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Outbreak investigation

Pivi Rajala-Schultz, DVM, PhD

Veterinary Preventive Medicine

Mondays exercise

Scenario: You are called up to a facility

that is experiencing a disease outbreak.

Your task is to identify the source/the

cause of the outbreak and to control it.

Mondays exercise

During the class period, you will be able to

ask questions about the case in order to

solve the problem

There will be 8 persons giving out the

information you request, i.e., one person

will work with 4 groups

Mondays exercise

After getting the description of the problem,reach a consensus in your group about whichpiece of information you want to request first

Raise up your hand to get the attention of thefacilitator when you are ready to request someinfo

You can only request one piece of information ata time

You will be able to solve the problem within theclass period by requesting relevant pieces ofinformation

Mondays exercise

You might want to take a calculator along

You are more than welcome to take e.g.,

Mercks manual

Definition of an outbreak

Outbreak is an epidemic a cluster ofdisease in which the number of cases

clearly exceeds the number expected

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Types of outbreaks

A point/common source epidemic:

A sharp increase in disease occurrence

No host-to-host spread

A propagating epidemic

A gradually-building increase

Host-to-host spread

Goals of outbreak investigation To identify key determinants of disease,

i.e. risk factors

Recognize the cause, source and mode oftransmission

Goals of outbreak investigation The ultimate goal is to stop the outbreak

and to prevent it from recurring

Outbreak investigation

In principal, very similar to clinical

examination of a patient in a clinic:

History, clinical and laboratory

examinations are often used to try to

identify the cause(s) of disease

Outbreak investigation

Descriptive phase:

Determine if an outbreak exists

Define the case

Describe occurrence of disease in time

and in place

Describe the demographic patterns

Outbreak investigation

Descriptive data is compared andanalyzed

What associations exist, e.g., what riskfactors appear to be associated with thedisease ?

Calculate attack rates, risk or oddsratios

What is the probable source of theetiologic agent?

Analytical phase:

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Outbreak investigation

What are you going to do?

How do you control the outbreak? What preventive options are available?

Intervention phase:

How/why do epidemics arise?

Introduction of a pathogenic agent or toxic

material

Adequate number of exposed andsusceptible individuals

An effective means of transmission

How do epidemics arise?

Very virulent agent very contagious

Novel in the area, i.e., a new agent

No immunity in hosts (lack of herd

immunity)

High population density => lot of contacts

between individuals, increased

transmission

Where do you look for the

source of the outbreak? Method of difference

Two situation appear to be generally similar,except for the disease rate

Anything else you can find thats markedlydifferent might be the cause

Method of agreement Two situations appear to be generally

different, except for the rate of disease

Look for something similar

Looking for the source of the

outbreak

Seek the exposure associated with thehighest incidence rates for the epidemic

disease

Tools for outbreak investigation

Attack rate =

Risk ratio / odds ratio

Number of cases

Number at risk

Risk or odds of disease in exposed group

Risk or odds of disease in unexposed group

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Finding the cause

Definite diagnosis is not always necessary

for solving the problem

However, often different samples arecollected to confirm the suspected cause

Diagnostic tests

Summary: steps in an outbreak

investigation Establish criteria for defining a case

Verify that there really is an epidemic

Describe the epidemic regarding Time, place and subjects

Formulate and test your hypotheses

ACT!! (establishing a diagnosis without

any action is not enough!)

Monkeypox

A rare viral disease, mainly found in the

rainforest countries in Africa

Caused by Monkeypox virus (orthopox)

First discovered in laboratory monkeys in

1958

Also in various rodent species

Monkeypox in the US

An emerging infectious disease in North

America

In early June 2003, monkeypox reported in

several residents in the US who became ill

after having contact with sick pet prairie

dogs

Monkeypox in the US

Prairie dogs became infected from ashipment of animals from Africa to Texas

on April 9, 2003

Imported animals included Gambian giant

pouched rats, rope squirrels, dormice

Monkeypox in the US

A total of 81cases of human monkeypoxreported to CDC from WI (39), IN (22), IL

(16), MS (2), KS (1), OH (1)

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Monkeypox in the US

Spread to humans:

Through a bite or direct contact with the

infected animals blood, body fluids orlesions

Majority of the cases in the US were

exposed to prairie dogs

Monkeypox

Case definition

Clinical criteria: Rash (vesicular, macular, pustular)

Fever

Chills/sweats, headache, backache etc

Epidemiologic criteria Exposure to an exotic/wild mammalian pet

obtained after April 15, with clinical signs

Monkeypox

Case definition

Epidemiologic criteria

Exposure to an exotic/wild mammalian pet w/o clinical

signs that had been in contact with animal with

monkeypox

Exposure to suspect, probable ro confirmed human

case

Laboratory criteria

Isolation of virus

Demonstration of virus with PCR, EM, IHC

Monkeypox epidemic curve