Epid 600 Class 13 Outbreaks

8/14/2019 Epid 600 Class 13 Outbreaks

1/48


2/48

An illness due to a specific infectious agent or its toxic

products that arises through transmission of that agent or

its products from an infected person, animal or inanimate

reservoir to a susceptible host; either directly or indirectly

through an intermediate plant or animal host, vector or the

inanimate environment

Infectious disease

Benensen AS, editor. Control of Communicable Diseases Manual. Sixteenth Edition, 1995. 2


3/48

Dynamics of disease transmission

Human disease results from interaction between the host, agent andthe environment. A vector may be involved in transmission.

Host susceptibility to the agent is determined by a variety of factors,including genetic background, nutritional status, vaccination, priorexposure, context

AGENT

HOST

VECTOR

ENVIRONMENT

Epidemiologic

Triad

3


4/48

Factors associated with increased risk of

human disease

Host Characteristics Agent Environmental Factors

Age Biologic (Bacteria, viruses) Temperature

Sex Chemical (Poison, smoke) Humidity

Race Physical (Trauma, radiation) AltitudeOccupation Nutritional (Lack, excess) Crowding

Marital Status Housing

Genetics Neighborhood

Previous Diseases Water

Immune Status Food

Air Pollution

4


5/48

The potential for a given agent to cause an outbreak depends

on the characteristics of the agent, including the mode of

transmissionof the agent

Two basic modes of transmission

Direct

Indirect

Certain diseases can be transmitted directly or indirectly

Modes of disease transmission

5


6/48

In an infectious setting, immediate and direct transfer of an

agent to a host by an infected person or animal

Touching, biting, or sexual intercourse are classic examples

Measles virus: airborne by droplet spread or direct contactwith nasal/throat secretions of infected persons

In a noninfectious setting, the host may have direct contact

with the agent in the environmentChildren ingesting lead paint from playground equipment

Direct mode of disease transmission

6


7/48

Vehicle-borne

Transmission through contaminated inanimate objects (toys, food,

water, surgical utensils, or biological products such as blood, tissues

or organs)

Vector-borne

Transmission through simple contamination by animal or arthropod

vectors or their actual penetration of the skin or mucous membranes

Airborne

Transmission occurs when microbial, particulate, or chemical agents

are aerosolized and remain suspended in air for long periods of time

Indirect mode of disease transmission

7


8/48

Interval from receipt of infection to the time of onset of clinical

illness (signs and symptoms)

Different diseases have different incubation periods

No precise incubation period but a range is characteristic for

a disease

What accounts for this delay?

Time needed for the pathogen to replicate to the critical

mass necessary for clinical diseaseSite in the body at which the pathogen replicates

Dose of the infectious agent received at time of infection

Incubation period

8


9/48

The spectrum of severity varies by disease:

1. Exposure, No infection2. Carrier - Individual harbors the pathogen but does not show

evidence of clinical illness; a potential source of infection

(can transmit the agent)

3. Subclinical Infection - Disease that is not clinically apparent; leadsto immunity, carrier, or non-immunity

4.

Clinical Infection - Apparent disease characterized by signs andsymptoms; results in immunity, carrier, non-immunity, or severe

consequences such as death

Outcomes of exposure to an agent

9


10/48

Endemic - The habitual presence (or usual occurrence) of a disease

within a given geographic area

Epidemic - The occurrence of an infectious disease clearly in excess

of normal expectancy, and generated

from a common or propagated sourcePandemic - A worldwide epidemic affecting an exceptionally high

proportion of the global population

Endemic

Epidemic

Time

Number

of Casesof

Disease

Endemic, epidemic, pandemic

10


11/48

Deaths in Greater London;

December 1 15, 1952

NumberofDeaths

December

1

1.000

800

600

400

200

0

Period of Dense Fog

2 3 4 5 6 7 8 9 10 11 12 13 14 15

11


12/48

Disease Outbreaks

Typically, sudden and rapid increase in the number of cases of a

disease in a population

Common Source

Cases are limited to those who share a common exposure

Food-borne, water

Propagated

Disease often passed from one individual to another

Measles, STDs

12


13/48

Essential Steps in an Outbreak

Investigation

13


14/48

Steps of an Outbreak Investigation

1. Establish the existence of an outbreak2. Verify the diagnosis3. Define and identify cases4. Describe and orient the data in terms of person, place

and time

5. Develop hypotheses6. Evaluate hypotheses7. Refine hypotheses and carry out additional studies8. Implement control and prevention measures9. Communicate findings

14


15/48

Before you decide whether an outbreak exists, you must first

determine the expected or usual number of cases for the given

area and time

Step 1: Establish the existence of an

outbreak

15


16/48

Data sources

Health department surveillance records for a notifiable

disease

Sources such as hospital discharge records, mortalityrecords and cancer or birth defect registries

for other diseases and conditions

If local data is not available, make estimates using data

from neighboring states or national data


outbreak

16


17/48

Whether or not an outbreak is investigated or control measures are

implemented is not strictly tied to verifying that an epidemic exists

Other factors may come into play, including:

Severity of the illness

Potential for spread

Political considerations

Public concern and pressure from community

Availability of resources


outbreak

17


18/48

Public Health Surveillance

The ongoing and systematic collection, analysis, and

interpretation of outcome-specific data for use in the

planning, implementation, and evaluation of public healthpractice.

