Disease Transmission and Epidemiological Models

PRINCIPLE OF EPIDEMIOLOGY DISEASE TRANSMISSION AND EPIDEMIOLOGICAL MODELS 2 Methods of disease transmission Disease transmission 3  Direct: from person to person e.g. STDs  Indirect: common vehicle: contaminated air or water supply  Vector: mosquito  The epidemiologic triad of a disease. 4 Factors Associated With Risk of Disease 5 organisms spread 6  characteristics of the organism  rate of growth  the route of transmission  the number of susceptible persons in the community. Clinical and Subclinical Disease 7 Three classes of infections 8 Severity of disease 9 virulence of the organism (how efficient the organism is at producing disease)  site in the body where the organism multiplies  host characteristics   immune response  Social contact NONCLINICAL (INAPPARENT) DISEASE 10 1. Preclinical disease: Disease that is not yet clinically apparent but is destined to progress to clinical disease. 2. Subclinical disease: Disease that is not clinically apparent and is not destined to become clinically apparent. This type of disease is often diagnosed by serologic (antibody) response or culture of the organism. 3. 4. Persistent (chronic) disease: A person fails to “shake off” the infection, and it persists for years, at times for life. Latent disease: An infection with no active multiplication of the agent, as when viral nucleic acid is incorporated into the nucleus of a cell as a provirus. In contrast to persistent infection, only the genetic message is present in the host, not the viable organism. Carrier Status 11  A carrier is an individual who harbors the organism    can still infect others,    not infected as measured by serologic studies (no evidence of an antibody response) no evidence of clinical illness the infectivity is generally lower than with other infections limited duration or chronic The case of Typhoid Mary    carried Salmonella typhi Over a period of many years, she worked as a cook in the New York City area She was considered to have caused at least 10 typhoid fever outbreaks that included 51 cases and 3 deaths. Endemic 12 the habitual presence of a disease within a given geographic area  the usual occurrence of a given disease within such an area (sometimes referred to as the “background rate of disease”).  Epidemic 13  the occurrence of a disease in a community or region, clearly in excess of normal expectancy and derived from a common or a propagated source Pandemic 14  Pandemic refers to a worldwide epidemic. Endemic vs. Epidemic 15 Disease Outbreaks: exposure 16  Common- vehicle exposure, all the cases that occurred were exposed to the suspected vehicle e.g. contaminated food.  single exposure  multiple exposures or Periodic  continuous single-exposure, common-vehicle outbreak 17  Explosive, sudden and rapid  limited to people who share the common exposure in the first wave of cases  rarely occur in persons not exposed Outbreak Waves 18 Immunity and Susceptibility 19   The amount of disease in a population depends on a balance between the number of susceptible and not susceptible people Immunity due to  Previous disease (antibodies)  Immunization  genetic basis   If the entire population is immune, no epidemic can develop. The case of formerly isolated populations:  measles epidemics in the Faroe Islands in 19th century Herd Immunity 20    the resistance of a group of people to an attack by a disease to which a large proportion of the members of the group are immune. If a large percentage of the population is immune, the entire population is likely to be protected, not just those who are immune Once a certain proportion of people in the community are immune, the likelihood is small that an infected person will encounter a susceptible person to whom he can transmit the infection Herd immunity and vaccination 21  When we carry out immunization programs, it may not be necessary to achieve 100% immunization rates to immunize the population successfully.  We can achieve highly effective protection by immunizing a large part of the population; the remaining part will be protected because of herd immunity. 22 Conditions for herd immunity 23   The disease agent must be restricted to a single host species transmission must be relatively direct   infections must induce solid immunity.    If we have a reservoir in which the organism can exist outside the human host, herd immunity will not operate because other means of transmission may be available. If immunity is only partial, we will not build up a large proportion of immune people in the community. random mixing happens Appropriate percent of immune cases  Measles 94% Her immunity: the case of polio 24 Incubation Period 25  the interval from receipt of infection to the time of onset of clinical illness (the onset of recognizable symptoms). time needed for the organism to replicate sufficiently until it reaches the critical mass needed for clinical disease to result  the site in the body at which the organism replicates (superficially or deep)  dose of the infectious agent received    Different diseases have different incubation periods. A precise incubation period does not exist for a given disease Incubation Period in disease control 26  the case of Black death:  quarantine as the only medical advance associated with the Black Death (plague) in medieval period.  How long was sufficient?  If the patient is transmitting the disease before the clinical manifestations, quarantine might not be much effective Incubation periods of viral diseases 27