Epidemiology Outbreak Investigation

investigation (three categories for preparation)

-know info on disease/condition -laboratory collection supplies -know laboratory collection procedures -supplies (flue outbreak, etc) -camera -telephone -journal articles/PDR/communicable disease -storage containers

consultation (three categories for preparation)

-know your role -know local contacts (media, actual site, hospitals, other health agencies) -have schedule

types of epi curves

-people are exposed to CONTINUOUSLY or INTERMITTENTLY to a harmful source -period of exposure may be brief or long

common source intermittent exposure

-exposure often results in an epi curve with irregular peaks that reflect the timing and the extent of exposure -produce irregularly jagged epidemic curves which reflect the intermittency and duration of an exposure and the number of persons exposed -small clusters or individual cases on an epicurve is spread out over a relatively long time period (gaps in curve) -example;contaminated roast beef served as an entree on Monday, sandwiches on Wednesday, and as a component of soup on Friday

comparing observed with expected

-for notifiable diseases, use health department data (annual flu surveillance) -find existing data locally (hospital discharge data, mortality statistics, cancer or birth defect) -apply rates from neighboring states/localities, etc -conduct a survey of the community to establish the background or historical level of disease

uncertainty in diagnosis (case definition)

-confirmed -probable -possible -suspect

interpreting an epidemic curve

-consider its overall shape -shape is determined by the period of time over which susceptible persons are exposed

epidemic curve

-each bar represents the number of cases of disease developing at a certain poin in time after the exposure along the vertical axis (y); the number of hours since exposure is shown along the horizontal axis (x) -purpose is to show the characteristics of an outbrak

establishing existence of an outbreak

-epidemic -most cases come to the attention of the health department in one of three ways 1) regular analysis of surveillance data 2) calls from a health care provider 3) public (interesting cases) -one of the first duties of an epidemiologist is to ensure that there is a true outbreak -compare the observed with the expected -how to determine what's expected -----compare the current numbers with the number from the previous few weeks or months or from a comparable period during the previous years

verifying the diagnosis

-establish what disease is occurring -is laboratory error the basis for the increase in diagnosed cases -review clinical findings and lab results -always summarize the clinical findings with frequency distributions -frequency distributions are useful in characterizing the spectrum of illness -developing a case definition -visit/investigate several patients with the disease -talk to patients -what you should ask patients

propagated epi curve

-progressive outbreak due to person to person transmission of disease, or indirect transmission through a vector -can last longer than common source outbreaks -may have multiple waves -relatively gradual rise in case numbers and a steeper decline -maybe more than one peak separated by distances approximately equal to the length of the incubation period -longer duration than other outbreaks -example; influenza

case definition

-standard set of criteria for deciding whether an individual should be classified as having the health condition of interest -includes clinical criteria (temperature, diarrhea, etc.) -setting of an outbreak (time, place, person) -base clinical criteria on simple and objective measures (fever > 101F) -may restrict case definition by time (within the past 2 months) -may restrict by place (residents of a certain county, employees working in a certain area of a plant, etc.) IMPORTANT -must apply criteria consistently and without bias to ALL persons under investigation -be careful not to include the exposure or risk in the case definition -example: "Disease Y among persons who were in Restaurant A" if the goal is to determine if Restaurant A is associated with the outbreak.

administration (three categories for preparation)

-travel -paper work -contact essential personnel

common source point source

-typically shows a sharp upward slope and a gradual downward slope -a common source outbreak where the period of exposure is brief, and ALL cases occur within one incubation period -steep upslope and a more gradual downslope -persons exposed to the same source over a relative brief period of time. -example; egg salad at a picnic

common source continuous exposure

-will often cause cases to rise gradually (and possible to plateau, rather than peak) -duration of exposure to a common agent is prolonged, beyond a brief period -epi curve will have a plateau instead of a peak -example; visitors to a park consume water over a period of several weeks

steps in outbreak investigation

1) prepare for feild work 2) establish the existence of an outbreak 3) verify the diagnosis 4) define and identify cases 5) perform descriptive epidemiology 6) develop hypothesis 7) evaluate hypothesis 8) reconsider/refine hypothesis 9) implement control and prevention measures 10) communicate findings

what should you ask patients (verifying the diagnosis)

1)what were their exposures before becoming ill 2)what do they think caused their illness 3) do they know anyone else with the disease 4)do they have anything in common with others who have the disease *conversations with patients help to develop hypotheses about disease etiology and spread

descriptive epidemiology

Describe The Events -number ill -number exposed -number in attendance -incidence and prevalence calculations -epidemic curve(s)

common source outbreaks

outbreak that results from the exposure of a susceptible group of people to a common agent of disease -example; salmonella outbreak from egg salad