Part III: Infectious Disease Epidemiology. Epidemics & Their Control. Epidemiological Triad for Infectious Disease. Agent. Environment. Host. Notifiable Diseases.
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Epidemiological Triad for Infectious Disease Agent Environment Host
Notifiable Diseases Acute Flaccid ParalysisAIDSAmoebiasisAnthraxBotulismBrucellosisCampylobacteriosisChancroidChickenpoxChlamydia, GenitalCholera Creutzfeldt-Jakob Disease Cryptosporidiosis Cyclosporiasis DiphtheriaGiardiasisGonorrheaGonococcal Ophthalmia Neonatorum Group B Streptococcal Disease of the Newborn Hantavirus Pulmonary SyndromeHepatitis AHepatitis BHepatitis CHepatitis Non-A, Non-B Human Immonodeficiency Virus Influenza,Laboratory-ConfirmedInvasive Haemophilus influenzae type b Disease Invasive Group A Streptococcal Disease Invasive Meningococcal DiseaseInvasive Pneumococcal DiseaseLegionellosisLeprosyListeriosis (all types)MalariaMeaslesMeningitis, PneumococcalMeningitis, Other BacterialMeningitis, ViralMumpsParatyphoidPertussisPlague* Poliomyelitis RabiesRubellaRubella, CongenitalSalmonellosisShigellosisSmallpox Syphilis, All Syphilis, CongenitalSyphilis, Early LatentSyphilis, Early Symptomatic (Primary and Secondary)Syphilis, OtherTetanusTuberculosisTularemia TrichinosisTyphoidVerotoxigenic E. coli1990 -Viral Hemorrhagic Fevers (Crimean Congo, Ebola, Lassa, Margurg) West Nile Virus Asymptomatic Infection West Nile Virus Fever West Nile Virus Neurological Syndromes West Nile Virus Unclassified/ Unspecified Yellow Fever
Emerging communicable disease • Cryptosporidium parvum 1976 • Ebola virus 1976 • Legionella 1977 • Campylobacter jejuni 1977 • E. Coli 0157:H7 1982 • HIV 1983 • Hepatitis C 1989 • nv CJD 1996 • Avian influenza ( Type A H5N1) 1997 • WNV 1999 • SARS 2003 • ……… Pandemic influenza???
Environment Agent Reservoir Contact (Host) Chain of Infection
Clinical Spectrum Severe Moderate Mild subclinical Iceberg of infectious disease Symptomatic disease Asymptomatic disease
Course of Disease Incubation period Illness Latent Period Infectious Infected
Immunity • Active • infection • immunization • Passive • immunoglobulin • intrauterine • Herd Immunity
Portals of Entry for Infection • Respiratory tract • Genitourinary tract • Gastrointestinal tract • Skin / Mucous membrane • Transplacental • Parenteral( percutaneaous , via blood)
Direct Direct contact (bite,soil,touch) Droplet spread Vertical transmission (through placenta) Indirect Airborne (droplet nuclei , dust particles) Vehicle borne (food, water) Vector borne ( mosquito) Mechanical ( flies) Modes of Transmission
Opportunities for interrupting transmission Quarantine Vehicle control Treatment Physical protection Infection control Improve general environment Chemoprophylaxis Immunization Isolation Herd immunity
Opportunities for interrupting transmission • Reservoir • Infection control measures,treatment,isolation • Susceptible contact • chemoprophylaxis, immunization (active and /or passive),quarantine , physical protection • Route of transmission • food preparation , water purification, vector control • Agent • Infection control measures, treatment
Epidemics • Epidemics are temporary increases in the incidence of disease in populations • E.g. Infectious Disease- SARS, West Nile Virus • E.g. Non-infectious Disease- Asthma deaths in the 1960s associated with increased use of pressurized aerosol broncholdilators • Increases in incidence are usually significantly above the level expected from past experience in the same population
Epidemics and Outbreaks The occurrence of cases of an illness clearly in excess of expectancy Examples of recent epidemics: • HIV / AIDS- Retrovirus; • Ebola- African filovirus via monkey; causes viral hemorrhagic fevers (VHF); often fatal; • Hanta virus (US)- Bunyavirus via rodents; causes severe infections of the lungs and kidneys; • Lyme disease (US)- Borrellia burgdorferi bacteria via tick bite; symptoms include: fatigue, fever, stiff muscles/joints, meningitis, arrhythmias, arthritis.
Definitions • Primary or index case- the first case (or group of cases) arising from the introduction of an agent in to a community • Secondary case- people who acquire infection from the primary case(s) • Incubation period- the time interval between infection of an individual and the onset of symptoms • Generation time- the time interval between the onset of primary and secondary cases • Derived infection- this is an infection arising by direction transmission from an infected contact
Types of Epidemic • There are two main types of epidemic: common source and propagated • Point or Common Source Epidemics-result from the exposure of a group of people to the same source of infection or noxious substance. Continuous exposure or intermittent exposure of the population to the causal agent produces a more extended and irregular epidemic curve. • E.g. outbreak of Salmonella typhimurium food poisoning amongst delegates ate a medical conference • E.g. outbreak of respiratory disease due to adulterated cooking oil in Spain
Propagated (Person-Person) Epidemics • Propagated epidemics are due to the transmission of infectious agent from one person to another. The epidemic curve usually shows a gradual rise and decline, often with further waves as each successive generation of cases infects a new generation. • E.g. An outbreak of measles occurred in a primary school. After two index cases , there were two epidemic waves at approximately 10-14 days intervals. The outbreak was modified by those vaccinated. The attack rate among those unvaccinated was 86%.
