African horse sickness is not in the U.S., and it is important that it stays that way. Texas A&M AgriLife faculty, various state and federal agencies and the U.S. horse industry are already monitoring the situation, ensuring surveillance and determining practices to prevent the deadly horse disease from crossing into the U.S.

African horse sickness comes out of Africa and is common from Morocco down to the middle of the continent. It has escaped the African continent several times, most recently reaching Thailand, where it is believed to have been introduced by the importation of infected zebras, Texas AgriLife said in an announcement.

African horse sickness is caused by an Orbivirus, which 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,” said Dr. Pete Teel, Texas A&M AgriLife Research entomologist with the Texas A&M department of entomology in College Station, Texas.

“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 said.

“The U.S. is constantly threatened by introductions of foreign animal diseases,” he said. “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 U.S. Department of Agriculture's Animal & Plant Health Inspection Service (APHIS), state agencies like the Texas Animal Health Commission and concerned and vigilant equine industry stakeholders are all part of the first line of defense, Teel said.

“If African horse sickness does come to the U.S., 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 the virus introduced, “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.”

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

African horse sickness symptoms are the same as those associated with respiratory and circulatory impairment, Texas AgriLife said. At the first signs of the disease, owners are advised to eliminate affected horses and vaccinate noninfected horses with polyvalent vaccine; then let them rest for two weeks.

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

Expertise

“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 said. “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 explained 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 U.S.,” Teel said. “They are involved in the transmission of two similar viruses causing diseases known as bluetongue and epizootic hemorrhagic disease in livestock and wildlife.”

Texas 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, the announcement said.

“A recent study of biting midges in an urban area of Brazos County found eight species including the principle vector species associated with bluetongue and epizootic hemorrhagic disease,” Teel said. “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,” he added.

Teel reassured that were African horse sickness to threaten the U.S., AgriLife Research and AgriLife Extension has diverse expertise, facilities and services to assist in preparation and response, including basic and applied research.

“It is important for us moving forward to continue working with the equine industry and equine veterinarians here in the U.S. and with international organizations such as the World Organization for Animal Health, who recently did a series of webinars on this event and are the international standard setting body for how to safely move horses around the globe,” said Dr. Elizabeth Parker, AgriLife Research associate director for operations and strategic initiatives.

Prevention

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, Texas AgriLife said.

In countries where African horse sickness is a problem, Teel said 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. However, this might not be as possible with wild horses or horses in pastures that may not be able to be handled or stabled.

Teel said 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 U.S., 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,” he said. “With bluetongue and (epizootic hemorrhagic disease), we might already have some insight on how efficient the midges might be as vectors.

“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,” Teel said.