US experts monitor threat of African horse sickness

Horses would be at risk if African horse sickness makes it into the US. Photo: File

American horses would be at risk if African horse sickness made it to its shores, with the nation having insects that are probably capable of spreading the deadly virus.

Officials at Texas A&M University’s AgriLife campus have discussed the threat of the disease as Thailand battles its first incursion of the disease.

It is suspected that imported zebras were behind the outbreak, which is being contained through a massive effort involving insect control measures and vaccination.

African horse sickness is not in the US, and officials say it is important it stays that way.

AgriLife faculty members, as well as various state and federal agencies and the horse industry, are already monitoring the situation, ensuring surveillance and determining practices to prevent the disease from crossing US borders.

African horse sickness comes out of Africa and is common from Morocco down to the middle of the continent. But it has escaped the African continent several times.

The Orbivirus that causes the infection is transmitted by certain insects.

“Equines including horses, mules, donkeys and zebras are moved all over the world for competitions, trade, breeding, zoologic and conservation purposes,” says Pete Teel, a research entomologist with the university.

“There is a real risk that this foreign animal disease could be introduced to the western hemisphere, including North America, where we have insects that will likely serve as effective vectors of this virus,” Teel told AgriLife Today.

“The US is constantly threatened by introductions of foreign animal diseases.

“Diligent surveillance, detection and planned responses at state and federal levels is essential, as is keeping an eye on what is happening globally. Having advanced knowledge and warnings is a huge advantage toward prevention before it gets into our country.”

Federal agencies, including the US Department of Agriculture Animal and Plant Health Inspection Service, state agencies such as the Texas Animal Health Commission, and concerned and vigilant equine industry stakeholders are all part of the first line of defense, Teel says.

“If African horse sickness does come to the US, do we have knowledge of insects that will likely transmit this virus? Yes,” Teel said. “Do we know everything we need to know about it? Probably not.”

He said, depending on the serotype of any introduced virus, “it will be necessary to determine whether the insects we have are competent vectors in the laboratory and capable vectors in the field. And further, to improve practices to protect equines from vector transmission and infection.”

What is African horse sickness?

African horse sickness is considered a Tier 3 disease by the National Bio and Agro-Defense Facility because of the potential negative impact on animal health.

Symptoms are the same as those associated with respiratory and circulatory impairment. At the first signs of the disease, owners are advised to eliminate affected horses and vaccinate non-infected horses with a polyvalent vaccine; then let them rest for two weeks.

US authorities have import requirements in place to reduce the likelihood of the virus being introduced. Imported horses must undergo an inspection before export. Horses imported from countries affected by the disease are required to undergo a 60-day quarantine on arrival in the US.

The tiny biting midge, left, as seen here compared to a mosquito, is a vector for African horse sickness. Photo: Dunpharlain CC BY-SA via Wikimedia Commons
The tiny biting midge, left, as seen here compared to a mosquito, is a vector for African horse sickness. Photo: Dunpharlain CC BY-SA via Wikimedia Commons

Horses are the most susceptible host, with close to 90% mortality of those affected, followed by mules and donkeys. African donkeys and zebras rarely display clinical symptoms, despite high virus titers in their blood. They are thought to be the natural reservoir of the virus. Thus, it is thought the virus was transported to Thailand through asymptomatic zebras.

“The primary vectors of African horse sickness are among species of biting midges, which are very small blood-feeding flying insects about 1/8th of an inch in length,” Teel explains.

“The immature stages of these insects complete their portion of the midge life cycle in association with wet habitats ranging from permanent and semi-permanent aquatic areas to very moist soils and decaying organic matter.”

He explains that these are biological vectors, meaning that the virus reproduces inside biting midges after blood meals are taken from infected animals; the resulting infected midges are then able to infect new animals.

“We have biting midges in the US,” he says. “They are involved in the transmission of two similar viruses causing diseases known as blue tongue and epizootic hemorrhagic disease in livestock and wildlife.”

He says Texas has biting midges and both diseases.

AgriLife Research entomologists have been involved in studying the taxonomy, ecology and management of biting midges associated with these and other pathogens, in part stimulated by the emergence of epizootic hemorrhagic disease in Texas deer farms.

“A recent study of biting midges in an urban area of Brazos County found eight species including the principal vector species associated with blue tongue and epizootic hemorrhagic disease,” Teel says. “This species has been studied under laboratory conditions and found to experimentally acquire and transmit the virus of African horse sickness.

“The discovery of African horse sickness in Thailand is another reminder that our global connectedness has risks and that state, national and global surveillance is the key to early warnings, to preparedness and to response.”

Elizabeth Parker, associate director for operations and strategic initiatives with AgriLife Research, says it is important for researchers to continue working with the equine industry and equine veterinarians in the US, as well as international organizations such as the World Organization for Animal Health, which is the international standard-setting body for how to safely move horses around the globe.


Currently, vaccines for African horse sickness are effective but not optimal because they contain live pathogens that can sicken horses, especially if not administered correctly, or lead to the creation of new genetic variants of the disease.

In countries where African horse sickness is a problem, Teel says prevention is provided with vaccines to the serotype present in that region, or the use of insecticides to keep the midges from biting.

With horses, it might mean keeping them stalled in areas where biting midges are a problem and using insecticides to keep the midges off them. But this might not be as possible with wild horses or horses in pastures who may not be able to be handled or stabled.

Moving forward

Teel says the investigation in Thailand should help scientists understand what happened and how the disease traveled – most likely infected animals moved to an area where there were native vectors.

“For the US, we need to be vigilant in understanding what this virus is, its cycle in nature as it is presently understood, and then to determine what vectors we have here and how they could potentially play a role in this.

“With blue tongue and epizootic hemorrhagic disease, we might already have some insight on how efficient the midges might be as vectors.

“And then we have the other part of vector transmission and that is called vector capacity. Is the timing right for the vectors to work in the field, as opposed to the lab, looking at seasonal phenology? We have a lot to learn and we need to stay on top of it.”

Reporting: Kay Ledbetter

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