Establishing post‐outbreak freedom from African horse sickness virus in South Africa's surveillance zone

Abstract

An African horse sickness (AHS) outbreak occurred in South Africa's AHS controlled area in autumn 2016. A freedom from disease survey was performed to establish the likelihood of ongoing circulation of the associated virus during the same period the following year. A single‐stage surveillance strategy was employed with a population‐level design prevalence of 1% to establish a survey population sensitivity of 95% (probability that one or more positive horses would be detected if AHS was present at a prevalence greater than or equal to the design prevalence). In March 2017, a total of 262 randomly selected horses from 51 herds were sampled from the 2016 outbreak containment zone. Three within‐herd and herd‐level design prevalence scenarios were used in evaluating the post‐survey probability of freedom. Depending on the underlying design prevalence scenarios, effectively ranging between 0.8% and 6.4%, and the use of informed or uninformed priors, the probability of freedom derived from this surveillance ranged between 73.1% and 99.9% (uninformed prior) and between 96.6% and 100% (informed prior). Based on the results, the authors conclude that it is unlikely that the 2016 AHS virus was still circulating in the autumn of 2017 in the 2016 outbreak containment zone. The ability to perform freedom from disease surveys, and also to include risk‐based methods, in the AHS controlled area of South Africa is influenced by the changing underlying population at risk and the high level of vaccination coverage in the horse population. Ongoing census post‐outbreak must be undertaken to maintain a valid sampling frame for future surveillance activity. The seasonality of AHS, the restricted AHS vaccination period and the inability to easily differentiate infected from vaccinated animals by laboratory testing impact the ability to perform a freedom from disease survey for AHS in the 12 months following an outbreak in the controlled area.

Post‐outbreak African horse sickness surveillance: A scenario tree evaluation in South Africa’s controlled area

“…Theoretically, one could remove the active components from the surveillance programmes for the AHS‐controlled area with negligible effect on overall sensitivity and probability of freedom. The resources required for active surveillance are substantial—the POSC cost approximately 15,500 USD (R210000) while the sentinel surveillance programme costs approximately 105,000 USD (R1.476 Million) per year (Grewar, Sergeant, et al., 2019; Grewar et al., 2017; Grewar, Weyer, et al., 2019). If these resources were spent on further improving passive surveillance, and in particular the probability of clinically suspect horses being presented for AHS testing, the surveillance programme would be simplified without losing sensitivity.…”

“…In practice, the FZ and SZ have the same AHS surveillance policy and they were merged for this evaluation (FZSZ). The 2016 AHS outbreak secondary containment zone, however, delineated the region where a structured freedom from disease survey was performed (Grewar, Sergeant, et al., 2019) and the combined FZSZ was separated into that part intersecting with the 2016 AHS secondary containment zone (FZSZ_CZ—A1 in Figure 1) and the remainder (FZSZ_NonCZ—A2 in Figure 1). The AHS PZ is considered the third surveillance area (B in Figure 1).…”

“…We describe the evaluation of AHS in terms of three components; ongoing passive surveillance (PSC), ongoing monthly sentinel surveillance (SSC) and a stand‐alone post‐outbreak freedom from AHS disease survey (POSC). The detailed processes of the active components (SSC and POSC) have been described (Grewar, Sergeant, et al., 2019; Grewar & Weyer, 2016) and only information pertinent to this quantitative evaluation of the system as a whole are expanded upon below.…”

“…A stand‐alone freedom from disease survey targeting the containment zone of the 2016 Paarl outbreak was undertaken in March 2017 (Grewar, Sergeant, et al., 2019). Data from this study were integrated into the surveillance data set used in this evaluation.…”

“…In the SSC programme, a total of eight horses were investigated to a negative conclusion between July 2016 and June 2018 (Grewar & Weyer, 2018; Grewar et al., 2016, 2017). Details of screening tests and investigations of the POSC have been published (Grewar, Sergeant, et al., 2019). We conclude that all suspect cases detected through any of the surveillance programmes were followed until AHS infection was ruled out.…”