PRINCIPLES OF OUTBREAK INVESTIGATION

Karoon Chanachai

Bureau of Disease Control and Veterinary Service

Department of Livestock Development, Thailand

An event (1)

On 28 May 2009, you were still at your work, when an email from your counterpart at the National Institute of Animal Health (NIAH) popped up. In an attachment, you found an official laboratory report from the Upper Northern Regional Veterinary Research and Diagnostic Center. It was a laboratory confirmed case of classical swine fever (CSF) in Mae Salong, Chiangrai province

An event (2)

Early September 2007, Khon Kean Provincial livestock office notified to Department of Livestock Development (DLD) that there was suspected porcine disease occurred in pig farms in Sila subdistrict, Muang district. Symptoms of infected pigs were fever, cough, blue ears, and petechial hemorrhage.

DO WE THINK THIS IS AN OUTBREAK?,

WHY?

Will we do anything after receiving this report ?

Definition of outbreak

Occurrence of more cases of disease than expected in a given area among a specific group of people over a particular period of time

or Two or more linked cases of the same

illness or

New diseases

What is th

e

level of e

xpected ?

Excess of Expected Level

More than • Median number of cases in previous 5

years or• Average number of cases + 2sd of

previous 5 yrN u m b e r o f D is e a s e A c a s e s b y m o n t h , P r o v in c e X

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

J a n F e b M a r A p r M a y J u n J u l A u g S e p O c t N o v D e c

# c

as

es

2 0 0 3 M e d ia n 1 9 9 8 - 2 0 0 2

A single case of disease that

has never been

occurred before.

1997:A 3-year old boy, case of Avian Flu (H5N1) in Hong Kong alerted the public health people around the world to start a full scale investigation.

Judged to be an outbreak

What? Who?When?Where?

Why?

How to react?

Descriptive epidemiology

Analytical epidemiology

Epidemiology for action

Specific demands when investigating outbreaks

• Unexpected event• Need to act quickly• Need for rapid control• Work carried out in the field

Systematic approach

Unexpected event !!!

Steps of an outbreak investigation Prepare for field work Establish the existence of an outbreak Verify the diagnosis Construct a working case definition Find case systematically and record

information Perform descriptive epidemiology Develop hypothesis Analytical studies to test hypotheses Special studies (e.g. environmental study) Implementation of control measures Communication, including outbreak report

Is this an outbreak?

Diagnosis verified ? clinical + laboratory

Link between cases?Expected numbers?

DetectionRoutine surveillanceClinical / LaboratoryGeneral publicMedia

Sequence of events in outbreak detection and confirmation (I)

0

10

20

30

40

50

60

70

80

90

PrimaryCase

1st casesnotified by Local

officerReport Lab

resultSamples

takenResponse

begins

DAY

CASES Opportunity for control

0

10

20

30

40

50

60

70

80

90

Sequence of events in outbreak detection and confirmation (II)

Prim Noti Rep LabSampResponse

begins

DAY

CASES

Potential cases prevented

Outbreak confirmed

Further investigation?

Immediate control measures?

Unknown aetiology(pathogen/source/transmission)Cases seriousCases still occurringPublic pressureTraining opportunity Scientific interest

Outbreak confirmed

Further investigation?

ProphylaxisQuarantine / isolationPublic warningHygienic measuresSurveillance

Assistance ?

Immediate control measures?

EpidemiologistMicrobiologistEnvironmental specialistMinistry / GovernmentPress officerOthers

FIELD

Outbreak Investigation

Team?

Preparing for field works

• Team members and roles

• Necessary Knowledge and equipments;

specimen collection & transport method,

etc.

• Lines of communication

An event (2)

Early September 2007, Khon Kean Provincial livestock office notified to Department of Livestock Development (DLD) that there was suspected porcine disease occurred in pig farms in Sila subdistrict, Muang district. Symptoms of infected pigs were fever, cough, blue ears, and petechial hemorrhage.

Review of Pig Diseases

There are many disease that cause a lot of death with fever, cough, blue ears, and skin petechial hemorrhage in pig.

The diseases can cause by PRRS, Swine fever, African swine fever, Mycoplasma and other bacterial infection.

……………….

………………

………………

Outbreak Investigation

Team?

