Managerial Epidemiology: Week 5

Chapter 12

Epidemiology of Infectious

Diseases

Learning Objectives

• State modes of infectious disease

transmission

• Define three categories of infectious

disease agents

• Identify the characteristics of agents

• Define quantitative terms used in

infectious disease outbreaks

• Describe the procedure for investigating a

disease outbreak

Infectious Diseases (Importance)

• They are a significant cause of morbidity and

mortality worldwide.

• Infectious agents are associated with some

types of cancer.

• Due to increasing world travel, infected

passengers can transmit the communicable

disease from within the time span of a long-

distance plane flight.

• 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

• 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

• A microorganism composed of a piece of

genetic material (RNA or DNA)

surrounded by a protein coat. To replicate,

a virus must infect a living cell.

• Viral hepatitis A, herpes, and influenza are

caused by viruses.

Rickettsia

• A genus of bacteria that can grow within

cells.

• Ectoparasites (e.g., fleas, lice, and ticks)

transmit the majority of rickettsial agents,

which cause a variety of diseases.

• Rickettsial agents produce typhus fever,

Q fever and Rocky Mountain spotted

fever.

Mycoses (Fungal Diseases)

• Mycoses cause diseases such as

coccidioidomycosis, ringworm, and athlete’s foot.

– Example of disease: A fall 2012 outbreak of fungal

meningitis was associated with a contaminated

steroid medication and associated with more than

400 cases and 30 deaths in at least 19 states.

• Opportunistic mycoses infect immunocompromised

patients.

– Candidiasis, cryptococcosis, and aspergillosis.

Protozoa

• Microscopic single-cell organisms.

• Responsible for diseases, such as

malaria, amebiasis, babesiosis,

cryptosporidiosis, and giardiasis.

• Example: malaria is transmitted by

mosquitos in endemic areas.

Helminths

• Organisms found most frequently in

moist, tropical areas.

• Include intestinal parasites such as

roundworms, pinworms, and

tapeworms.

• Are responsible for trichinellosis and

schistosomiasis.

Arthropods

• Act as insect vectors that carry a disease agent from its reservoir to humans.

• Examples: mosquitos, ticks, flies, mites, and other insects.

• Transmit diseases such as Dengue fever, Lyme disease, viral encephalitis, Rocky Mountain spotted fever, trypanosomiasis, and leishmaniasis.

Characteristics of Infectious

Disease Agents

• Infectivity – The capacity of an agent to enter and

multiply in a susceptible host and produce infection or disease.

– Polio and measles are diseases of high infectivity.

– Measured by the secondary attack rate.

Characteristics of Infectious

Disease Agents

• Pathogenicity

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

– Measles is a disease of high pathogenicity, whereas polio is a disease of low pathogenicity.

– Measured by the ratio of the number of

individuals with clinically apparent disease to

the number exposed to an infection.

Characteristics of Infectious

Disease Agents (cont’d)

• Virulence

– Refers to an agent’s capacity to induce

disease in the host.

– Sometimes used as a synonym for

pathogenicity.

– Measured by the ratio formed by the number of

total cases with overt infection divided by the

total number of infected cases.

– If fatal, use case fatality rate (CFR).

Characteristics of Infectious

Disease Agents (cont’d)

• Toxigenicity

– Refers to the capacity of the agent to

produce a toxin or poison.

– The pathologic effects of agents for

diseases such as botulism and shellfish

poisoning result from the toxin produced

by the microorganism rather than from

the microorganism itself.

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.

• Host responses to infectious agents

– immunity may decrease as we age.

– nutritional status of the host

– Genetic factors

Disease-Specific Defense

Mechanisms

• Immunity (resistance) of the host to a disease agent.

• Types of immunity:

– Active: A disease organism stimulates the potential host’s immune system to create antibodies against the disease. Long lasting, but requires time to develop.

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

Active Immunity

• Natural, active or natural immunity

– Results from an infection by the agent.

– Example: A patient develops long-term immunity to measles because of a naturally acquired infection.

• Artificial, active or vaccine-induced immunity

– 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 confer protection against a disease.

– Example: Prophylaxis against hepatitis by administration of immune globulin to individuals who have been exposed.

