Unit 12-13:Infectious DiseaseEpidemiology
Unit 12-13 Learning Objectives:
Understand primary definitions used in infectious disease epidemiology.Differentiate between direct and indirect modes of infectious disease transmission.Understand the concept of herd immunity.Understand the necessary conditions, goals, and utility for outbreak investigations.Understand unique features for studying infectious diseases from an epidemiological perspective.Understand the major components of conducting an outbreak investigation.
Unit 12-13 Learning Objectives (cont.):
Interpret epidemic curves in terms of possible outbreak patterns and sources.Recognize the primary epidemiological study designs used in outbreak investigations.Understand measures of effect used in outbreak investigations.Review a real-world example of an outbreak investigation.
DEFINITIONS AND BACKGROUND
DefinitionsEndemic: Habitual presence of a disease in a given geographic area.Epidemic: Occurrence of a group of illnesses of similar nature within a given community or region in excess of normal expectancy, and derived from a common or from a propagated source.Pandemic: A worldwide epidemic.Herd immunity: Resistance of a group of to an attack by a disease to which a large proportion of members of the group are immune.
Definitions (cont.)Virulence: Severity of the disease produced by the organism.Carrier: Individual who harbors the organism but is not infected, as measured by serologic studies or evidence of clinical illness.Classic Example: Typhoid Mary was a carrier of Salmonella typhi who worked as a cook in NYC in different households over many years considered to have caused at least 10 typhoid fever outbreaks that included 51 cases and 3 deaths.
Definitions (cont.)Infectivity: Capacity of agent to enter and multiply in a susceptible host (hence produce infection/disease) (polio and measles have high infectivity)Pathogenicity: Capacity of agent to cause clinical disease in the infected host (measles has high pathogenicity)Toxigenicity: Capacity of agent to produce a toxin or poison (e.g. toxin produced by the microorganism rather than microorganism itself (such as botulism and shellfish poisoning))
Definitions (cont.)Resistance: Ability of agent to survive adverse environmental conditions (hepatitis agents generally very resistant whereas influenza viruses are typically fragile). Note: resistance is also applied to the host.Antigenicity: Ability of agent to induce antibody production in the host (e.g. re-infection with measles virus is very rare). The related term immunogenicity refers to infections ability to produce specific immunity.
Modes of Disease TransmissionDirect: Person-to-person contact (propagated)Indirect: a)Common vehicle such as contaminated air or water supply (can occur from single, multiple, or continuous exposure)b)Vector such as a mosquito (e.g. West Nile virus)The likelihood of disease transmission depends on several related factors (see upcoming slides).
Timelines for Infection and DiseaseDynamics ofinfectiousnesslatentperiodinfectiousperiodnoninfectious-- removed-- dead-- recoveredDynamics ofdiseasesusceptiblesusceptibleTime of Infectiontimetimeincubationperiodsymptomaticperiodnoninfectious-- dead-- recovered-- immune-- carrier
Timelines for Infection and DiseaseDefinitions from Previous Slide:Latent period: time interval from infection to development of infectious (note: this definition differs from that used for non-infectious diseases).Infectious period: time during which the host can infect another host.Incubation period: time from infection to development of symptomatic disease.Symptomatic period: period in which symptoms of the disease are present.
Factors Affecting Disease Transmissionand Symptomatic Clinical DiseaseHostVectorAgentEnvironment-- Susceptibility-- Immune response-- Resistance--- Portal(s) of entry-- Virulence Toxigenicity-- Infectivity Resistance-- Pathogenicity AntigenicityVECTOR-- Prevalence-- Portal(s) of entry-- Balance of immune to susceptible individuals--- Opportunity for exposure (e.g. crowding)
Characteristics of Herd Immunity--If a large percent of the population is immune, the entire population is likely to be protected.--Once a high proportion of all people in the community are immune, the likelihood is small that an infected person will encounter a susceptible person.--Due to herd immunity, highly protective immunization can occur without requiring 100% immunization rates (estimated 94% immunity for measles to interrupt the chain of transmission).
