Abstract:
South Africa is endemic for African horse sickness (AHS), an important health and tradesensitive disease of equids. The country is zoned with movement control measures facilitating an AHS-free controlled area in the south-west. Our objective was to quantitatively establish the risk of entry of AHS virus into the AHS controlled area through the legal movement
of horses. Outcomes were subcategorised to evaluate movement pathway, temporal, and
spatial differences in risk. A ‘no-control’ scenario allowed for evaluation of the impact of control measures. Using 2019 movement and AHS case data, and country-wide census data, a
stochastic model was developed establishing local municipality level entry risk of AHSV at
monthly intervals. These were aggregated to annual probability of entry. Sensitivity analysis
evaluated model variables on their impact on the conditional means of the probability of
entry. The median monthly probability of entry of AHSV into the controlled area of South
Africa ranged from 0.75% (June) to 5.73% (February), with the annual median probability of
entry estimated at 20.21% (95% CI: 15.89%-28.89%). The annual risk of AHSV entry compared well with the annual probability of introduction of AHS into the controlled area, which
is ~10% based on the last 20 years of outbreak data. Direct non-quarantine movements
made up most movements and accounted for most of the risk of entry. Spatial analysis
showed that, even though reported case totals were zero throughout 2019 in the Western
Cape, horses originating from this province still pose a risk that should not be ignored. Control measures decrease risk by a factor of 2.8 on an annual basis. Not only do the outcomes
of this study inform domestic control, they can also be used for scientifically justified trade
decision making, since in-country movement control forms a key component of export
protocols.
