outbreak investigation hsph infectious disease epidemiology april 29, 2015 natasha hochberg, md, mph...

Outbreak Investigation

HSPH Infectious Disease Epidemiology

April 29, 2015

Natasha Hochberg, MD, MPHBoston University School of Medicine

Boston University School of Public Health

Boston Medical Center

http://www.bu.edu/

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Fes.wikipedia.org%2Fwiki%2FBoston_Medical_Center&ei=vfY4Vb2MOoe0sASswIGADg&bvm=bv.91427555,d.cWc&psig=AFQjCNFd-7grGevz1wAH133SkTLi53nEJg&ust=1429883032806580

Recent/Current Outbreaks

• H5N2 in turkeys in United States

• Leptospirosis in Argentina

• Undiagnosed fatal blinding illness in Nigeria

• Chikungunya in the Americas

• Measles from Disney theme parks (US)

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Outbreaks are not new

Learning Objectives

• Describe real and artificial reasons for an increase in case numbers

• Report the steps taken in an outbreak investigation

• Create a case definition and understand how different case definitions serve different purposes

• Interpret epidemic curves and identify likely date of exposure

• Understand factors that affect choice of study design in an outbreak

Factors affecting outbreaks

• Changes in environment

• Agriculture/food processing

• International transport

• Changes in human behavior

• Density of populations

• Underlying medical problems

Why investigate an outbreak?

Agent-related– 1983: Is AIDS caused by an infectious agent?– 2004: How infective is SARS?

Source-related– 2003: How did monkeypox get to the Midwest?

Vehicle-related– 2001: Can anthrax spores leak out of a sealed envelope?– 2007: What widely distributed food is contaminated with Salmonella?

Control-related– 1990s: Do “safe cooking” directions actually do anything to help protect

consumers from E. coli O157:H7 infection?– 2006: Are two doses of mumps vaccine better than one?

What is an outbreak?

3389 cases of West Nile virus infection. Is this an outbreak?

• A. Yes

• B. No

• C. It depends

Is this an outbreak?

3389 cases of West Nile virus infection

• In 2002 in the United States• Compared with 149 in 1999 through 2001


2820 cases of tuberculosis

• In 1999 in Guatemala• Compared with 3059 in 1998

Definitions

Epidemic: “The occurrence of cases of an illness clearly in excess of the normal expectancy.”

Outbreak: Same as epidemic but more limited geographical area (really a political distinction)

Cluster: Aggregation of cases in a given area over a particular period without regard to whether the number of cases is more than expected


48 E. coli O157:H7 infections• In June and July 1994 in New Jersey, • Compared with 4 cases in June and July 1993

Causes of an increased number of cases

• Artificial increase

• Real increase

Source: Mead et al, Arch Internal Med, 1997:157:204-8.

Sometimes it’s not so clear

2001

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(9)

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0.5

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1.5

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2.5

2002 2003 Jan-Oct2004

Nov 04-Feb 05

Period of Lumbar Puncture

Incidence per

100,000 Person Years

Steps in outbreak investigation

• Confirm outbreak and diagnosis• Define a case• Find and count cases• Descriptive epidemiology• Generate hypotheses• Design and conduct epi study• Laboratory and environmental investigation• Implement and evaluate control measures• Communicate findings• Consider additional studies

June,1995Arizona Department of Health Services

reports to CDC:

• 19 cases of Salmonella serotype Stanley infection in Arizona residents in April and May, 1995.

• By comparison:– 1992—0 cases– 1993—1 case– 1994—1 case

Concurrent outbreaks in other states, Canada, Finland


• Confirm outbreak and diagnosis• Define a case• Find and count cases• Descriptive epidemiology• Generate hypotheses• Design and conduct epidemiologic study• Laboratory and environmental investigation• Implement and evaluate control measures• Communicate findings• Consider additional studies

Confirm outbreak and diagnosis

• No change in lab procedures at ADHS

• No known change in clinical practices in AZ or elsewhere

• No change in reporting

Define a case

• Definition always includes:– Person

• Clinical criteria

– Place– Time

• Definition sometimes includes:– Laboratory criteria– Epidemiologic criteria

By including a laboratory component to a case definition,

one wouldA. Increase sensitivity

B. Increase specificity

C. Not sure

Using Multiple Case Definitions

• Confirmed or definite– Most specific, least misclassification– Useful for analytic study

• Probable:– Symptoms without lab confirmation

• Possible– Most sensitive– Use for gauging overall impact of outbreak

What case definition would you use?

