African horse sickness is not in the United States and it’s important it stays that way. Texas A&M University-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 our borders.
Horses would be at risk if African horse sickness makes it into the United States. African horse sickness is common from Morocco down to the middle of the African continent. But it has escaped the African continent several times, most recently into Thailand, where it’s believed to have been introduced by importation of infected zebras. African horse sickness is caused by a virus, more specifically an Orbivirus, which is transmitted by certain insects.
Equines including horses, mules, donkeys and zebras are moved all around the world for competitions, trade, breeding, zoologic and conservation purposes. 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.
The United States 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 moves into our country.
Federal agencies, including the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, state agencies, and concerned and vigilant equine-industry stakeholders are all part of the first line of defense.
The tiny biting midge, as seen here compared to a mosquito, right, is a vector for African horse sickness.
If African horse sickness does come to the United States we have knowledge of insects that will likely transmit this virus. But we don’t know everything we need to know about it. Depending on the serotype of the virus that were to be introduced it would be necessary to determine whether the insects we have are competent vectors in the laboratory and capable vectors in the field. And we would need to improve practices to protect equines from vector transmission and infection.
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. African horse sickness 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. They are to vaccinate noninfected horses with polyvalent vaccine and then let them rest for two weeks.
The Animal and Plant Health Inspection Service along with the U.S. Fish and Wildlife Service have import requirements in place to reduce the likelihood of African horse sickness introduction. Per Animal and Plant Health Inspection Service regulation, imported horses undergo inspection prior to export. Horses imported from African horse sickness-affected countries are required to undergo a 60-day quarantine upon arrival in the United States.
Horses are the most susceptible host, with almost 90 percent mortality of those affected, followed by mules and donkeys. African donkeys and zebras rarely display clinical symptoms, despite high virus titers in blood. They are thought to be the natural reservoir of the virus. That’s why it’s 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 one-eighth of an inch in length. The immature stages of the 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. Those 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 are biting midges in the United States. They’re involved in the transmission of two similar viruses, causing diseases known as blue tongue and epizootic hemorrhagic disease in livestock and wildlife. AgriLife Research entomologists have been involved in studying the taxonomy, ecology and management of biting midges associated with those and other pathogens. A recent study of biting midges in an urban area of Brazos County found eight species including the principle vector species associated with blue tongue and epizootic hemorrhagic disease. That 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. It’s important for us moving forward to continue working with the equine industry and equine veterinarians here in the United States and with international organizations such as the World Organization for Animal Health; it recently did a series of webinars and is the international standard-setting body for how to safely move horses around the globe.
Vaccines for African horse sickness are effective but currently 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, 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 if in areas where biting midges are a problem, and using insecticides to keep midges off them. But that might not be possible with wild horses, or horses in pastures that may not be able to be handled or stabled.
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. We have a lot to learn and we need to stay on top of it.