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Keeping African horse sickness out of U.S. is critical

by Wyoming Livestock Roundup

To date, African horse sickness (AHS) is not in the U.S., and officials say it is important it stays this way.

            Federal agencies, including USDA Animal and Plant Health Inspection Service (APHIS), state agencies such as the Texas Animal Health Commission, Texas A&M AgriLife Extension Faculty and other concerned equine industry stakeholders are currently monitoring the disease, ensuring surveillance and determining practices to prevent AHS from crossing into the U.S. 

African horse sickness

            According to Merck Veterinary Manual, AHS is an insect-borne, viral disease endemic to sub-Saharan Africa. It can be acute, subacute or subclinical and is characterized by clinical signs and lesions associated with respiratory and circulatory impairment. 

            AHS is caused by the African horse sickness virus (AHSV), which consists of nine immunologically distinct serotypes. 

            “Culicoides spp are the principal vectors of all nine serotypes of AHSV, with C. imicolausually considered to be the most important,” explains Merck. “Consequently, AHS is seen during warm, rainy seasons, which favor propagation of the vectors, and disappears when cold weather stops or significantly reduces vector activity.”

            Merck notes mortality depends on the virulence of the AHSV strain and susceptibility of the host.

            “A mixed pulmonary and cardiac form is most commonly seen in outbreaks,” states Merck. “In naïve populations of horses, which are most susceptible, mortality may reach 90 percent.”

AHS vectors    

“The primary vectors of AHS are among species of biting midges, which are very small, blood-feeding, flying insects about one-eighth of an inch in length,” explains Dr. Pete Teel, Texas A&M AgriLIfe research entomologist. “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.”

Teel notes these midges are biological vectors, meaning 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.

“There is a real risk 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 explains. “In fact, we have biting midges in the U.S. They are involved in the transmission of two similar viruses causing diseases known as blue tongue and epizootic hemorrhagic disease (EHD) in livestock and wildlife.”

Teel points out the state of Texas, as well as several other states, have biting midges, blue tongue and EHD.

“Entomologists at Texas A&M AgriLife 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 EHD in Texas deer farms,” Teel explains. 

He continues, “A recent study of biting midges in an urban area of Brazos County, Texas found eight species including the principle vector species associated with blue tongue and EHD. This species has been studied under laboratory conditions and found to experimentally acquire and transmit AHS.”

Teel reassures if AHS were to threaten the U.S., AgriLife Research and Extension has diverse expertise, facilities and services to assist in preparation and response, including basic and applied research.

Preventing AHS

According to Teel, current vaccines for AHS exist and are effective. 

However, he also notes they are not optimal because they contain live pathogens, which can sicken horses, especially if not administered correctly, or they may lead to the creation of new genetic variants of the disease.

In countries where AHS 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.

Teel says, prevention of AHS for horses living in the U.S. might mean keeping them stalled in areas where biting midges are a problem and using insecticides to keep midges off them. 

“But this might not be possible with wild horses or horses in pastures that may not be able to be handled or stabled,” he notes. 

Teel continues, “The U.S. 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.”

Moving forward

Teel notes AHS has most recently been reported in Thailand. 

“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,” he says. 

“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 determine what vectors we have here and how they could potentially play a role in this,” he adds. “With blue tongue and EHD, we might already have some insight on how efficient the midges might be as vectors.”

Hannah Bugas is the managing editor for the Wyoming Livestock Roundup. Send comments on this article to

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