Potential insect vectors in Texas
by Kay Ledbetter, Texas A&M AgriLife Marketing and Communications
African horse sickness is not in the U.S., and it is important 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 our 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, most recently into Thailand, where it is 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 over the world for competitions, trade, breeding, zoologic and conservation purposes,” said Pete Teel, Ph.D., Texas A&M AgriLife Research entomologist with the Texas A&M Department of Entomology, College Station.
“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 Animal and Plant Health Inspection Service, 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 that were to be 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.”
What is African horse sickness
African horse sickness is considered a Tier 3 disease by the National Bio and 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. 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.
The APHIS and the U.S. Fish and Wildlife Service have import requirements in place to reduce the likelihood of AHSV introduction. Per APHIS regulation, imported horses undergo inspection prior to export. Horses imported from AHS-affected countries are required to undergo a 60-day quarantine upon arrival in the U.S.
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. Thus, it is thought the virus was transported to Thailand through asymptomatic zebras.
Texas A&M expertise on African horse sickness
“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 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 blue tongue and epizootic hemorrhagic disease in livestock and wildlife.”
He said 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 principle vector species associated with blue tongue 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.”
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 Elizabeth Parker, DVM, AgriLife Research associate director for operations and strategic initiatives, College Station.
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 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. But 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 blue tongue and EHD, 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.”