CHAPTER 25 Epidemiology

Principles of Epidemiology The Science of Epidemiology Epidemiology is the study of disease in populations. To understand infectious disease, the epidemiologist studies the interactions of the pathogen with the host population.

Figure 25.1 shows deaths due to infectious disease in the United States.

A well-adapted pathogen lives in balance with its host, taking what it needs for existence and causing only a minimum of harm. Such pathogens may cause a chronic, or long-term, infection in the host. Pathogens can be selective forces in the evolution of the host, just as hosts are selective forces in the evolution of pathogens. Such pathogens often cause acute infections, characterized by rapid and dramatic onset and, often, rapid recovery.

The Vocabulary of Epidemiology The prevalence of a disease in a population is defined as the proportion of diseased individuals in a population in a given time period. The incidence of a disease is the number of cases of an individual disease in a population in a given time period. An endemic disease is constantly present at low incidence in a specific population. In epidemics, an unusually high incidence of disease occurs in a specific population. A pandemic is a worldwide epidemic.

Figure 25.2 shows worldwide incidence of endemic, epidemic, and pandemic disease.

Sporadic cases of disease may occur when individual cases are recorded in geographically separated areas, implying that the incidents are not related. A disease outbreak occurs when a number of cases are observed, usually in a relatively short period of time, in an area previously experiencing only sporadic cases of the disease. Finally, diseased individuals who show no symptoms or only mild symptoms have subclinical infections. Subclinically infected individuals are frequently identified as carriers of a particular disease. Infectious diseases cause morbidity (illness) and may cause mortality (death). An infectious disease follows a predictable clinical pattern in the host.

Table 25.1 shows worldwide deaths due to infectious diseases in 2002.

Disease Reservoirs and Epidemics To understand how diseases spread, the pathogen reservoir must be known. Some pathogens exist in soil, water, or animals. Others exist only in humans and are maintained solely by person-to-person contact. An understanding of disease carriers and pathogen life cycles is critical for controlling disease (Table 25.2).

Infectious Disease Transmission A pathogen can be transmitted directly from one host to another, or indirectly by living vectors or inanimate objects (fomites) and common vehicles such as food and water. Environmental factors may play a role in survival of a pathogen, and such variables as weather patterns may influence exposure to a pathogen (Figure 25.3).

A common-source epidemic is one resulting from infection of a large number of people from a single contaminated source. A host-to-host epidemic results from person-to-person contact and is characterized by a gradual rise and fall in number of cases (Figure 25.4).

The Host Community Hosts and pathogens coevolve with time and arrive at a steady state that favors the continued survival of both (Figure 25.5).

With herd immunity, a large fraction of a population is immune to a given disease, and it is difficult for the disease to spread (Figure 25.6).

Disease cycles occur when a large, recurring, nonimmune population such as children entering school is exposed to a pathogen.

Current Epidemics The AIDS Pandemic

AIDS is one of the most thoroughly studied disease pandemics. AIDS will continue to be a major public health problem, especially in developing countries. There is no effective cure or immunization to prevent AIDS, although we now know a great deal about its pathology and spread. Table 25.3 shows the incidence of HIV/AIDS infections worldwide in 2003.

Figure 25.7 shows annual diagnosed cases of AIDS since 1981 in the United States.

The distribution of AIDS cases by risk group and sex in adolescents and adults in the United States for the year 2002 is shown in Figure 25.8.

Hospital-Acquired (Nosocomial) Infections Many common microorganisms have the potential to be pathogens in a hospital environment. This is called nosocomial infection. Hospital patients are unusually susceptible to infectious disease and are exposed to a variety of infectious agents, including opportunistic pathogens, in the hospital environment. Treatment of these infections is complicated by antibiotic resistance.

Table 25.4 gives the number of intensive care unit nosocomial infections in the United States, by site and organism.

Severe Acute Respiratory Syndrome Zoonosis is infection with a disease that occurs primarily in animals but can be transferred to humans. Infection with a novel zoonotic virus, SARS-CoV, causes severe acute respiratory syndrome (SARS). Person-to-person spread is by respiratory means. Control of this high mortality virus is through rapid diagnosis and isolation of victims.

Epidemiology and Public Health Public Health Measures for the Control of Disease

Food and water purity regulations, vector control, immunization, quarantine, disease surveillance (Table 25.5), and pathogen eradication are public health measures that play a major role in reduction of disease incidence.

The Centers for Disease Control and Prevention (CDC) in the United States, through the National Center for Infectious Diseases (NCID), operates a number of surveillance programs (Table 25.6).

Global Health Considerations Infectious diseases account for nearly 30% of all worldwide mortality. Most infectious diseases occur in developing countries.

Figure indicates that most African deaths result from infectious diseases; whereas in the Americas, cancer and cardiovascular diseases are the leading causes of mortality.

Travelers to endemic disease areas should be immunized when possible and should take appropriate precautions to prevent infection (Table 25.7).

Emerging and Reemerging Infectious Diseases

Changes in host, vector, or pathogen conditions, whether natural or artificial, can result in conditions that encourage the explosive emergence or reemergence of certain infectious diseases. Recent dramatic examples of global emerging and reemerging infection are shown in Figure Table 25.8 shows emerging and reemerging epidemic infectious diseases.

Bacterial genetic mechanisms can enhance virulence and promoting emergence of new epidemics. One group of virulence-enhancing mechanisms are the mobile genetic elements, bacteriophages, plasmids and transposons.

Table 25.9 lists some virulence factors carried on these mobile genetic elements that contribute to pathogen emergence.

Global surveillance and intervention programs must be developed to prevent new epidemics and pandemics.

Biological Warfare and Biological Weapons Biological warfare is the use of biological agents to incapacitate or kill a military or civilian population in an act of war or terrorism. Bioterrorism is a threat in a world of rapid international travel and easily accessible technical information. Biological agents can be used as weapons by contemporary military forces or by terrorist groups.

Aerosols or common sources such as food and water are the most likely mode of inoculation. Commonly considered biological weapons agents are listed in Table

Prevention and containment measures rely on a well-prepared public health infrastructure.

Anthrax as a Biological Weapon Bacillus anthracis has emerged as an important pathogen because of its use as a bioweapon. Highly infective weaponized endospore preparations have been used as bioterror agents. Inhalation anthrax has a fatality rate of about 90% in untreated individuals. Effective treatment relies on timely observation and diagnosis of symptoms. Treatment does not guarantee survival of those infected by inhalation anthrax.