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Dynamics of Disease Transmission • • • • • • • • • • • • • • • • • • • Learning Objectives Students should learn about Epidemiological triad of disease transmission Factors effecting disease transmission Routes of transmission Models of disease causation Iceberg concept of infection Dynamics Of Disease Transmission Like all creatures ,human falls victim to death & disease. Disease is defined in Webster new collegiate dictionary as a condition in which body health is impaired. This led to fundamental questions Where do diseases come from ? Is ill health inevitable? What can be done to prevent the onset of disease? Dynamics Of Disease Transmission Diseases has been classically described as the result of an epidemiologic triad Host. Agent. Environment: Physical, chemical, biological. Factors like stress are harder to classify Dynamics Of Disease Transmission The balance of forces that determines an individual state of health at a given time is a kind of dynamic equilibrium. A potentially harmful change in any of the components of the system may not lead to detectable disease if other parts of the system have the capacity to compensate for the insult. Models of Disease Causation (1) The Epidemiologic Triangle (Triad) (2) The Web of Causation (3) The Wheel (2) The Web of Causation • • • • This model is best used in the study of chronic diseases, where the disease agent is often not known. Effects never depend on single isolated cause but as a result of chains of causation, interlinked. The web of causation provides a model which shows a variety of possible interventions that could be taken to prevent & control disease. Removal or elimination of just one link or chain may be sufficient to control disease. (3) The Wheel • • • • The wheel consists of a host (human), which has genetic make-up as its core. Surrounding the host is the environment (Biological – Physical – Social). For hereditary diseases (sickle cell anemia, G-6-P-D), the genetic core will be large For measles or influenza, the state of immunity of the host & the biological environment will contribute more. Carriers • It occurs either due to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state. • • • It is “an infected person or animal that harbors a specific infectious agent in the absence of discernible (visible) clinical disease and serves as a potential source of infection to others. Three elements have to occur to form a carrier state: • • • The presence in the body of the disease agent. The absence of recognizable symptoms and signs of disease. The shedding of disease agent in the discharge or excretions. Incubation and Latent periods Incubation period: time from exposure to development of disease. In other words, the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question. • • • • Latent period: the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period). Herd immunity Level of immunity in a population which prevents epidemics even if some transmission may still occur Presence of immune individuals protects those who are not themselves immune Disease occurrence in populations Sporadic: occasional cases occurring at irregular intervals • Endemic: continuous occurrence at an expected frequency over a certain period of time and in a certain geographical location • Epidemic or outbreak: occurrence in a community or region of cases of an illness with a frequency clearly in excess of normal expectancy • Pandemic: epidemic involves several countries or continents, affecting a large population • • • • • Epidemic “The unusual occurrence in a community of disease, specific health related behavior, or other health related events clearly in excess of expected occurrence” (epi= upon; demos= people) Epidemics can occur upon endemic states too. Endemic It refers to the constant presence of a disease or infectious agent within a given geographic area or population group. It is the usual or expected frequency of disease within a population. (En = in; demos = people) • • Pandemic and Exotic An epidemic usually affecting a large proportion of the population, occurring over a wide geographic area such as a section of a nation, the entire nation, a continent or the world, e.g. Influenza pandemics. Exotic diseases are those which are imported into a country in which they do not otherwise occur, as for example, rabies in the UK. -------------------The end-----------------------