Managerial Epidemiology
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.
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Diagram of Epidemiologic Triangle
Microbial Agents of Infectious Disease
Bacteria
Viruses
Rickettsia
Mycoses (fungal diseases)
Protozoa
Helminths
Arthropods
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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Iceberg Concept (cont’d)
Measures of Disease Outbreaks
Attack rate
Secondary attack rate
Case fatality rate
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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
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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.
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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.)
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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.
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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%
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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
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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