African Horse Sickness in Horses: Symptoms, Transmission, and Outbreak Risk

African horse sickness (AHS) is a severe viral disease of equids, including horses, mules, donkeys, and zebras. It is caused by African horse sickness virus, an orbivirus, and it mainly affects the lungs, blood vessels, and heart. Horses and mules tend to become much sicker than donkeys and zebras, which can carry infection with milder signs.

This disease is especially concerning because it can move quickly and cause very high death rates in horses that have not been previously exposed. Clinical forms include a pulmonary form with sudden respiratory distress, a cardiac form with swelling of the head and neck, a mixed form with both patterns, and a milder horse sickness fever form seen more often in partially protected or less susceptible animals.

For U.S. pet parents, the key point is that AHS is considered a foreign animal disease and has never been detected in the United States. That lowers everyday risk for most horses in the U.S., but it also means any suspected case is treated as an emergency for both the horse and the broader equine community.

African horse sickness is caused by African horse sickness virus (AHSV). The virus is transmitted mainly by biting midges in the genus Culicoides. Other biting insects, including some mosquitoes, may also play a role. This means risk is tied more to insect exposure than to normal nose-to-nose contact between horses.

The disease is considered infectious but not directly contagious in the usual barn sense. A horse does not typically spread AHS to another horse through casual contact the way some respiratory infections do. However, blood-contaminated equipment such as needles can create transmission risk, so strict biosecurity matters.

Outbreak risk rises when three things overlap: a susceptible horse population, competent insect vectors, and introduction of the virus through infected animals or animal movement. USDA notes that AHS has never been found in the United States, but insect species capable of transmission are present here. That is why import controls, quarantine, surveillance, and rapid reporting are such important parts of prevention.

Your vet will start with an urgent physical exam and a travel, import, and exposure history. Fever, respiratory distress, frothy nasal discharge, and swelling of the head or tissues above the eyes can raise concern, especially if there is any link to affected regions, imported equids, or an unusual cluster of severe illness.

AHS cannot be confirmed by symptoms alone. Laboratory testing is needed, and real-time RT-PCR is a key test used to detect the virus. Additional testing may include virus isolation, serology, and strain or serotype identification through approved veterinary diagnostic pathways. Sample choice and timing matter, so your vet may coordinate with state or federal animal health officials.

Because AHS is a reportable foreign animal disease in the U.S., suspected cases trigger more than routine diagnostics. Your vet may advise immediate insect exposure reduction, movement restriction, and official reporting while testing is underway. That response protects your horse and helps reduce outbreak risk for nearby equids.

Prevention focuses on keeping the virus out and reducing insect transmission risk. In the U.S., that starts with import controls, quarantine, and disease surveillance. For individual horse facilities, your vet may recommend practical vector-control steps such as stabling horses during peak midge activity, using fine mesh screening where feasible, removing standing water when possible, and applying veterinarian-guided insect repellents or insecticide strategies.

Good biosecurity also matters. Do not share needles or blood-contaminated equipment between horses. If a horse has recent travel or import history and develops fever, swelling, or respiratory signs, isolate that horse from the rest of the barn and call your vet right away.

Vaccination is used in some parts of the world, but vaccine decisions are highly region-specific and tied to disease status and regulatory policy. In the United States, where AHS has not been detected, prevention is centered on rapid recognition, official reporting, movement control, and vector management rather than routine field vaccination. Your vet can help you build a prevention plan that fits your horse's travel risk, housing setup, and local insect pressure.