How do we know when we have an

excess over what is expected?

Thacker, Berkleman. Epidemiologic Reviews 1988;10:164-90 18


19/48

Disease for which regular, frequent, and timely information

regarding individual cases is considered necessary for the

prevention and control of disease

Notifiable disease

19


20/48

Two goals in verifying a diagnosis

Ensure that the problem has been properly diagnosed

Ensure that the outbreak really is what it has been reported to be

Review clinical findings and laboratory results for affected people

Visit or talk to several of the people who became ill

For outbreaks involving infectious or toxic chemical agents, be certain

that the increase in diagnosed cases is not the result of a mistake inthe laboratory.

Step 2: Verify the diagnosis

20


21/48

Establish a case definition- a standard set of criteria for deciding

whether a person should be classified as having the disease under

study

In many outbreaks, a working definition of the disease syndrome

must be drawn up that will permit the identification and reporting of

cases

As the investigation proceeds and the source, mode of transmission

and/or etiologic agent becomes better known, you can modify the

working definition

Step 3: Define and identify cases

21


22/48


A case definition includes four components

1. Clinical information about the disease2. Characteristics about the people who are affected

(person)

3. Information about the location (place)4. A specification of time during which the outbreak

occurred (time)

22


23/48

To increase sensitivity & specificity of reporting, we use three classifications ofcases that reflect the degree of certainty regarding diagnosis:

1. Confirmed

2. Probable

3. Possible

The case definition is used to actively search for more cases beyond the earlycases and the ones that presented themselves.

Confirmed Case Probable Case Possible Case

LaboratoryVerification

Clinical

Features

+

+ ++ +


23


24/48

The following information should be collected from every affected

person in an outbreak:

1. Identifying information - name, address, phone

2. Demographic information - e.g., age, sex, race, occupation

3. Risk factor information

4. Clinical information

Verify the case definition has been met for every case

Date of onset of clinical symptoms to create an epidemic curve


24


25/48

The first cases to be recognized are usually only a small proportion of

the total number

To identify other cases, use as many sources possible

Passive Surveillance - Relies on routine notifications by healthcare

personnel (recall Notifiable Diseases)

Active Surveillance - Involves regular outreach to potential reporters

to stimulate reporting of specific conditions; investigators are sent tothe afflicted area to collect more information

Contact physician offices, hospitals, schools to find persons with

similar symptoms or illnesses

Send out a letter, telephone or visit the facilities to collect information


25


26/48


27/48

The time course of an epidemic is shown by the distribution of the

times of onset of the disease, called the Epidemic Curve

Graph of the number of cases of the health event by their date of

onset

Provides a simple visual display of the magnitude and time trend of

the outbreak

May stratify epidemic curves by place (residence, work, school, etc.)

or by personal traits (age, gender, race, etc.) to assess whether time

of onset varies in relation to place or person characteristics

Step 4: Describe and orient the data in

terms of time

27


28/48

Assessment of the outbreak by place provides

Information on the geographic extent of the problem

A spot map indicating place of occurrence of cases may

show clusters or patterns that provide clues to the natureand source of the outbreak

Patterns reflecting water supply, wind currents, or

proximity to a restaurant, swimming pool, school room or

workplace

If the size of overall population varies between

comparison areas, a spot map of the area may be

misleading because it only shows number of cases


terms of place

28


29/48

Examine risks in subgroups of the affected population

according to personal characteristics, as well as

interaction between characteristics

Age, race, sex, occupation, social group, medical status

Characterizing an outbreak by person helps to determine

which subgroups of the population are at risk


terms of person

29


30/48

Examples of epidemic curves

30


31/48

Though we generate hypotheses from the beginning of

the outbreak, at this point, the hypotheses are sharpened

and more accurately focused.

