1 Food and Agriculture Organization of the United Nation Regional Office for Asia and the Pacific Emergency Center for Transboundary Animal Diseases Special Concern for Animal Outbreak Investigation & Herd Factors Field Epidemiology in Action 14 September 2009, Khonkaen Pawin Padungtod Regional Project Coordinator, ECTAD - RAP [email protected]
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1 Food and Agriculture Organization of the United Nation Regional Office for Asia and the Pacific Emergency Center for Transboundary Animal Diseases Special.
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
Special Concern for Animal Outbreak Investigation & Herd Factors
Field Epidemiology in Action14 September 2009, Khonkaen
Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
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Aggregate (pooled) testing• To determine the infection status at group level instead of
an individual– A pool of cloacal swabs from 20 chickens in a village – 10 pools, each pool contains 5 individual samples– Bulk milk tank test for antibiotic residues– Individual fecal culture from 30 cows in a herd
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
25
Herd-level sensitivity (HSe)• Probability of obtaining at least one positive
result from an infected herd• HSe = 1 – [1 – (f * Se)]A
– f = sampling fraction– Se = individual sensitivity– A = number of infected animal
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
26
Herd-level sensitivity (HSe)
0.00
0.20
0.40
0.60
0.80
1.00
0.1 0.3 0.5 0.7 0.9
Sampling fraction
He
rd s
en
sit
ivit
y
0.99
0.8
0.6
HSe = 1 – [1 – (f * Se)]A
A=4
f=0.2HSe0.590.500.40
f=0.6HSe0.970.930.83
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
27
Herd-level specificity (HSp) • Probability of obtaining only negative results
from a test applied to every individual in a non-infected herd
• HSp = Spn
– Sp = Individual specificity– n = number of animals tested
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
28
Herd-level specificity (HSp)
0.00
0.20
0.40
0.60
0.80
1.00
5 10 15 20 25 30 35 40 45 50
Number of animal tested
Herd
sp
ecif
icit
y
0.999
0.99
0.9
HSp = SpnHSp0.990.900.35 HSp
0.960.670.01
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
Summary• Factors affecting transmission of diseases
– Between herd• Neighboring farms/villages• Prevalence in the area• Agent survival
– Within herd (R0)• b = Probability of transmission per contact (case /contact)• k = Frequency of contact (contact / time)• d = Duration of shedding (time)
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Food and Agriculture Organization of the United Nation
Regional Office for Asia and the Pacific
Emergency Center for Transboundary Animal Diseases
Summary
• Factors related to investigation result– Factors affecting precision
• Sample size
– Factors affecting validity• Diagnostic test characteristics
– HSe– HSp
• Sampling method– Simple random– Multi stages sampling