Characteristics of Herd Immunity--For herd immunity, disease agent must be restricted to a single host species, and transmission must be relatively direct from one member of host species to another (e.g. no reservoir outside the human host in which the organism can exist).--Herd immunity operates optimally when there is random mixing of the population.
Conditions for an OutbreakAgent and susceptible hosts are present in sufficient numberThe agent can effectively be conveyed from a source to susceptible hostsWhy do outbreaks occur?Increase in amount or virulence of agentRecent introduction of the agent into a new settingEnhanced mode of transmission, exposing more susceptible individualsFactors that increase host exposure or involve introduction through new portals of entry
Outbreak InvestigationGOALS:1.Rapidly identify the source and reservoir of the outbreak.2.Implement interventions to control and eliminate the outbreak.3.Develop policies to prevent future outbreaks.Note: It is important to maintain an open mind when investigating the source of an outbreak natural biologic toxins, heavy metals, and chemical poisoning may mimic signs and symptoms of infectious agents.
Outbreak InvestigationUtility of Performing Outbreak Investigations:1.Has goal of determining cause of outbreak, eliminating or interrupting transmission, and providing post-exposure prophylaxis.2.Uncovers new infectious agents and diseases.3.Identifies spread of a known virus to a new geographic area (e.g. West Nile encephalitis in New York City).4.Improves epidemiological understanding by uncovering new means of disease transmission.5.Leads to public health regulations/recommendations to prevent future disease outbreaks.
Unique Features of Infectious Diseases 1. A case may also be a source:For most non-infectious (e.g. chronic) diseases, a persons risk of disease in not influenced by the disease status of others (e.g. a persons risk of CHD is not influenced by the fact that his/her neighbor experienced a myocardial infarction). However, for infectious diseases such as influenza, a persons risk is greatly affected by the number of influenza patients around, and if many of the people one meets have been vaccinated.
Unique Features of Infectious Diseases2. Some people may be immune:For most non-infectious risk factors (e.g. toxins or radiation), there will be levels when all exposed will become ill. However, for some infectious diseases, such as measles, once a person has had the disease, he/she will never get it again, even if exposed in the middle of an epidemic.Thus, in terms of measuring incidence, not everyone is at risk of developing the disease.
Unique Features of Infectious Diseases3. There is sometimes a need for urgency:Many chronic diseases are concerned with the impact of environmental (e.g. lead exposure) and behavioral (e.g. smoking) risk factors that require big time-consuming public health programs.However, with outbreaks of some infectious diseases, such as Ebola or Legionnaires disease, the time frame for investigation and preventive action may be a matter of hours or days this may give little time for elaborate analyses.
Unique Features of Infectious Diseases4. Preventive measures (often) have a clear scientific basis.For many chronic diseases such as CHD, the relative impact of factors such as diet and cholesterol on disease etiology (and hence prevention), remains equivocal and a matter of debate.However, for many infectious diseases, the causative agent and characteristics of transmission are well established this leads to clear (but not always easy) targets for disease prevention.
Endemic: Malaria is still endemic in many parts of the tropics and subtropics in which socioeconomic development is deficient.Epidemic: The usual reason for immediate outbreak investigations.Pandemic: HIV has probably attained the status of pandemic, especially in sub-Saharan Africa.Herd immunity: Will discuss in more detail later in lecture.Virulence is not directly related to probability of infection, but is a measure of clinical severity of being infected. It is often expressed as a ratio: number of cases of overt infection / number of total infections.Pathogenicity is similar to virulence. Primary difference is the denominator. Pathogenicity is the ratio of the number of person developing clinical illness to the number of exposed (rather than infected) individuals.Hepatitis A is spread by person-to-person via fecal-oral route. However, common source outbreaks have also been reported such as from contaminated water and food contaminated from infected food handlers. Propagated refers to progressive transmission from person to person.Note: The latent period in infectious disease differs from that used in non-infectious dx epi (period from development of asymptomatic disease to development of symptoms. In infectious dx epi, the latent period refers to time interval from infection to infectiousness (e.g. in sexually transmitted diseases). The incubation period in infectious dx epi (time from infe