Use existing knowledge (if any) on the disease, or find

analogies to diseases of known etiologyHypotheses should address

Source of the agent

Mode of transmission

Exposures associated with disease and should beproposed in a way that can be tested

Step 5: Develop hypotheses

31


32/48

Generally, after a hypothesis is formulated, one should be able to

show that

All additional cases, lab data, and epidemiologic evidence are

consistent with the initial hypothesis; and

No other hypothesis fits the data as well

Observations that add weight to validity

The greater the degree of exposure (or higher dosage of the

pathogen), the higher the incidence of disease

Higher incidence of disease in the presence of one risk factor

relative to another factor

Step 6: Evaluate hypotheses

32


33/48

Reminder....attack rate

An attack rate is the proportion of a well-defined

population that develops illness over a limited period of

time, such as during an epidemic or outbreak

Useful for comparing the risk of disease in groups with

different exposures

Remember..an attack rate is an incidence proportion

(even though it is called a rate)

Often expressed as a percent

33


34/48

Attack rate

Attack Rate = Number of new cases occurring in a given time period

Population at risk at the start of the time period

= Number of people at risk who develop a certain illness

Total number of people at risk

34


35/48

Calculating an Attack Rate in a

food-borne outbreak

In a foodborne outbreak occurring among people attending a social

function or common geographical site

Calculate an attack rate for people who ate a particular item

(exposed) and an attack rate for those who did not eat the item

(unexposed)

The attack rate is calculated by dividing the number who

became ill and consumed the item by the total number

of people who consumed that item

35


36/48

Identifying the source of an outbreak

Look for an item with

A high attack rate among those exposed

AND

A low attack rate among those not exposed (so the ratio of attack

rates for the two groups is high)

Ideally, most of the people who became ill should have been

exposed to the proposed agent so that the exposure could explain

most, if not all, of the cases.

36


37/48

Additional epidemiologic studies

What questions remain unanswered about the disease?

What kind of study used in a particular setting would answer these

questions?When analytic studies do not confirm the hypotheses reconsider the

original hypotheses orlook for new vehicles or modes of

transmission

Step 7: Refine hypotheses and carry out

additional studies

37


38/48

Laboratory and environmental studies

Epidemiologic studies can implicate the source of infection,

and

guide appropriate public health action

But sometimes laboratory evidence can clinch the

findings

Environmental studies often help explain why an outbreak

occurred and may be very important in certain settings

Step 7: Refine hypotheses and carry out

additional studies

38


39/48

Case control methods applied to a

food-borne outbreak

The usual approach is to apply the case-control

methodology to determine what exposures ill people had

that well people did not have

List all of the relevant items on the menuDetermine the proportions of ill and of non-ill persons who

ate each of the items by questionnaire

Identify the food item with the largest difference in attack

rates between cases (ill) and controls (non-ill)

39


40/48

The practical objectives of an epidemic investigation are to

stop the current epidemic and establish measures that would

prevent similar outbreaks in the future

Preliminary control measures should be implemented as soon

as possible

Step 8: Implementing control and

prevention measures

40


41/48

Ro = cD

Ro = Reproductive Rate

(number of secondaryinfections/infected case)

= average probability susceptible partner will be infected

over duration of relationship

c = average rate of acquiring new partners

D = average duration of infectiousness

41


42/48

To sustain an epidemic

Ro > 1 but also

> 0 (transmission must be possible)can block with barriers

c > 0 (new susceptibles) can reduce contacts

D > 0 (maintain infectiousness)

can treat infection

42


43/48

Therefore, elements of epidemic control

The elements of epidemic control include:

Controlling the source of the pathogen (if known)

e.g., Remove or inactivate the pathogen

Interrupting the transmission

e.g., Sterilize environmental source of spread; vector controlControlling or modifying the host response to exposure

e.g., Immunize the susceptibles; use prophylactic

chemotherapy

43


44/48

At the end of the investigation, communicate findings to

others who need to know

Prepare a final report

Provide information on the nature, spread, and controlmeasures employed

The report can take several forms:

1.An oral briefing for local health authorities2.A written report to a journal [note: MMWR]3. Formal presentation of recommendations

Step 9: Communicate findings

44


45/48

Diarrhea at a high school dinner

In November 2006, between 200-300 students and teachers reported

gastroenteritis after attending a school function in Denmark

The cause was determined to be primarily enterotoxigenic E. coli.

Enterotoxigenic E. coliis transmitted through fecally contaminated food or

water, and mainly diagnosed as travelers diarrhea in industrialized

countries

So how did this happen at a dinner in Denmark?

Pakalniskiene et al. A foodborne outbreak of enterotoxigenic E. coli and Salmonella Anatum infection after a high-school dinner in Denmark, November 2006.

Epidemiol Infect. 2008; 1-6

45


46/48


Pasta salad with pesto has the highest

number of exposed cases (98%), and a

high attack rate; bread rolls come in at a

close second: exposure=96%



46


47/48


So which is it? Bread rolls or pesto?

Eating more

portions ofpasta had a

dose responseeffect on the

risk ratio; this

was not true ofthe bread rolls



47


48/48


None of the food preparers had a history of recent illness or

foreign travel; nor did their stool samples test positive forE coli

Investigators concluded that basil in the pesto was the likely

culprit

The basil had been imported from a country that uses surface

and run-off water for irrigation

Basil from that same country has been linked to various other

outbreaks as well

[aside: the purported producer of the basil denies having grown

basil in the four years prior to the investigation]



48

Documents

Epid 600 Class 13 Outbreaks