Chapter 12

Epidemiology of Infectious Diseases

Learning Objectives

• State modes of infectious disease transmission.

• Define categories of infectious disease agents.

• Identify the characteristics of agents.

Infectious Diseases (Importance)

• They are significant cause of morbidity and mortality worldwide.

• Infectious agents are associated with some types of cancer.

• They cause disease outbreaks in institutions.

Epidemiologic Triangle

• A model used to explain the etiology of infectious diseases.

• Recognizes three major factors in the pathogenesis of disease: agent, host, and environment.

Diagram of Epidemiologic Triangle

Microbial Agents of Infectious Disease

• Bacteria• Viruses and

rickettsia

• Mycoses (fungal diseases)

• Protozoa• Helminths• Arthropods

Bacteria

• Once were the leading killers, but now are controlled by antibiotics.

• Remain significant causes of human illness.

• Tuberculosis and salmonellosis are common diseases caused by bacteria.

• Emergence of antibiotic-resistant strains a growing concern.

Viruses and Rickettsia

• Viral hepatitis A, herpes, and influenza are caused by viruses.

• Rickettsial agents produce Q fever and Rocky Mountain spotted fever.

Mycoses (Fungal Diseases)

• Valley fever (coccidioidomycosis), ringworm and athlete’s foot.

• Opportunistic mycoses infect immunocompromised patients.– Candidiasis, cryptococcosis, and

aspergillosis.

Protozoa

• Cause malaria, amebiasis, babesiosis, cryptosporidiosis, and giardiasis.

Helminths

• Found in tropical areas.

• Include intestinal parasites such as roundworms, pinworms, and tapeworms.

• Are responsible for trichinellosis and schistosomiasis.

Arthropods

• Act as insect vectors.• Examples: mosquitos, ticks, flies, mites. • Transmit diseases such as malaria and

encephalitis.

Characteristics of Infectious Disease Agents

• Infectivity– The capacity of an agent to produce infection

or disease.

– Measured by the secondary attack rate. • Pathogenicity

– The capacity of the agent to cause disease in the infected host.

– Measured by the proportion of individuals with clinically apparent disease.

Characteristics of Infectious Disease Agents (cont’d)

• Virulence– Refers to the severity of the disease. – Measured by the proportion of severe or

fatal cases. If fatal, use case fatality rate.

• Toxigenicity– The capacity of the agent to produce a toxin

or poison.

Characteristics of Infectious Disease Agents (cont’d)

• Resistance– The ability of the agent to survive adverse

environmental conditions.

• Antigenicity– The ability of the agent to induce antibody

production in the host. Related to immunogenicity.

Host: Definition (Refer to Glossary)

• A person (or animal) who permits lodgment of an infectious disease agent under natural conditions.

Host

• Once an agent infects the host, the degree and severity of the infection will depend on the host’s ability to fight off the infectious agent.

• Two types of defense mechanisms are present in the host: nonspecific and disease-specific.

Nonspecific Defense Mechanisms

• Examples include skin, mucosal surfaces, tears, saliva, gastric juices, and the immune system.

• Nonspecific defense mechanisms such as immunity may decrease as we age.

Disease-Specific Defense Mechanisms

• Immunity (resistance) against a particular agent.

• Types of immunity:– Active: administration of a microorganism to

invoke an immunologic response that mimics the natural infection.

– Passive: short-term immunity provided by a preformed antibody.

Active Immunity

• Natural, active--results from an infection by the agent.

• Artificial, active--results from an injection with a vaccine that stimulates antibody production in the host.

Passive Immunity

• Natural, passive--preformed antibodies are passed to the fetus during pregnancy and provide short-term immunity in the newborn.

• Artificial, passive--preformed antibodies are given to exposed individuals to prevent disease.

Environment

• The domain external to the host in which the agent may exist, survive, or originate.

• The environment consists of physical, climatologic, biologic, social, and economic components that affect the survival of the agents and serve to bring the agent and host into contact.

