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A RISK BASED MODEL TO GUIDE DECISIONS ON ZONIFICATION TO STOP VACCINATION IN A FREE COUNTRY
WITH VACCINATION
O. Daza, N. Guzman, D. Gareca, J.L. Gonzales
Bolivia
Geographical regionsAmazonsChacoValleysHighlands
Area: 1.098.581 Km2
8 637 358 cattle 5 386 939 Sheep
2 572 226 camelids 1 722 606 goats
FMD situation
2003 2006
2012 2013
2014
Free without vaccinationFree with vaccination
• Eradication programme (started 2000) based on surveillance, control of movements and mass vaccination of the cattle population
• The goal is to gradually extend the free without vaccination zones in a risk based manner
Objective
To develop a model to quantify the risk of FMD introduction into zones where vaccination is discontinued and evaluate the efficacy of control measures to minimise this risk.
Methods: Scenario tree model
OS16
Parameters Description Reference
Vaccination Province vaccination coverage -Proportion
SENASAG
Herd level infection Prevalence = 0.015Design prevalence used for surveillance
SENASAG
Risk of infection Adjusted relative risk for non-vaccinated Vs vaccinated
Gonzales et al 2014. Vaccine
Anima level infection Within herd prevalence 0.29 (0.04-0.50) Gonzales et al 2014. Vaccine
OS16
Cattle movementsParameters Description Reference
Destination Proportion cattle going to area of interest
SENASAG – Movement records
Herd inspection (clinical) Sensitivity clinical inspection 0.35 (0.25 – 0.40)
Gonzales et al 2014. Vaccine
OS16
Parameters Description Reference
Movement inspections
Proportion of animals inspected at road control posts 0.10 (0.05-0.015)
SENASAG.
Clinical inspection
Sensitivity clinicalinspection 0.35 (0.25 – 0.40)
Gonzales et al 2014. Vaccine
Purpose of movement
Proportion of cattle moved for breeding
SENASAG –Movement records
Results
Zones
No vaccination
No Vaccination
Once per year
Twice per year
One Mass vaccination + One < 24 months
Two zones as examples
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
Prevalence 3% Prevalence 1.5%
Prob
abilt
y of
intr
oduc
tion
0
0.1
0.2
0.3
0.4
0.5
0.6
Inspection 10% Inspection 50% Inspection 80%
Prob
abili
ty o
f int
rodu
ctio
n
Valleys Santa Cruz Punata
Yearly probability of introduction
0
0.0005
0.001
0.0015
0.002
0.0025
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Prob
abili
ty o
f int
rodu
ctio
n
00.010.020.030.040.050.060.070.080.09
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Prob
abili
ty o
f int
rodu
ctio
n
Monthly probability of introductionValleys Santa Cruz
Punata
Conclusions
• Clear differences in risk of introduction in different regions/zones can be observed
• This differences can be used to stop vaccination or target vaccination to specific zones
• Cattle moved from areas free with vaccination are considered the main risk for areas where vaccination is not applied
• Work is being done on improving data collection on inspection parameters from control posts and fairs in the country
• Future work: Combine risk of introduction with risk of transmission