Arizona Salmonella Stanley outbreak case definition

• An illness occurring in 1995 in a person who was in Arizona for the 3 days before illness onset and in which S. Stanley was isolated.

June,1995Arizona Department of Health Services

reports to CDC:

• 19 cases of Salmonella serotype Stanley infection in Arizona residents in April and May, 1995.

• By comparison:– 1992—0 cases– 1993—1 case– 1994—1 case

Find and count cases

• During an outbreak, enhanced surveillance is needed. – Usually, active surveillance is used

• In Arizona, active surveillance included:– Letters to clinical laboratories– Telephone calls to laboratories of large hospitals– Letters to all infection control practitioners

• 22 cases identified

Epidemic Curves

• An epidemic curve (epi curve) is a graph of the number of cases by date of illness onset

• Can provide information on the:– Pattern of spread (Common source, point source,

propagated)– Magnitude– Outliers– Time trend (peak of outbreak, date of first/last case)– Exposure and/or disease incubation period

Epidemic Curves (Epi curves)

• Two-dimensional graph of the number of cases by date of illness onset

• Simple display of an epidemic's magnitude and time course

Drawing an Epi Curve- Histogram with time on X axis and number of cases on Y axis- Include title and axis labels

Date of symptom onset

Number of cases

Drawing an Epi Curve

Changing the time interval can markedly change the shape of the curve: - Intervals between ¼ to ½ of an incubation period work best at revealing the time pattern


Number of cases

Drawing an Epi Curve

Begin before the onset of the outbreak to show -the background number of cases and/or -the source case for propagated outbreaks


Number of cases

Pattern of Spread

• Point Source: A group of persons are all exposed to an infectious agent or a toxin from the same source– Exposed over a relatively brief period so everyone

who becomes ill does so within one incubation period• Common-source outbreak: Exposed from same source

– Period of exposure is prolonged– Can be intermittent

• Propagated: Transmission from person to person (“secondary transmission”)

Point Source

• Steep upslope

• More gradual decline

• Cases occur within one incubation period

Common source continuous exposure: Salmonella

Salmonella incubation period 1-3 days

Propagated outbreak: Measles

• Propagated = secondary transmission

• One case = source for subsequent cases

• Series of peaks separated by the incubation period

Assuming a 1 day incubation period,

what type of transmission pattern does this epi curve

represent?

A. Propagated

B. Common Source

C. Point source

D. Hmmm. . ..

Propagated

Common Source

Point source

Uh, yeah

0% 0%0%0%

Bonus Points

• What are some problems with this epi curve?

Pay attention to the outliers

0

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Date of Onset of Illness

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ases

Point source outbreak: Rubella

• Epi curve can be used to estimate:

• Point source/ exposure for a disease with known incubation period or

• Incubation period of disease with known date of exposure

0

1

2

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4

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6

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of

Cas

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Point source outbreak: RubellaUsual incubation period 14-21 days

17.5 days before peak of curve

Time of exposure: count back the median incubation period from the peak of the outbreak

0

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14 days before first case

Time of exposure: count back the minimum incubation period from the first case

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21 days before last case


Time of exposure: count back the maximum incubation period from the last case

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likely time of exposure


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Epidemiologic week, 1995# ca

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kDescriptive epidemiology

Arizona S. Stanley epidemic curve

Descriptive epidemiology

TIME – Epidemic curves

PERSON– Age– Sex– Race/ethnicity– Health status– Etc.

PLACE – Residence– Work/school– Travel, events– Etc.

Descriptive Epi: Line Lists

Descriptive epidemiologyArizona S. Stanley outbreak

PERSON– Age: 25 years median (2 months to 81 years)– Sex: 68% female– Race: 91% white

PLACE– Cases in the 3 most populous counties– No obvious geographic clustering


• Confirm outbreak and diagnosis• Define a case• Find and count cases• Descriptive epidemiology• Generate hypotheses• Test hypotheses• Laboratory and environmental investigation• Implement and evaluate control measures• Communicate findings• Consider additional studies

Generate hypotheses

Sources of hypotheses regarding vehicles of exposure:

• Descriptive epidemiology• Existing scientific knowledge--“the usual suspects”• Open-ended patient interviews

– 4 patients, all ate lots of salad– 1 vegetarian– 3 ate alfalfa sprouts– 1 with animal contact

• Outliers

Epidemiologic Study: Outcome & Determinant

• Outcome (case definition)– Laboratory test?