Reservoirs of Infectious Diseases

• The environment can act as a reservoir that fosters the survival of infectious agents.

• Examples: contaminated water supplies or food; soils; vertebrate animals.

Animal Reservoirs

• Animals can be reservoirs of infectious agents.

• Zoonoses--infectious diseases that are potentially transmittable to humans by vertebrate animals. Examples: rabies and the plague.

Direct Transmission from Reservoir

• Spread of infection through person-to-person contact.

• Portal of exit--site where infectious agents leave the body, e.g., respiratory system, skin lesions.

Direct Transmission (cont’d)

• Portal of entry--locus of access to the human body, e.g., mouth and digestive system.

• Agent must exit in large enough quantities to survive in the environment and overcome the defenses at the portal of entry into the host.

Inapparent Infection

• No symptoms of infection present.• Important because disease can be

transmitted to unsuspecting hosts.• In asymptomatic individuals, clinicians can

look for serologic evidence of infection.– Example: Increase in antibodies and enzymes

in patients with hepatitis A virus.

Incubation Period

• The time interval between exposure to an infectious agent and the appearance of the first signs and symptoms of disease.

• Provides a clue to the time and circumstance of exposure to the agent.

• Useful for determining the etiologic agent.

Herd Immunity• Immunity of a population, group, or

community against an infectious disease when a large proportion of individuals are immune either through vaccinations or prior infection.

Generation Time

• Time interval between lodgment of an infectious agent in a host and the maximal communicability of the host.

• Can precede the development of active symptoms.

• Useful for describing the spread of infectious agents that have large proportions of subclinical cases.

• Applies to both inapparent and apparent cases of disease.

Colonization and Infestation• Colonization--agents multiply on the

surface of the body without invoking tissue or immune response.

• Infestation--the presence of a living infectious agent on the body’s exterior surface, upon which a local reaction may be invoked.

Iceberg Concept of Infection• Active clinical disease accounts for only a

small proportion of host’s infections and exposures to disease agents. (Refer to next slide.)

Iceberg Concept (cont’d)

Indirect Transmission• The spread of infection through an

intermediary source:– Vehicles--e.g., contaminated water, infected

blood, food.– Fomites--inanimate objects laden with

disease-causing agents. – Vectors--living insects or animals involved

with transmission of the disease agent.

Measures of Disease Outbreaks

• Attack rate

• Secondary attack rate

• Case fatality rate

Attack Rate

• Similar to an incidence rate.

• Used when the nature of the disease or condition is such that a population is observed for a short period of time.

• Formula:

___Ill __ X 100 during a time period

Ill + Well

Secondary Attack Rate

• An index of the spread of disease in a family, household, dwelling unit, dormitory or similar circumscribed group.

• A measure of contagiousness.

• Useful in evaluating control measures.

Secondary Attack Rate: Definition

• The number of cases of infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts.

• Number of new cases in group - initial case(s) Number of susceptible persons

in the group - initial case(s)• Initial case(s) = Index case(s) + coprimaries• Index case(s) = Case that first comes to the

attention of public health authorities.• Coprimaries = Cases related to index case so

closely in time that they are considered to belong to the same generation of cases.

Secondary Attack Rate (%) (Multiply fraction by 100.)

Case Fatality Rate (CFR)

• The number of deaths caused by a disease among those who have the disease.

• Examples of diseases with a high CFR are rabies and AIDS.

Formula for CFR

• Number of deaths due to disease “X” x 100 Number of cases of disease “X”

• Sample calculation: Assume that an outbreak of plague occurs in an Asian country. Health authorities record 98 case of the disease, all of whom are untreated. Among these, 60 deaths are reported.

• CFR = (60/98) x 100 = 61.2%

Investigation of Infectious Disease Outbreaks

• Define the problem.• Appraise existing data.

– Case identification– Clinical observations– Tabulation and spot maps– Identification of responsible agent

Investigation (cont’d)

• Formulate a hypothesis.• Test the hypothesis.• Draw conclusions and formulate practical

applications.

