Epidemiology-T2.3 ed & T2.4 - PDF

2.0 DYNAMICS OF DISEASE TRANSMISSION BASIC EPIDEMIOLOGY MHBE 0113 1 CHAPTER 2.3 & 2.4 Factors Associated with Increased Risk Pathway of Disease Infection 2 BASIC EPIDEMIOLOGY MHBE 0113 CONTENT: Factors associa...

2.0 DYNAMICS OF DISEASE TRANSMISSION BASIC EPIDEMIOLOGY MHBE 0113 1 CHAPTER 2.3 & 2.4 Factors Associated with Increased Risk Pathway of Disease Infection 2 BASIC EPIDEMIOLOGY MHBE 0113 CONTENT: Factors associated with increase risk for human disease Host Factors Agent factors Environmental Factors  Pathway of Disease Infection -Chain of Infection Sources of Infection Mode of Transmission Portal of Entry Susceptible Host Learning Outcome: At the end of the lesson, the student will be able to, 1.Briefly describe the natural history of disease and levels of prevention 2.Explain the stages of clinical disease 3.Explain Incubation Period 4.Explain disease prevention 5.Explain the concept of disease causation Factors associated with increase Risk of human disease Host Factors -demographic characteristics Agent Factors - characteristics of disease-causing agents - include virulence, infectivity, mode of transmission etc Environmental Factors Such as climate, pollution, geographical location, social and behavioral factors Directly or indirectly influence disease transmission 6 7  CHAIN OF INFECTION 1. Source of Infection 2. Mode of Transmission 3. Portal of Entry 4. Susceptible Host 8 Natural History of Disease  The natural history of disease -refer to the process of disease in individual over time -includes all disease related phenomena before the onset of disease (i.e stage of susceptibility)until after resolution (i.e., stage of recovery, disability or death) Subclinical Stage  Corresponds to time during the etiologic agent is present within the body but has not yet caused discernible(identifiable) signs or symptoms  Both INFECTIOUS AND NON- INFECTIOUS diseases are characterized by subclinical stages of disease  ID- incubation period  Non-ID –latent period/induction period Incubation Period  Incubation periods vary considerably according to agent-disease pairs.  Some diseases have -short incubation period e.g salmonella 6-48 hrs ; cholera 2-4 hours -intermediate e.g chickenpox 2-3 weeks -extended e.g AIDS 10 yrs Stage of Clinical Disease  begins with patient’s first symptoms and end either with recovery, disability and death.  Depending on host factors, access to health care,and the diagnostics  The onset of symptoms-not the time of diagnosis-marks the beginning of the clinical stage of disease Disease Prevention  Notonly to prevent the occurrence of disease, such as risk factor reduction, but also to arrest its progress and reduce its consequences once established PRIMA SECONDA TERTIA RY RY RY Directed towards Directed towards Directed towards stage the of the subclinical the susceptibility stage, people clinical stage who carry the agent but asymptomatic Goals Goals: -prevent : -Goals:Early - To prevent and diseas occurring of detection and minimize -reduce e incidence - prompt Reduce progression of prevalenc and severity of treatment disease e disease Exampl Example: Example: -e: Needle -Needle prick injury- -to prevent or progra exchange antiviral give treatment the progression of minimize vaccinatio m -the Screening for cervical ca n disease - Screening people with diabetic retinopathy in order to promptly treat the progression of https:// www.cdc.gov/csels/dsepd/ss1978/lesson1/se ction9.html CONCEPT OF DISEASE CAUSATION  Is an event, characteristic or condition that precedes the disease and without which the disease could not have occurred.  Exposure to microbe, chemical substance, physical trauma, radiation or other exposure.  Many diseases do not have a single cause and thus exposure to a ‘causal agent’ does not inevitably result in disease.  Although the cause of disease is always statistically associated with its occurrence a statistically association cannot be taken as a proof of cause.  A sufficient cause is a set of factors or conditions that inevitably produce disease.  The factors or conditions that form a sufficient cause are called component cause.  Component causes include host factors, agents and environmental factors.  Sometimes an event or exposure is associated with both the occurrence of the disease and another exposure which is statistically associated with the disease. This is called confounding.  A confounding variable is a factor that is significantly associated both with the occurrence of a disease in population and with one of its causes or determinants.  A determinant is an attribute or circumstance that affects the liability of an individual to be exposed to or, when exposed, to develop disease. Case definition A case definition is a set of standard criteria for deciding whether a person has a particular disease or other health-related condition.  By using standard case definition we ensure that every case is diagnosed in the same way, regardless of when or where it occurred or who identified it.  A case definition may have several sets of criteria, depending on how certain the diagnosis is. Types of risk Absolute : incidence of disease in any defined population.  Relative : ratio of the incidence rate in the exposed group to the incidence rate in the non-exposed group.  Attributable : difference between the incidence rates in the exposed and non-exposed groups. HILLS CRITERIA FOR DISEASE CAUSATION  Strength of association  Time sequence  Distribution of the disease  Gradient  Consistency  Specificity  Biological plausibility  Experimental Models  Preventive trials  Strength of association  The stronger the association the more likely it is to be causal.  This is usually measured in terms of relative risk.  Time sequence  If an agent causes a disease then exposure must always precede its onset.  Distribution of the disease  Thespatial or geographical distribution of the disease should be similar to that of the suspected causal agent.  Gradient  The incidence of disease should correlate with the amount and duration of exposure to the suspected cause (population dose–response).  Consistency  The same association between a disease and a suspected causal agent should be found in studies of different populations.  Failure to find consistency may be explained by differences in study design.  Caution is needed before rejecting a causal hypothesis in such circumstances.  Specificity  Specificitywas amongst the criteria that could be used to distinguish chance associations from cause that a single true cause should lead to a single effect, not multiple effects.  Biological plausibility  The association between the disease and exposure to the suspected causal agent should be consistent with the known biological activity of the suspected agent.  Sometimes an association is observed before the biological process is identified.  The fact that there is no known biological explanation for an association should not on its own lead to rejection of a hypothesis.  Experimental Models  The disease can be reproduced in experimental models with animals.  The fact that exposure to an agent can produce a disease in animals similar to that seen in humans gives credence to a causal hypothesis.  However, failure to produce the disease amongst animals cannot be used as evidence to reject the hypothesis.  Preventive trials  Control or removal of the suspected agent results in decreased incidence of disease.  REFERENC ES  Smart, B. (2016). Concepts of Causation in the Philosophy of Disease. In Concepts and Causes in the Philosophy of Disease (pp. 44-69). Palgrave Pivot, London.  https://www.cdc.gov/csels/dsepd/ss1978/lesson1/sectio n 8.html  Rothman KJ,GreenlandS.Modem Epidemiology,2nd Edition.Philadelphia:Lippincot-Raven,1998.  MacMahonB,Trichopoulos D.Epidemiology. Principles &Methods,2nd Edition.Little Brown and Co,1996.  THANK YOU 28