– Clinical symptoms?

– Confirmed, probable, possible?

– May have small number of cases

• Determinant (aka exposure): characteristic upon which the outcome depends– Can be a behavior (e.g. food consumption, injection) or

environmental exposure (e.g. water, air)

– Often several suspected possible determinants (e.g. food items in the case of common vehicle; contacts in hospital setting)

What is the determinant?

Think about what you ate for lunch two weeks ago. . .

Cohort vs. Case-Control

• Retrospective Cohort – – Retrospective – study begins after cases have occurred– Source population defined and enumerated – Good if outbreak suspected to have occurred at a specific event

(party) or place (cruise ship)– Can try to include all persons or selection

• Case control – – Include persons based on their disease status– For large event (state fair), community-wide or uncommon

diseases reported from a population– Include all or select group of cases– Most common study type in outbreaks

Arizona case-control study

• Cases (outcome): A person with onset of illness in which a clinical specimen yielded S. Stanley from routine serotyping of Salmonella isolates submitted to the Arizona Health Dept laboratory

• Controls:

– 2 per case, matched on neighborhood and age 1 year if 5 years of age to 20 years if >60 years of age

– Found by systematic telephone dialing (adding or subtracting to case’s telephone number)

– Excluded if had symptoms since April 25

– Interviewed about most recent same 3 days of week as case

• Possible determinants: Medical history, 24 specific foods, environmental exposures

– In 3 days before illness for cases

– Most recent same 3 days of the week for controls

Arizona study results

Exposure Matched OR p-value

Alfalfa sprouts 12.00 0.02

Applesauce 0.50 0.87

Chicken 1.40 0.91

Egg 0.74 0.84

Lettuce 1.00 0.74

Onion 1.17 1.00

Roast beef 0.33 0.50

Turkey 1.85 0.43

Reptile contact 1.25 0.91

And many others, none associated with illness

Other evidence

Investigations in other outbreak sites

– Independent case-control studies conducted in Michigan and Finland

– Both found associations with alfalfa sprouts and only alfalfa sprouts

Case interviews in other states

– 95 people interviewed in 16 states

– 82% said they definitely or probably ate alfalfa sprouts before illness

Other evidence cont’dTraceback of alfalfa seeds in the US

Other evidence cont’dTraceback of alfalfa seeds for 50 patients in 6

states to both a US and Finnish supplier

Laboratory Investigation

• Can include:– 1. identification of agent – 2. environmental testing – 3. molecular analysis of isolates from cases

and environment

Molecular subtyping of S. Stanley isolates (Pulsed Field Gel Electrophoresis)

Outbreak isolates Non-outbreak isolates

Environmental Investigation

• Can include: traceback, culturing• Can support epidemiologic findings, but

– Samples may not be available from time of exposure– Where to sample may not be straightforward– Finding an organism in an environmental sample

does not always mean that was the source– Some organisms are hard to culture or test for– Patient may have contaminated environment, rather

than vice-versa

Implement control measures

• Wash sprouts?

• Decontaminate seeds?

Communicate findings

Volume 175 · Number 4 April 1997

Sprouts are now one of the “usual suspects”– Recognized as vehicle for many foodborne outbreaks– Salmonella, E. coli O157:H7, and other agents– Alfalfa, mung, radish, other sprouts

Consider additional studies

Jaquette et al, Applied Environmental Microbiol 1996;62:2212-5

Growth of S. Stanley on alfalfa seeds during soaking (1), germination (2), and sprouting (3), and survival during storage (4)

Summary

• Describe real and artificial reasons for an increase in case numbers– Artificial: __________________

• Report the steps taken in an outbreak investigation

• Case definition consists of ________________________________

Summary• Epi curves:

– Point source: All cases occur within _________________

– Propagated: See evidence of person-person transmission from ____________________

• Can count back to find likely date of exposure in point source outbreak:– From peak of epidemic, count back _____ – From first case, count back ________– From last case, count back _________

• Considered study design types

Thank You

outbreak investigation hsph infectious disease epidemiology april 29, 2015 natasha hochberg, md, mph...

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