Epidemiologically Significant Categories of Infectious Diseases

• Foodborne illness• Water- and foodborne diseases• Sexually transmitted diseases• Vaccine-preventable diseases• Diseases spread by person-to-person contact• Zoonotic diseases• Mycoses (fungal diseases)• Arthropod-borne diseases

Foodborne Illness

• One of the most common infectious disease problems in the community.

• Examples include:– Staphylococcus aureus--present in

contaminated food. Produces a toxin. – Trichinosis--associated with inadequately

cooked pork products.

Foodborne Agents

Source: Data are from Heymann DL, Control of Communicable DiseasesManual, 18th ed. Washington, DC: American Public Health Association; 2004.

Water and Foodborne Diseases

• Examples include:– Amebiasis--intestinal disease.– Cholera--acute enteric disease.– Giardiasis– Legionellosis– Schistosomiasis--infection caused by adult

worms in the bloodstream. The cycle involves alternate human and snail hosts.

Schistosoma mansoni Life Cycle

Source: Reprinted from Centers for Disease Control and Prevention. Schistosomiasis in US Peace Corps Volunteers—Malawi, 1992.

MMWR, vol 42, p 568, July 30, 1993.

Sexually Transmitted Diseases: AIDS

• Risk factors in U.S.– Male-to-male sex– Injection drug use

• In 2005, the case rate was 19.8 per 100,000 for 33 U.S. states.

• The human immunodeficiency virus (HIV) is an acute problem worldwide.– For example, approximately 24.7 million

cases were living with HIV in Sub-Saharan Africa in 2006.

Vaccine-Preventable Diseases

• Vaccines are routinely given to children for the prevention of several diseases, including:– Chickenpox, Diphtheria, Haemophilus

influenzae type b infections, hepatitis A, hepatitis B, influenza, measles, meningococcal meningitis, mumps, pertussis, paralytic poliomyelitis, pneumococcal disease, rotaviral enteritis, rubella, and tetanus.

Diseases Spread by Person-to-Person Contact

• One example is tuberculosis.

• Resurgence of TB (from late 1980s until mid-1990s) due to:– Increase in persons infected with HIV.– Increase in homeless population.– Importation of cases from endemic areas.

U.S. TB Cases, 1980-1992

Source: Reprinted from Centers for Disease Control and Prevention. Tuberculosis morbidity—United States, 1992.

MMWR, vol 42, p 696, September 17, 1993.

U.S. TB Cases, 1995-2005+

• TB incidence on the decline for most racial/ethnic groups.

• Extensively drug-resistant tuberculosis (XDR TB) was the focus of media attention in 2007.

Zoonotic Diseases

• Zoonosis--a disease that under natural conditions can be spread from vertebrate animals to humans. Example: Q fever.

• Zoonotic diseases may be either:– Enzootic--similar to endemic in human

diseases.– Epizootic--similar to epidemic in human

diseases.

Mycoses

• Coccidioidomycosis (San Joaquin Valley fever )--caused by the fungus Coccidioides immitis.– Usually attacks the lungs.– Cases of infection usually have had contact

with contaminated soil.

• Other examples: blastomycosis, ringworm, athlete’s foot, candidiasis, cryptococcosis, and aspergillosis.

Arthropod-Borne Diseases

• Include arboviral diseases.

• Blood-feeding arthropod vectors transmit disease agents to vertebrate hosts.

• Examples of vectors: sand flies, ticks, mosquitoes.

• Examples of diseases: encephalitis, Lyme disease, chikungunya fever

Emerging Infections

• Suddenly increase in incidence or geographic scope.

• Many infections appear when an existing pathogen gains access to new host populations. – Examples: U.S. hantaviral pulmonary

syndrome, Lyme disease, AIDS, hepatitis C, hemorrhagic fever.

• Environmental changes may contribute to their emergence.

Emerging Infectious Diseases

Emerging Infectious Diseases (cont’d)