African Horse Sickness

African Horse Sickness is a significant infectious but non-contagious disease caused by AHS Virus (AHSV), of the family Reoviridae, genus Orbivirus. There are nine recognised serotypes, some of which infer cross-protection.

Horses, donkeys, hinnies, mules and zebra are natural hosts, with zebra considered to be reservoir hosts. Dogs may also develop clinical signs of disease, usually associated with the ingestion of contaminated horsemeat, although any role in the epidemiology of the disease is unclear.

The virus is spread by female Culicoides midges, with C. imicola (thought to be the principal vector) and C. bolitinos known to transmit the virus in the field. However, there are numerous other Culicoides species which may have potential to spread the virus, such as C. pulicaris and C. obsoletus (both present in Ireland). AHSV was isolated from both of these species during the Spanish AHSV-4 outbreak in 1988. However, their specific ability to transmit the virus or not remains undetermined.

Culicoides species are active at temperatures of ≥12°C, and AHSV viral replication is thought to commence at temperatures of >15°C.

AHS is endemic in much of sub-Saharan Africa. However, outbreaks have occurred historically in the Middle East, Europe, and Asia. A serious outbreak in Spain occurred between 1987-1990, linked with the import of zebras from Namibia, requiring years of control efforts. More recently, in 2020, an outbreak of AHS in Thailand was linked to illegally imported zebras from Africa. Over 500 horses died, and Thailand did not officially regain AHS-free status until 2023.

No, AHS is not a zoonotic disease meaning it cannot spread and cause illness in humans.

The clinical signs of AHS vary depending on the form of the disease in question. There are four forms of AHS described:

• Pulmonary Form: The most acute form. Horses suffer from high fever and severe respiratory failure, presenting with coughing, difficulty breathing and frothy nasal discharge coming from their lungs. This form is almost always fatal, with horses typically dying within 24–48 hours.
• Cardiac Form: Horses suffer from high fever and significant heart disease, presenting with swelling of the face, head, neck, and brisket region. Mortality rates are reported to be at least 50%.
• Mixed Form: This is the most common form of the disease seen. It combines features of both cardiac and pulmonary forms, with mortality rates reaching 70-80% or higher.
• Horse Sickness Fever: Mild, non-fatal form with fever and lethargy being the most common way it presents. It is mostly seen mostly in zebras and donkeys.

A live attenuated vaccine is available in South Africa and widely used there. However, it is not currently authorised for use in the EU due to safety concerns around its potential to revert to virulence, and the inability to distinguish vaccinated from infected animals.

AHS is a Category A Disease under the Animal Health Law, and any suspects are immediately notifiable in Ireland under S.I. No. 130/2016. If you have any concern that one of your animals has African Horse Sickness, do not delay in acting.

To report a suspect case, please contact your Regional Veterinary Office (during business hours) or the National Disease Emergency Hotline (out of hours) on 01-4928026, which is operational 365 days of the year.

An outbreak of AHS in would have very serious consequences for the Irish equine industry, resulting in loss of our official freedom from AHS which would take years to regain. Furthermore, significant travel and trade restrictions to some other countries would be implemented during this time including:

• A 150 km-radius restriction zone would be established around the outbreak location.
• Movement of all equine animals into, within, and out of this zone would be heavily restricted.
• All exports of live equine animals from Ireland would be suspended until official AHS-freedom was regained.
• If the outbreak spread, large numbers of Irish horses would become ill, and many would die.

To protect against AHS, the Department of Agriculture, Food, and the Marine (DAFM) has introduced a comprehensive post-import protocol for any horses imported into Ireland from South Africa.

On arrival into the country, all horses imported from South Africa must:

• Be kept indoors and isolated from other animals on the farm, as well as being protected against midges.
• Be subject to a post-import wellness check and blood test by RVO staff within the first five days of arrival into the country.
• Be subject to a follow-up wellness check and blood test by RVO staff 10 days later.
• Exit from voluntary isolation only after two sets of appropriate blood test results.

Anyone involved in importing or caring for horses recently arrived from South Africa should follow all government guidelines. Please contact your Regional Veterinary Office in advance of importing any equine animals so that you can be fully familiar with the protocol detailed above.

Please refer to the resources below:

AHS - Infection with African horse sickness virus

African Horse Sickness - WOAH

African Horse Sickness Update 09 Sept 2020

Equine Disease Alert No 2/2020 (EIA & AHS)

African Horse Sickness Update 01 - 2020

Risk Assessment of the Introduction of African Horse Sickness into the Republic of Ireland (12 March 2011)

African Horse Sickness Leaflet for Vets

African Horse Sickness Leaflet for Horse Owners