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U~!kլSulz,; "IcR~!/DR=w@D/oGHpGpT;.m-гneI̻HR2_]$g Vױ¯W׫ǫ%y[lסc'DFXB$9"zB$~.z{_/_z _dҜw)B V5(  d  0 bClip (MS_ClipArt_Gallery.20,Microsoft Clip Gallery0bDocument Word.Document.80.Microsoft Word Document/ 0LDTimes New Roman8b8b`b0bb0  @n?" dd@  @@`` `T    ?2$2Lar,ü $2$s@8BBZq伈%Z c $@g4%d%db0b Hppp@  <4BdBd8b8b? -O =/INFECTIOUS DISEASE EPIDEMIOLOGY   OVERVIEW   !INFECTIOUS DISEASE EPIDEMIOLOGY "!I Definition - Infectious disease is an illness caused by transmission of a specific infectious agent or its toxic products from an infected person or animal to a susceptible host, either directly or indirectly. Basis of infection - Infection occurs only when the agent is encountered by a susceptible host in an environment that is favorable. - Infectious disease occur as a result of the interaction of the agent, the transmission process and the host. - These diseases can have a wide range of effects, varying from inapparent infection to severe illness and death.>J[ 4BASIC TERMINOLOGY IN INFECTIOUS DISEASE EPIDEMIOLOGY 54^Infection Disease Sub-clinical infection/disease Colonization Contamination Carrier Reservoir __ 'FACTORS INVOLVED IN INFECTION PROCESS6(Characteristics of causative agent: - morphology, structure with regard to chemical composition, reproduction method, etc. Properties related to its perpetuation: - host range, viability as a free state, growth requirement, vulnerability to therapeutic agent, etc. Those that govern contact with human host: - close proximity, repeated contact etc. Host-parasite relationship X$](k+*"FACTORS INVOLVED INDISEASE PROCESS #"Infectivity - indicated by attack rate - number of index cases number of population at risk Pathogenecity - the capability to cause disease which depends on: ~ properties of agent ~ how extensive is the tissue damage ~ toxin production - number of disease cases total number infected Virulence - the ability to produce serious illness or death - indicated by case fatality rate L W  Y$ V%MEANS OF TRANSMISSIONDirect transmission - touching - kissing - sexual intercourse - other contact ( e.g. childbirth, breast feeding, medical procedures etc) - droplet spray (e.g. sneezing, coughing) - transfusion of blood - transplacental Indirect transmission - vehicle-borne ( e.g. contaminated food, water, towels, kitchen utensils etc.) - vector-borne ( insects, animals) - air-borne, long distance ( dust, droplet nuclei) - parenteral ( injection with contaminated syringes)@3INFECTIOUS DISEASE AGENTS AND THEIR CHARACTERISTICS 43RESERVOIR OF THE AGENT  Human - Some infectious disease only occur in human. - Typhoid fever, Cholera, Shigellosis etc. Animals - Many diseases have their reservoir in animals. - Salmonellosis, Brucellosis, Listeriosis, Plague etc. Soil - Some infectious agents are found principally in the soil. - Histoplasma , Cryptococcus, Clostridium etc. Water - Several infectious agents have a natural reservoir in water. - Legionnella, Pseudomonas, etc.lZhk_THE SUSCEPTIBLE HOST  Susceptibility of host depends on resistance. Resistance refers to some total of all body mechanism which interpose as barrier to the progress of invasion or multiplication of infectious agent or damage by their toxin. Resistance prevents occurrence of infection. Resistance is both specific and non-specific. Specific resistance refers to immunity Immunity can either be specific and non specific - specific result from immunologic experience either active or passive - non-specific is innate or inherent immunity.   X INCUBATION PERIOD The time period between time of infection to the time of disease menifestation. Thought of as the time required for multiplication of the infectious agent within the the host up the the threshold point at which the infectious agent population is large enough to produce symptoms in the host. Each infectious agent has its characteristic range of incubation periods in a particular host species. The usual frequency curve of incubation time takes the form of a logarithmic normal curve. The measure of variation in incubation periods (dispersion factor), is the antilogarithm of the logarithmic standard deviation. Dispersion factors for most of the disease range from 1.2 to 1.5 and are independent of mean length of incubation.  