Animal disease - major outbreak of African horse sickness

Impact 5
risk indicator
ID 56c
Risk theme Human, animal and plant health
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


African horse sickness (AHS) is a vector-borne animal disease that is spread by midges and affects horses, donkeys, zebras and mules. It does not affect humans but can be fatal in 90% of the horses, donkeys and mules that it infects. There have been no cases of AHS in the UK to date, with the majority of outbreaks occurring in Sub-Saharan Africa where the zebra acts as a reservoir. However, cases have also been detected in countries such as Spain, Portugal, Thailand, India and Pakistan. AHS is a notifiable disease throughout the UK and anyone who suspects disease. must immediately report it. The Animal and Plant Health Agency monitors AHS outbreaks internationally and publishes outbreak assessments considering the risk posed to the UK animals on GOV.UK. The UK's strict trade rules are the most important risk mitigation measure.


The reasonable worst-case scenario assumes that an AHS infected horse is imported into the UK and bitten by midges, which would carry the virus to other horses. Although the infected horse will probably die within a few days it would not be necessarily suspected by the owner and samples may not be submitted to the reference laboratory at The Pirbright Institute. By the time the virus is identified, it would be well established in geographically dispersed midge populations around the UK.

Control measures include movement restrictions, culling of infected horses and may include preventive vaccination. The restriction zones for AHS are very large, up to 150km radius, because of the movement of infected midges. The outbreak would last for a minimum of 6 months (depending on the season and the presence of midges) and result in long-lasting trade restrictions, affecting the international movement of equine animals and a range of very high-value commodities. The likelihood and impacts of an outbreak of AHS continue to be assessed.

Key assumptions

It is assumed that a horse infected with a low- to medium- pathogenicity strain of AHS is imported into England in early spring (April) and is bitten by midges at the beginning of the midge season. The disease would replicate undetected in midge populations and spread to equine animals. Sporadic illness and deaths in infected horses may not be attributed to AHS for the first few weeks, allowing for a period of undetected spread as horses are moved around the country, facilitating a wider geographic dispersal of the disease.


If the outbreak occurred at a time of increased global demand for AHS vaccine, the control strategy may be reliant on vector controls for midges and an increase in culling until vaccines become available.

Response capability requirements

Specialist staff including equine vets (who may be private vets), animal technicians, licensed slaughterers, and carcass disposal personnel would be required to conduct surveillance and dispose of infected animals. In addition: sufficient laboratory capacity to monitor the situation, local authority staff to conduct enforcement activities, modelling experts, epidemiologists, wildlife experts, entomologists, administrative staff and trained policy staff to support, would be needed. Some parts of the horse-owning sector are difficult to reach and additional specialists may be needed.

Sufficient approved personal protective equipment (PPE) and approved disinfectant would be necessary across government and operational partners.


The minimum period to regain country-free status for international trade is 2 years from the last confirmed outbreak. Any trade permitted to continue would be subject to rigorous health certification and restrictions. During the outbreak, movement restrictions would have a devastating impact on the horse racing and breeding sector, and rare populations of wild ponies could be severely impacted. The rural economy would also suffer heavy losses.