Text of Chapter 12 Epidemiology of Infectious Diseases. Learning Objectives State modes of infectious...
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 athletes 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 (contd) 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 (contd) 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 hosts 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 (contd) 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 bodys exterior surface, upon which a local reaction may be invoked.
Iceberg Concept of Infection Active clinical disease accounts for only a small proportion of hosts infections and exposures to disease agents. (Refer to next slide.)
Iceberg Concept (contd)
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 aut