An entry risk assessment of African horse sickness virus into the controlled area of South Africa through the legal movement of equids.
Authors: Grewar John D, Kotze Johann L, Parker Beverly J, van Helden Lesley S, Weyer Camilla T
Journal: PloS one
Summary
# African Horse Sickness Risk Assessment in South Africa's Controlled Zone South Africa maintains an African horse sickness (AHS)-free zone in the south-west despite endemic virus circulation elsewhere in the country, relying on movement controls and zoning protocols to protect this trade-critical status. Grewar and colleagues developed a stochastic model using 2019 movement data and AHS surveillance records to quantify the annual probability of virus entry through legal equine movements, stratifying risk by location, pathway and season. Monthly entry risk ranged from 0.75% in June to 5.73% in February, yielding an annual median probability of 20.21% (95% CI: 15.89%–28.89%), which aligns reasonably with the ~10% introduction risk observed historically. Direct non-quarantine movements posed the greatest risk despite comprising the bulk of legitimate trade, and notably, horses from the Western Cape—which reported zero cases in 2019—still contributed measurable risk to the controlled area, suggesting that absence of detected disease does not guarantee absence of infection. The study demonstrates that current control measures reduce annual risk by a factor of 2.8, providing quantitative evidence for the effectiveness of movement restrictions and informing both domestic biosecurity protocols and export certification requirements for equine professionals and regulatory bodies managing AHS risk.
Read the full abstract on PubMed
Practical Takeaways
- •Risk of AHS entry into protected zones varies seasonally, peaking in February—timing of movement control enforcement should reflect these temporal patterns
- •Direct non-quarantine movements represent the primary vector; stricter screening and quarantine protocols for this movement type will yield the greatest risk reduction
- •Geographic origin alone is insufficient for risk assessment—implement epidemiological surveillance and testing protocols to identify infection risk in low-case provinces
Key Findings
- •Annual median probability of AHSV entry into the controlled area was 20.21% (95% CI: 15.89%-28.89%), with monthly risk ranging from 0.75% in June to 5.73% in February
- •Direct non-quarantine movements accounted for the majority of movements and posed the greatest risk of AHSV entry
- •Current control measures decreased annual risk by a factor of 2.8 compared to no-control scenarios
- •Horses from Western Cape province posed measurable risk despite zero reported AHS cases in 2019, highlighting importance of asymptomatic transmission pathways