Investigation of an Outbreak • Stages In Investigation • Descriptive enquiries into the facts of the outbreak • Investigative reservoirs and vehicles of infection • Analysis of the data collected • Formulation of a causal hypothesis • Testing its validity in the control of the outbreak
Descriptive enquiries • Verify the diagnosis by clinical and laboratory investigations • Verify the existence of the epidemic by comparison with previous incidence of the disease in the same population • Compile a list of all cases • Investigate patients and others who might be involved in the case (i.e. age, sex, occupation, address etc.) • Ensure all clinical and laboratory investigations required to confirm the identity of the infection in patients were carried out
Investigate reservoirs and vehicle of infection • Human- an epidemic may originate from an individual who a minor clinical episode or is a carrier of disease • Animal- enquire about the contacts patients may have had with sick animals or animal products • Environment- investigate sources of foods consumed by individuals and the circumstances of their production, storage, preservation and preparation. Arrange for laboratory examination of food, water supplies and other environmental sources and typing of any organisms that are isolated
Analysis of the data collected • Plot of the epidemic curve- this may give some clue to the mode of spread and probable time of initial exposure • Plot the cases on a map- this will detect clustering and the distribution of cases • Analyze the incidence rates in different groups- for example by age, sex or occupation. Attack rates must be calculated among both exposed and non-exposed • Look for a quantitative relationship- this may exist between the degree of exposure (or dose) and attack rate
Formulation of a causal hypothesis • Factors for hypothesis: • The properties of the agent, it’s reservoirs, favored vehicles and the nature of illness • The probable source and route of transmission • Time and duration of exposure of the patients to the agent in relation to the onset of their illness • Attack rates of the different sub-groups of the population at risk
Testing validity in the control of the outbreak • Seek support for the causal hypothesis by further investigation of cases, if necessary, to confirm the proposed explanation of their illness • Implement appropriate control measures on the assumption that the hypothesis is correct and monitor their success in reducing the incidence of further cases
Epidemic Curves • # cases by time of onset • Shape of the curve gives you clues: • Agent known: use incubation period to look back at exposure • Agent unknown (but common event likely): postulate agent by determining the incubation period • Draw the curve relative to specific sites/groups
Salmonellosis in passengers on a flight from London to the US by time of onset, March 13-14, 1984 Supper 4 a.m. Index case | | Hours since exposure
Epidemic Curve:Propagated or Person-Person Tertiary Cases Secondary Cases Index Case Days since exposure
Occurrence/Cessation of an Outbreak • There are numerous conditions that might change and precipitate an outbreak, although all lead to an increase in the number of exposed and susceptible persons required for an outbreak. Some include: • The new appearance of or sudden increase in an infectious agent or toxic material • The arrival of susceptible to an environment which has an endemic pathogen • The introduction of an effective route of transmission from source to susceptible (e.g. climatic change and malaria)
Question 1. The quarantine or surveillance period prescribed for a specific infectious disease is based upon: (a) Incubation period of the disease (b) Epidemic cycle of the disease (c) Period of infectivity of the disease (d) Normal maximum duration of acute symptoms (e) Proportion of susceptibles in the contact population
Question 1. Answer (a) Incubation period of the disease
Question 2. The isolation period prescribed for a specific infectious disease is based upon: (a) Incubation period of the disease (b) Epidemic cycle of the disease (c) Period of infectivity of the disease (d) Normal maximum duration of acute symptoms (e) Proportion of susceptibles in contact population
Question 2. Answer (c) Period of infectivity of the disease
Question 3. When is the isolation of infectious disease cases most useful as a method of preventing new cases? • During the period before the onset of clinical disease • When the ratio of unapparent to apparent cases is high • During period of greatest communicability occurs after onset of the disease • Isolating cases of infectious disease is never a useful method of preventing new cases • Isolating cases of infectious disease is always a useful method of preventing new cases
Question 3. Answer (C) During period of greatest communicability occurs after onset of the disease
Question 4. An outbreak of measles occurred in an elementary school with an enrolment of 300 pupils. During October and November, 72 pupils in the school were absent with measles. (a) Compute the attack rate for October and November The 72 pupils with measles had a total of 100 brothers and sisters living at home. Of the siblings, 20 subsequently developed measles during October through December. (b) Compute the secondary attack rate among the siblings.
Question 4. Answer • 24% • 20%
Question 5. With one exception, all the following diseases are notifiable in Saskatchewan. The one exception is: (a) HIV infection (b) Gonorrhea (c) Non‑gonococcal urethritis (d) Chancroid (e) Chlamydia
Question 5. Answer (c) Non‑gonococcal urethritis
Question 6. In Canada, which of the following methods of transmission apply BEST to each of the four diseases mentioned below? (a) Person‑to‑person (b) Foodborne (c) Airborne (d) Waterborne (e) Bloodborne ____ salmonellosis ____ hepatitis A ____ psittacosis ____ hepatitis B
Question 6. Answer B salmonellosis A hepatitis A C psittacosis E hepatitis B
Question 7. The Culex tarsalis mosquito is the vector in: (a) Malaria (b) Typhus (c) Western equine encephalitis (d) Yellow fever
Question 7. Answer (c) Western equine encephalitis
Question 8. The A.I.D.S. (Acquired Immune Deficiency Syndrome) is due to a HIV infection. The mode of transmission for HIV infection is similar to: (a) Hepatitis A (b) Tuberculosis (c) Typhoid (d) Hepatitis B (e) None of these
Question 8. Answer (d) Hepatitis B
Question 9. Notification of infectious disease is the duty of: (a) The attending physician (b) A teacher or principal (c) A nurse providing professional services (d) The manager of a medical laboratory (e) All of the above