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Special Concern for Animal Outbreak Investigation & Herd Factors

Special Concern for Animal Outbreak Investigation & Herd Factors Field Epidemiology in Action 14 September 2009, Khonkaen Pawin Padungtod Regional Project Coordinator, ECTAD - RAP Pawin.padungtod@fao.org Unit of Analysis Country Province Village/Farm House/Flock Animal Herd

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Special Concern for Animal Outbreak Investigation & Herd Factors

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  1. Special Concern for Animal Outbreak Investigation & Herd Factors Field Epidemiology in Action 14 September 2009, Khonkaen Pawin Padungtod Regional Project Coordinator, ECTAD - RAP Pawin.padungtod@fao.org

  2. Unit of Analysis • Country • Province • Village/Farm • House/Flock • Animal Herd

  3. Herd factors in animal disease outbreak investigation • Factors related to disease spreading • Between herds : Herd level source of infection • Within a herd : Reproductive number • Factors related to investigation result • Factors affecting validity • Factors affecting precision

  4. Herd level source of infection Neighboring farms Introduction Resurgence Neighbour Wind Water Disease-free farm Animals Animal products Humans Vehicles B. Dufour, P. Hendrikx. 2009.Epidemiological surveillance in animal health. OIE publication www.oie.int

  5. Herd level source of infection:Neighboring farm/country • Health status • Distance • Cross farm/border activities Nicoline DeHaan and Jonathan Rushton. 2009.FAO

  6. Herd level source of infection:Introduction • Prevalence in the area • Individual Prevalence (Animal) • Aggregate Prevalence (Farm/Village) M. Gilbert et al. 2006. EID 12(2) : 227

  7. Herd level source of infection:Resurgence • Time period from last outbreak • Agent survival T,Tiensin et al. 2005. EID. 11(11) : 1164

  8. Transmission within a herd Transmissibility of the pathogen (SAR) The extent of receptivity of the host Animal husbandry

  9. Source of infection within a herd Reservoir - Receptive but not susceptible Carrier - Receptive and susceptible National geographic magazine

  10. Source of infection within a herd Incubation Clinical sign Recovery Intensity of transmission Time Incubatory carrier Convalescence carrier Chronic carrier

  11. Basic reproductive number (R0) Number of new case arise from an index case in naïve population. R0 = b * k * d b = Probability of transmission per contact (case /contact) k = Frequency of contact (contact / time) d = Duration of shedding (time) National geographic magazine

  12. Basic reproductive number (R0 = 1)

  13. Basic reproductive number (R0)

  14. Basic reproductive number (R0) R0< 1 Disease subside R0= 1 Endemic R0> 1 Disease outbreak FMD R0 = 60

  15. Basic reproductive number and herd immunity Disease spread freely if all Animals are susceptible Disease cannot spread if certain Proportion of the animals are not susceptible

  16. Factors Affecting R0 Addition of susceptible Die-off or departure Vector abundance Population density Individual mobility

  17. R0 example

  18. P P P Factors Affecting Investigation Result :Validity & Precision P

  19. Factors Affecting Precision Sample size Number of population (N) Expected number of disease animal/herds (prevalence*N) Confidence level Population Sample

  20. Effect of sample size on precision

  21. Internal validity Free from systematic error (bias) Validity

  22. External validity Good representation of the population – sampling method Population Multi-stages sampling Simple random sampling Cluster 1 Cluster 2 1st stage Farm 1 Farm 2 Farm 3 Farm 1 Farm 2 Farm 3 2nd stage 3rd stage Animal Animal Animal Animal Animal Animal Animal Validity

  23. Factors Affecting Validity • Diagnostic test characteristic • Sensitivity : P(T+|D+) • Specificity : P(T-|D-)

  24. 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

  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

  26. Herd-level sensitivity (HSe) f=0.6 HSe 0.97 0.93 0.83 HSe = 1 – [1 – (f * Se)]A A=4 f=0.2 HSe 0.59 0.50 0.40

  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

  28. Herd-level specificity (HSp) HSp 0.99 0.90 0.35 HSp = Spn HSp 0.96 0.67 0.01

  29. 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)

  30. 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

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