EpidemiologistMicrobiologist, PathologistEnvironmental specialistMinistry / GovernmentPress officerOthers

Descriptive epidemiology

Assess situationExamine available information

Preliminary hypothesis ?Case definition

Case finding

(Passive cases)

(Active cases)

Reported cases

Cases in community (active cases) • Mild or no symptom

• Treated by owner

• Self recovering • DeathThese cases can be potential spreader

Case definition

Standard set of criteria for deciding if an individual animal/flock should be classified as suffering from the disease under investigation.

Clinical criteria, restrictions of time, place, person

Unit of interest Simple, practical, objective Sensitivity

Case definition:

Suspect farm was defined as pig farm in village Moo.13 and surrounding villages, Sala subdistrict, Khon Kean province that have

• more than 30% of pigs having cough, petechial hemorrhage or reproductive or respiratory tract problems or

• mortality rate more than 10%

during the period 1 August – 30 November 2007

Case definition: categorisation

Suspect Farm that met suspect case definition

Confirmed The confirmed farm was defined having at least

one of pig positive to ………

Methodology:

Case finding Active case finding was conducted by

doing a census among all pig farms in the village 13 and nearby villages.

Interviewed the owners of farms location, number of pig, onset date, clinical signs, farm management, movement, etc..

Laboratory study Collected serum, nasal swab and dead pigs

specimens from every farm in infected area Sent to Northeastern Veterinary Research and

Development Center (Khon Kean province) and National Institute of Animal Health (Bangkok)

Sample will be tested by PCR technique for PRRS Classical Swine Fever Virus Porcine Circovirus Type 2 African Swine Fever Bacterial culture

Methodology:

Identify & count cases

Obtain information

Perform descriptive epidemiology

Clearly identifiable groups

surveillance

Laboratories

Identify & count cases

Obtain information

Perform descriptive epidemiology

Identifying information

• Demographic

information

• Clinical details• Risk factors

Identify & count cases

Obtain information

Perform descriptive epidemiology

Orient cases in

- time

- place

- person (animal)

Animal

Place

Time

Cases

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10

0

200

400

600

800

1000

1200

0-4 '5-14 '15-44

'45-64

'64+

Age Group

Evaluate information

Pathogen? Source? Transmission?

Point source

0

5

10

15

20

Continuing common source

Example: food contaminationExample: Insecticide contamination

Examples of epidemic curves:

COMMON SOURCE

0

1

2

3

4

5

6

7

8

9

10

Common sourc

e

Example: Brucellosis outbreak among goat farms

0

5

10

15

20

25

30

Examples of epidemic curves:

PROPAGATED SOURCE

Disease A: Incubation period 7-10 days(from literature review)

Estimation of “exposure period” by using Epidemic curve

Number of cases

Date of onset

Point source outbreak

Min. IP

Median IP

0

1

2

3

4

5

6

7

8

9

10

Exposure period

RESULTS

Descriptive results

An outbreak of PRRS in Sila subdistrict, and Bungneam subdistrict, Muang

Sila sub-district: There were 28 small scale farms in village Moo. 13 and

14. The infected farms were located too close to each other

in 2 km. along the railway. 23 farms had met suspect case definition (attack rate

82%) 607 pigs died and 65 severe illness have been culling

from 1,142 pigs population (mortality rate 57%)

Descriptive results

Bungneam sub-district: 10 km. far from the Sila Only one from 20 farms had met suspect

case definition and confirmed PRRS. The owner was brought his boars to Sila for

natural breeding and bought growing pigs back to slaughter and sell in community.

Percentage of symptoms of PRRS cases in Muang district, Khon Kean province, August - November 2007

0 10 20 30 40 50 60 70 80 90 100

abortion

seizure

petechial hemorrhage

cyanosis at ears

diarrhea

respiratory distress

symptoms

percentage

Number of PRRS infected farms by date of onset, Muang district, Khon Kean province, August – November 2007

0

1

2

3

4

5

6

18 -

24 ju

l

25 -

31 ju

l

1 - 7

aug

8 - 1

4 aug

15 -

21 a

ug

22 -

28 a

ug

29 a

ug - 4

sep

5 - 1

1 sep

12 -

18 s

ep

19 -

25 s

ep

26 s

ep -

2 oc

t

3 - 9

oct

10 -

16 o

ct

17 -

13 o

ct

24 -

30 o

ct

31 o

ct - 6

nov

7 no

v - 1

3 nov

Onset date

No. of farms

Sila subdistrict

Bungneam subdistrict

Laboratory results

Places Results (positive/specimens)

