Evaluation of African horse sickness surveillance in the controlled area of South Africa

Abstract

Surveillance is one of the core components of freedom from disease status declarations made by countries with regards to African horse sickness (AHS). This is especially true for South Africa which has a controlled area defined specifically for trade purposes. Three AHS surveillance activities are evaluated in this thesis: the surveillance during the 2016 Paarl AHS outbreak; the stand-alone freedom from disease survey undertaken in 2017; and a two-year surveillance sensitivity and probability of freedom analysis based on multiple surveillance components (passive surveillance, active sentinel surveillance and the 2017 survey) within three distinct zones in the AHS controlled area. Outbreak surveillance in 2016 established affected population proportions and these results were included as within and between-herd estimates for evaluation of surveillance in the post-outbreak period. The stand-alone 2017 survey established that the point in time probability of freedom ranged between 73.1% and 100% in March 2017. Scenario tree analysis showed that, at a design prevalence of 1 animal in 1% of herds, the median posterior probability of freedom from AHS in the AHS controlled area after the 24-month post-outbreak period was between 98.3% - 99.8%. The final median probability of freedom had been realised by the 9th month after the 2016 outbreak had been resolved. The inclusion of active surveillance provided minimal additional confidence in surveillance outcomes. Freedom from AHS was achieved fairly soon after the outbreak concluded in 2016 and this freedom was driven by the passive surveillance component. Surveillance challenges arise, in the South African context, as a result of high numbers of vaccinated animals within the population at risk, the seasonality of AHS and limitations of the DIVA (differentiating infected from vaccinated animals) capabilities of existing routine laboratory tests. Current global standards require a two-year post-incursion period of AHS freedom before re-evaluation of free zone status. Our findings show that the length of this period could be decreased if adequately sensitive surveillance is performed. In order to comply with international standards, active surveillance will remain a component of AHS surveillance in South Africa. Passive surveillance, however, can provide substantial evidence supporting AHS freedom status declarations, and further investment in this surveillance activity would be beneficial.