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Outbreak Investigation

Outbreak Investigation. Marshall Tulloch-Reid Epidemiology Research Unit Tropical Medicine Research Institute. Objectives. Define disease epidemic Understand factors that may influence the prevalence of disease in a community

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Outbreak Investigation

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  1. Outbreak Investigation Marshall Tulloch-Reid Epidemiology Research Unit Tropical Medicine Research Institute

  2. Objectives • Define disease epidemic • Understand factors that may influence the prevalence of disease in a community • Understand the steps involved in the evaluation of any disease outbreak

  3. Outbreak Investigation • Epidemiology usually thought of as related to infectious conditions, perhaps because early epidemiology was so closely related to infectious epidemics. • Similarly disease outbreaks need not result from infectious agents but can also result from environmental toxins and other factors such as lifestyle.

  4. Outbreak Investigation • Outbreak / Epidemic – a rise in the incidence rate or prevalence of a disease or syndrome. • Can be infectious or non-infectious • Can occur suddenly or over a long period of time

  5. Outbreak Investigation • The prevalence of an infectious disease in a population is determined by the number of people who are susceptible and at risk and the number who are not susceptible or immune • Herd Immunity –resistance of a group to an attack by a disease to which a large proportion of its members are immune

  6. Outbreak Investigation • Incubation period -The interval from receipt of the infection to the time of onset of clinical illness • Carrier – a person or animal that harbors a specific infectious agent in the absence of discernable clinical disease and serves as a potential source of infection

  7. Outbreak Investigation • A standard approach to investigation can be employed regardless of disease or cause, though study design and information collected may vary according to the outbreak under investigation. • Steps need not be taken in sequence and several efforts may occur simultaneously

  8. Outbreak Investigation 1. Verify the diagnosis • Obtain detailed information from the person(s) making initial report • Review / repeat case history, laboratory tests and related investigations to verify diagnosis • Visit site

  9. Outbreak Investigation 2. Confirm the epidemic • Is there really a change in the prevalence or the incidence of this condition? • The observed cases should be greater than expected

  10. Outbreak Investigation 2. Confirm the epidemic Are there other explanations for this change? • Change in reporting practices • Increased interest in disease by health care providers or by the public • New diagnostic capabilities • Laboratory error

  11. Typhoid Fever, Jamaica • Typhoid Fever is endemic to Jamaica and Westmoreland in particular. • Usually 0 – 10 cases are reported each year from the parish between 1980 – 1988 • 27 typhoid cases were confirmed (culture positive) with symptom onset in July 1990

  12. Outbreak Investigation 3. Identify Infected persons and their characteristics • Formulate the case definition • Collect demographic information on the persons affected • Look for additional cases in community

  13. Outbreak Investigation 3. Identify Infected persons and their characteristics Case Definition The case definition is usually simple and chosen to identify most people who may have to condition of interest.

  14. Typhoid Fever, Jamaica CASE DEFINITION Vomiting Diarrhea Fever

  15. Outbreak Investigation 3. Identify Infected persons and their characteristics Examine the characteristics of these subjects by person, place and time.

  16. Outbreak Investigation For each suspected case determine - Person – Who is affected Place – Where are they Time – When are they affected Symptoms - What sort of symptoms

  17. Outbreak Investigation Person • Age • Sex • Religion • Occupation • Other characteristics

  18. Outbreak Investigation Place – may require that distribution be plotted on a map • Unique location • Unique event • A defined community • A specific distribution area

  19. Outbreak Investigation Time • Usually number of cases, by date of onset, is plotted against time. • Can help identify source of infection, pattern of spread and any evidence of recurrence

  20. Outbreak Investigation Symptoms • What are the presenting symptoms? • Which symptoms are most common?

  21. Outbreak Investigation 3. Identify Infected persons and their characteristics Activate case finding measures • Health Care Professionals • Hospitals, Laboratories • Schools, Neighborhoods, Media

  22. Typhoid Epidemic, Jamaica • Public health team was assembled with staff from neighboring parishes • Senior Medical Officer (Cornwall) put in charge of investigation. • Principal Medical Officer (Epidemiology), 2 advisors from CAREC, Epidemiologist from CDC