PRRSV CSFV AFSV PCV2

Sila subdistrict 10/30 0/16 0/11 1/18

Bungnean subdistrict 1/3 0/1 - 0/1

Chumpae district 0/5 0/1 - 1/1

Autopsy

Lung congestion, mucous exudate Brain congestion, edema petechial hemorrhage at stomach button ulcer at ileum Lymph node congestion pericarditis petechial hemorrhage at kidneys

Bacterial culture Corynebacterium spp. 1/1 Salmonella spp. 1/1 Mycoplasma hyopneumoniae 10/23 Mycoplasma hyorhinis 1/6 Pasteurella multocida 1/1

Laboratory results

Porcine Reproductive and Respiratory Syndrome (PRRS)

Infectious viral disease of swine, characterized by reproductive failure in sows and respiratory distress in piglets and growing pigs.

Easily transmitted through direct contact to susceptible pigs and vertically to fetuses.

Also know as Blue Ear Disease, Porcine Endemic Abortion and Respiratory Syndrome (PEARS) and Swine Infertility Respiratory Syndrome (SIRS).

Incubation period 5 – 20 days.

Porcine Reproductive and Respiratory Syndrome (PRRS)

Differential diagnosis Swine fever

Porcine circovirus

Two 2 strains found in Thailand: US-strain (33.6%) & EU-strain (66.4%) (Thanawongnuwech et a, 2004)

The disease can be found in pig farms and hardly to eliminate because the virus can survive from 42 – 157 days after infected in different organs (Thanawongnuwech, 2005)

Farm management

Very low bio-security in Sala subdistrict Almost of the farm connect to each other 70% of the farms in Sala subdistrict used

swill feeding from the market or restaurant

74% of the farms bought weaning pig from unspecified sources

65% of them breed their pig by using natural breeding services from other farms outside the community

Risk factors possible

Sila district Bought weaning pigs from unspecified

sources Swill feeding Natural breeding Farm to farm contamination

Develop hypotheses

Compare hypotheses with facts

• What is the risk factor of this outbreak?• What is the disease causing the outbreak? • What is the source and the vehicle?• What is the mode of transmission?

Case-controlCohort

Analytical epidemiological studies

Test specific hypotheses

Limitations

Even though, control measures were fully implemented by local officer. Some owners had sold out their pigs to other areas after detected the abnormal signs without notification to the government officer.

All of infected farms are located along the railway which posses to the State Railway of Thailand, so the owners did not want to change their life or improve their farms because it will cost expenses and other important point is they will be expel from this area anytime.

Conclusions and Discussions

PRRS outbreak occurred in small scale pig farms in Sila and Bungneam subdistrict, Muang district, Khon Kean Province.

There will be co-infection of PRRSV, PCV type2 and bacteria in this outbreak:

The virus can easily spread because we still found PRRSV in piglet’s carcass 1 month after outbreak and all pigs were looked healthy.

Recommendation

Treated mild cases and culling severe illness pigs to minimize loss

Quarantine pigs in infected area for 4 months Prohibit to bring new piglets Pigs were allowed only movement to

slaughterhouse Education to farm holders by group activities,

recommended for farm holders to improve the farm sanitation to prevent outbreak in future

Implement control measures

May occur at any time during the outbreak!!

Prevent recurrence

Control the source of the pathogen

Interrupt transmission

Modify host response

Control the source of pathogen

Remove source of contamination

Remove animal from exposure

Inactivate/ neutralise the pathogen

Isolate and/or treat or cull infected

animal

Interrupt transmission

Interrupt

environmental

sources

Control vector

transmission

Improve

sanitation

Modify host response

Immunise susceptible animal

Use prophylactic chemotherapy

At the end

Prepare written report

Communicate

Convince policy

Evaluate performance

Steps of an outbreak investigation

Prepare for field work Establish the existence of an outbreak Verify the diagnosis Construct a working case definition Find case systematically and record

information Perform descriptive epidemiology Develop hypothesis Analytical studies to test hypotheses Special studies (e.g. environmental study) Implementation of control measures Communication, including outbreak report

Imp

lemen

t con

tr ol m

easures

Thank you