African Horse Sickness Cases in Thailand Quadruple

The death toll in the African Horse Sickness (AHS) outbreak in Thailand has nearly quadrupled in a little more than a week, reaching 154 deaths and a fatality rate of 94% in horses showing clinical signs, according to reports by the Thai government and the Thailand Equestrian Federation (TEF).

African horse sickness is a viral disease spread by various vectors including Culicoides–tiny, blood-sucking midges—and certain mosquitoes. While some healthy horses are testing positive in Thailand, those showing clinical signs have little chance of survival, an on-site veterinarian reported.

“The Pak Chong province has had a high number of deaths,” said Thapana Jarutummasiri, DVM, an equine veterinarian at TP Equine Ambulatory Services working throughout Thailand. That’s where most deaths have occurred, he added.

However, the disease has spread outside its initial zone—possibly due to horse movement before the disease had been diagnosed, he said. “We now have 37 stables affected in six provinces.”

“We are all scared and anxious,” said Karen Wainwright, a UK citizen who’s lived in Thailand for 30 years. So far, there have been no deaths among the 50 horses at the boarding center in Bangkok where she keeps her retired dressage horse, Tango, whom she imported from Denmark several years ago. At 130 kilometers (about 80 miles) from the outbreak’s epicenter, though, she and her fellow owners at the stable fear the spread.

“I know the owner of a farm (in Pak Chong) who lost 15 horses,” she said. “I also know the owner of another farm in that area who so far has not lost any horses, but she’s working night and day to protect them. We are all doing what we can, but with no treatment and currently no vaccine, we are all very worried.”

The source of the unprecedented Thai outbreak remains unknown—a fact that frustrates veterinarians and upsets owners. “We’re all very angry,” Wainwright told The Horse. “We want justice for the dead horses.” The lost horses had been “much-loved and well cared for,” and their owners are “heartbroken,” she added.

Most owners believe the virus was imported along with an infected herd of zebras, said Jarutummasiri. However, so far, the government has not confirmed those suspicions. “We asked (the Department of Livestock Development) about imported zebra but that’s still under investigation,” he said. That investigation requires cooperation with the Department of National Park, Wildlife and Plant Conservation and the Ministry of Natural Resources and Environment, he added.

Other owners wonder if the virus came from zebras that were in transit to China. “It seems there’s wildlife trade to China passing through Thailand with a resting place in Pak Chong,” said Sara Uggelberg, an expatriate Swede who owns a Swedish Warmblood named Rally.

Uggelberg lives in the northwestern province of Chiang Mai—“hopefully far from the outbreak of AHS,” she said. Still, she’s “taking all the precautions” she can. “I have very strong insecticides on my horse. The stable is sprayed with stronger insecticide, too.”

Recent additional testing of affected horses by the Pirbright Institute, in Surrey, U.K., which specializes in viral disease research, revealed the AHS virus causing the Thai epidemic is related to the Type 1 AHS virus, according to results published by the TEF.

Type 1 is “weird” because it’s never left the African continent, said Simon Carpenter, PhD, head of the Entomology Group at the U.K. Biotechnology and Biological Sciences Research Council Pirbright Laboratory, in Woking, Surrey. The outbreak in the late 1980s in Spain and Portugal and that across the Middle East and parts of India in the 1950s and 1960s were caused by strains of serotypes 9 and 4, he explained.

Treating Type 1 AHS will require the use of a live attenuated virus vaccine, he told The Horse.

While vaccination of horses will be critical to control the outbreak, an additional concern could be the existence of “reservoirs”—herds of healthy animals that survive the disease, added Carpenter. AHS has zero to low fatality in zebras and donkeys, so if midges bring infection to these equids in Thailand, eradication will be more difficult.

Thailand has a small donkey population, said Jarutummasiri. However, it has nearly 600 mules used in military service. Jarutummasiri’s main concern isn’t the domestic animal, but the wild equid. “What worries me is the 300-plus zebras we have in Thailand,” he said.

Still, Jarutummasiri has confidence in the government’s management of the epidemic. “The livestock department is discussing vaccination protocols to control the situation, and I know they’ll put in their full effort and handle this in the most effective way possible,” he said.