Midge and mosquito

The tiny biting midge (Culicoides impunctatus), left, is a vector for African horse sickness. At right, shown for comparison, is a mosquito (Ochlerotatus annulipes).

African horse sickness virus is not in the U.S., but it is a growing concern as a potential threat. Texas A&M University AgriLife faculty, along with state and federal agencies and U.S. horse industry stakeholders, are monitoring the situation, utilizing surveillance and working on practices to prevent the horse disease from crossing our borders.

The virus 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 into Thailand, where it is believed to have been introduced by importation of infected zebras. African horse sickness virus, more specifically an orbivirus, 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, a Texas A&M AgriLife Research entomologist with the Texas A&M Department of Entomology in 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.”

The U.S. Department of Agriculture Animal and Plant Health Inspection Service (APHIS), Texas Animal Health Commission and concerned industry stakeholders are all part of the first line of defense, Teel said.

About the virus

“If African horse sickness virus 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.”

African horse sickness virus 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 virus 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 polyvalent vaccine, then let them rest for two weeks.

The APHIS and the U.S. Fish and Wildlife Service have safeguards in place to reduce AHSV introduction. Per APHIS regulation, all imported horses undergo inspection before arrival in the U.S. Horses imported from countries with active cases undergo a 60-day quarantine before being imported.

Texas A&M expertise on the virus

“The primary vectors of African horse sickness virus are among species of biting midges, which are very small blood-feeding flying insects about one-eighth 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.”

In layman’s terms, the biting midges transfer the virus from infected animals to new host animals, spreading the virus.

“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 virus.

“The discovery of African horse sickness virus 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 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, AgriLife Research associate director for operations and strategic initiatives in College Station.

Prevention

Currently, vaccines for the virus 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 virus is a problem, Teel said prevention is provided with vaccines to the serotype present in that region and with 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.

Moving forward

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.”

Portions of this article were first published in AgriLife Today by Kay Ledbetter. Article is used with permission.

ZACH DAVIS is the agriculture and natural resources extension agent with Texas A&M AgriLife Extension. He can be reached at 940-349-2889 or via email at zadavis@ag.tamu.edu.

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