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