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.

• Example

–Direct contact with the blood or bodily fluids of an infected person as in the spread of sexually transmitted diseases.

Indirect Transmission from

Reservoir • Spread of infection through an

intermediary source: vehicles, fomites, or vectors.

– Examples of vehicles - Contaminated water, infected blood on used hypodermic needles, and food.

– Examples of fomites – Inanimate objects, such as a doorknob or clothing – laden with disease-causing agents.

– Examples of vectors – flies and mosquitos

Portals of Exit and Entry

• Portal of exit—sites where infectious agent may

leave the body, e.g., respiratory passages, the

alimentary canal, and the openings in the

genitourinary system, and skin lesions.

• Agent must exit in large enough quantities to survive

in the environment and overcome the defenses at the

portal of entry into the host.

• Portal of entry--locus of access to the human body,

e.g., mouth and digestive system and the mucous

membranes or wounds in the skin.

Inapparent Infection

• A subclinical infection that has not yet penetrated the clinical horizon--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.

• Applies only to clinically apparent cases 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

• The tip of the iceberg, which

corresponds to active clinical disease

accounts for only a small proportion

of host’s infections and exposures to disease agents.

Iceberg Concept (cont’d)

Measures of Disease

Outbreaks

• Attack rate

• Secondary attack rate

• Case fatality rate

Attack Rate

• The proportion of a group that experiences the

outcome under study over a given period.

• Similar to an incidence rate.

• Used when the occurrence of disease among a

population at risk increases greatly over 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)

• Proportion formed by the number of

deaths caused by a disease among those

who have the disease during a time

interval.

• Provides an index of the virulence of a

particular disease within a specific

population.

• Examples of diseases with a high CFR are rabies and untreated bubonic plague.

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 during the month of January.

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%

Basic Reproductive Rate (R0)

• A measure of the number of

infections produced on average by an

infected individual in the early stages

of an epidemic when virtually all

contacts are susceptible.

• Can be used as a measure of the

transmissibility of influenza.

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

• Fungal diseases (mycoses)

• Arthropod-borne diseases

Foodborne Illness

• One of the most common infectious

disease problems in the community.

• Examples include:

– Staphylococcus aureus--present in

contaminated food that have been

stored at improper temperatures.

– Trichinosis--associated with

inadequately cooked pork products.

Foodborne

Agents

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.

Sexually Transmitted

Diseases: HIV/AIDS

• High-risk populations in the U.S. – Men who has sex with men (MSM)

– African Americans, Hispanics or Latinos

– Injection drug use

• In 2008, the estimated prevalence of AIDS diagnoses in the general U.S. population was 157.7 per 100,000 population.

• The human immunodeficiency virus (HIV) is an acute problem worldwide. – Approximately 34.2 million people were living with

HIV in 2011.

Vaccine-Preventable Diseases

• Vaccines are routinely given to children

(0-6 years) 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

• By 2010, TB incidence had declined.

• Most affected groups were foreign-born

individuals and racial and ethnic minorities.

• Current high-risk populations

– Migrant farm workers

– Homeless persons

• 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.

• Examples: Anthrax, brucellosis,

leptospirosis, Q fever, and rabies.

• Zoonotic diseases may be either:

– Enzootic--similar to endemic in human

diseases.

– Epizootic--similar to epidemic in human

diseases.

Fungal Diseases (Mycoses)

• Three major types:

– Opportunistic infections among persons who

have weakened immune systems

– Hospital-associated and Community-acquired

infections

– Coccidioidomycosis (San Joaquin Valley fever )

• Manifests as a lung disease and is caused by the

fungus Coccidioides immitis.

• Cases of infection usually have had contact with

contaminated soil.

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: Dengue fever,

Lyme disease, malaria, viral encephalitis,

West Nile Virus, and plague

Emerging Infections

• Infectious disease that have recently been identified and taxonomically classified.

• Refers to certain ‘old’ diseases that have experienced a resurgence because of a changed host-agent-environment conditions.

– Examples: HIV/AIDS, hepatitis C virus infections, Lyme disease, E.coli O157:H7 foodborne illnesses, and hantavirus pulmonary syndrome.

Emerging Infectious Diseases