  23. Outbreak Investigation Typhoid Epidemic, Jamaica • There were 2 outbreaks of typhoid fever • Confined to the parish Westmoreland • Most were under 25; 58% M 42% F • Cases were receiving water from the Roaring River water distribution system

  24. Typhoid Cases by Week of Onset, Westmoreland Jamaica, 1990

  25. Fever 99% Diarrhea 48% Constipation 9% Vomiting 30% Headache 45% Malaise 37% Abdominal pain 30% Anorexia 15% Typhoid Epidemic, JamaicaSymptoms

  26. Outbreak Investigation 4. Define and investigate population at risk • Population at risk is often defined by the disease under investigation and the demographic data that has been collected

  27. Typhoid Epidemic, Jamaica Persons at risk were • Men and women in Westmoreland • No access to potable water • Received water from the Roaring River water distribution scheme • Poor sanitation in homes • Vendors in the market / itinerant vendors

  28. Outbreak Investigation 5. Investigate the disease • Study design and methods will be dependent on the disease under consideration and may change over time

  29. Outbreak Investigation 5. Investigate the disease • Assemble a team for evaluation • Decide on study type • Design questionnaire, decide on laboratory tests and investigations • Decide on means of administering questionnaire / collecting specimens

  30. Outbreak Investigation Study type • Dependent on disease under investigation • Case control studies are very useful • Fast, cheap and provide rapid results

  31. Outbreak Investigation

  32. Outbreak Investigation • Odds Ratio = Odds of Disease in persons exposed divided by the odds of disease in the persons who were not exposed = (115/89)/(5/36) = 1.29 / 0.14 = 9.2

  33. Outbreak Investigation Attack Rate (incidence proportion) Number of persons from the population who develop disease = 120 / 254 = 0.47 Attack Rate = 47%

  34. Typhoid Fever, Jamaica • All suspected patients were interviewed • Stool samples for S.Typhi collected as well as blood tests for the antigen • Environmental testing in rivers and sewage for the organism

  35. Typhoid Fever, Jamaica Case Control Study • Standard questionnaire was administered to persons with typhoid fever • Controls were one, two or three well neighbors matched for age and sex and similar questionnaire also administered

  36. Typhoid Fever, Jamaica Case Control Study – 1 • Exposure to river water (washing dishes, bathing or drinking) - OR (95% CI) = 3.5 (1.4, 9.0) • Buying at Savannah-la-Mar Market • Eating and cooking at home protective

  37. Typhoid Fever, Jamaica Case-control study – 2 Piped water at home [OR = 2.4(95% CI = 1.3 - 4.1)] Purchasing 2 or more items from vendors [(OR = 1.9 (1.1 – 3.2)] Eating ice cream cake [OR =2.2 (1.3 -3.7)]

  38. Typhoid Fever - Jamaica • Case control study – 3 Only confirmed cases –onset October, 1990 • Not boiling water during September, 1990 [OR = 3.7 (1.2 – 12)] • Storage of water, refrigeration, hand washing and source of water were not associated with illness

  39. Outbreak Investigation 6. Manage the Epidemic • Allocate resources • Treat cases • Implement control measures – education, vaccination, remove exposure

  40. Typhoid Fever, Jamaica • Proper monitoring of water supply • Repair of sewage system in Hospital • Closure and relocation of market • Repair of market sanitary facilities • Registration and education of food vendors • Public Education program – boil drinking water, use of bleach, knowledge of typhoid fever and its transmission

  41. Outbreak Investigation 7. Write a report • It is important to document the information collected • Information can be used by other investigators or for future epidemics

  42. Typhoid Fever, Jamaica Figureoa JP, Campbell-Forrester S. Disasters in Health. Typhoid Epidemic in Jamaica, 1990. West Indian Med J 1992 April; Suppl.1:71-75

  43. Outbreak Investigation 8. Continue Surveillance • Monitor disease in community – may need to have additional notification systems in place • Design additional studies for further evaluation of the disease and its pathogenesis

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