DISEASE PATTERNS Sporadic - Occasional occurrence of diseases at irregular interval. Endemic - Occurrence of disease in a defined population at the usual rate at moderately regular interval. Hyperendemic - Gradual increase in the occurrence of a disease beyond the endemic level but not currently at epidemic proportions. Epidemic - Sudden increase in incidence of a disease above the expected. Outbreak - Epidemic of smaller magnitude or geographical extent. Pandemic - Epidemic that has spread across continents.<e yA9/ :  TYPES OF EPIDEMIC Common source epidemic - Point source - Intermittent - Continuous Propagated epidemic - Person-to-person transmissions Mixed epidemic - Common followed by propagated Other - Zoonotics.     BASIC THEORY OF EPIDEMIC  A single infected person in an otherwise susceptible population will start an epidemic only if the density of susceptibles exceed a threshold. Epidemic cannot begin a low density population. Even if begun, it cannot be sustained without an influx of susceptibles. Epidemics wax and wane as a function of the supply of susceptibles. At the end of an epidemic, the population will consists of - susceptibles below threshold density - no infectives - removals bn  8 @    POINT SOURCE OUTBREAK A subset of common source epidemic. Cases are all exposed to the same source in a brief period of time. Cases occur in clusters that are usually very closely related to time and space. The source may be infectious agent or a non-infectious agent such as toxin or chemical, contaminating foods, drinks or environment. Everyone who becomes ill develop the disease at the end of one incubation period. The epidemic curve has a log-normal distribution shape.PROPAGATED EPIDEMIC  Epidemic that spread gradually from person to person, usually growing as it spread. Transmission is usually by direct person-to-person contact. Transmission may also by vehicle-borne or vector-borne. Cases occur over more than one incubation period. The epidemic curve tend to have a successive series of peaks reflecting increasing number of cases in each generation. The epidemic usually wanes after a few generations. The shape of epidemic curve varies depending upon: - population size and composition - proportion of susceptibles - number of cases at the start of the epidemic - contact rate between infectives and susceptibles - infectivity of the disease agent - incubation period of the disease P  H   : EPIDEMIC CURVE :A histogram of cases-by-time of onset distribution. The shape of epidemic curve can give indication as to the types of epidemic. From  log-normal epidemic curve can determine: - incubation period (given exposure time is known). - exposure period ( when incubation period is known). - estimated date of common exposure and hence estimated incubation period (etiology unknown). - duration of the epidemic. INVESTIGATING AN EPIDEMIC Determine that an epidemic exists by comparing rates with previous data on the disease Establish an etiological diagnosis when possible Formulate a working hypothesis of the reservoir, and means of spread of the disease Test the hypothesis by further investigation. Measure illness rates on exposed and unexposed. Isolate agent or do serological tests if possible Institute control measures or recommend them to those in a position to institute them Measure the effectiveness of the control measures.6EPIDEMIOLOGIC TECHNIQUES IN INVESTIGATION OF EPIDEMIC  76Descriptive epidemiology: - Most frequently applied initially. - All pertinent data are collected and described according to time, place and person. - Help to generate hypothesis as to the possible etiology and probable mode of transmission. Analytic epidemiology: - To investigate relationship between cause and effect. - To evaluate risk factors and disease - Study designs include: ~ Case-cohort ~ Case-control ~ Cohort 6,  -INVESTIGATING EPIDEMIC BY CASE-CONTROL STUDY .-The investigator selects cases with the disease, and appropriate controls without the disease. Obtain data regarding post exposure to possible etiologic factors in both groups. The investigator then compares the odds of exposure between the two groups. 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