya:
    was born in Latin America
    has a father who had pulmonary TB disease
  4. Ms. Parker:
    volunteers in the emergency room of an inner-city hospital
    works in a day care center
  5. Mr. Dudley:
    released from prison last year
    injects morphine

Answers

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 for HIV.

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 once infected.

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?

Answers


 
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.

  1. Mr. Sims:
    injects heroin
    is HIV infected
  2. Mr. Allen:
    has diabetes
    has high blood pressure
  3. Ms. Li:
    has chest x-ray findings suggestive of previous TB disease
    has heart problems
  4. Mr. Vinson:
    is obese
    became infected with M. tuberculosis 6 months ago

Answers

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, 1993.

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).
 

Table 2.2
Relative Risk* for TB by Race and Ethnicity, 1993
Race/Ethnicity TB Case Rate
(number of TB cases for every 100,000 persons in this race/ethnicity)
Relative Risk
Asians/Pacific Islanders 44.5 12

Non-Hispanic blacks

29.1 8

Hispanics

20.6 6

American Indians/Alaskan Natives

14.6 4
Non-Hispanic whites 3.6 1

* 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 Settings).

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 by 36%.

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 that

  • 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

2.11. Which racial and ethnic groups are disproportionately affected by TB?

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.

Answers

 


Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination - http://www.cdc.gov/tb

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