African Horse Sickness: what you need to know

Emergency Animal Diseases (EADs) pose a serious threat to the health and productivity of Australia’s feedlot industry. This blog series will outline a number of EADs and is designed to help you stay informed, prepared, and ready to respond. 

What Is African Horse Sickness (AHS)? 

African Horse Sickness (AHS) is a severe, often fatal viral disease that affects all members of the horse family, including horses, donkeys, mules, and zebras. Horses are the most severely affected, with fatality rates between 70–95% in some outbreaks. 

AHS is caused by the African Horse Sickness Virus (AHSV), a double-stranded RNA virus belonging to the Orbivirus genus in the Reoviridae family. There are nine known serotypes (AHSV-1 to AHSV-9). 

Importantly, AHS has never occurred in Australia. Australia remains free from the disease and maintains robust biosecurity systems to ensure it stays that way. 

Although AHS primarily affects equids, dogs can contract a fatal form of the disease through consumption of infected meat or develop mild infection after being bitten by infected insects. Other species like elephants, camels, and some wild ruminants have tested positive for antibodies but do not show clinical signs. 

How Does African Horse Sickness Spread? 

AHS is not contagious between horses and is spread exclusively by biting insects, particularly Culicoides midges. Once infected, insects can carry the virus for life. 

The spread of AHS is influenced by: 

• Insect activity and population dynamics 

• Environmental factors such as wind, rain, and high temperatures 

• Insect flight range (spread is typically local unless aided by strong winds) 

• Seasonal conditions that favour insect breeding 

The virus does not replicate at temperatures below 15°C, but warm climates significantly enhance transmission. Newly emerged female insects feed on infected blood and transmit the virus through their saliva during future blood meals. 

While direct mechanical transmission by other biting flies like stable flies and buffalo flies has not been proven, their involvement cannot be entirely ruled out. 

What Are the Signs of African Horse Sickness in Horses? 

The incubation period for AHS in horses ranges from 3 to 14 days, usually 5–9 days. Infected horses may remain infective for up to 40 days. 

There are four classical clinical forms of AHS, each varying in severity and symptoms: 

• Peracute (Pulmonary) Form: Sudden onset, severe respiratory distress, and often sudden death 

• Subacute Oedematous (Cardiac) Form: Swelling (oedema), especially in the head and neck, with fever and depression 

• Acute (Mixed) Form: Signs of both pulmonary and cardiac involvement 

• Mild (Horse Sickness Fever) Form: Usually seen in zebras or previously exposed equids, with mild fever and recovery 

Common clinical signs include: 

• Oedema 

• Petechial haemorrhages (tiny bleeding spots) 

• Vascular congestion and respiratory distress 

• Frothy discharge from the mouth and nostrils 

• Fever and sudden death within 24 hours of clinical signs 

AHS and Biosecurity: Why Prevention Is Essential 

Australia’s freedom from AHS is maintained through strong quarantine measures, vector monitoring, and surveillance. The vector-based transmission route makes insect control and movement restrictions crucial in at-risk regions. 

A single incursion could have devastating impacts on Australia’s horse population, equine industries, and international markets. 

How to Report African Horse Sickness in Australia 

AHS is a notifiable disease, meaning any suspected cases must be reported immediately. 

Contact your veterinarian or call the Emergency Animal Disease (EAD) Watch Hotline: 1800 675 888 

Early reporting helps protect Australia’s horses, industries, and international standing.