African Horse Sickness Update

With mortality rates topping 90%, AHS has left a devastating wake of destruction across Thailand and Malaysia. Learn about this disease, the recent outbreak, and what it could mean for U.S. horse owners.

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African Horse Sickness Update
A horse with African horse sickness, which has a higher than 90% mortality rate. | Photo: Courtesy Manuel Rodriguez

Hélène Sundermann’s horses are stuck in Thailand. To celebrate the end of her master’s program at an agricultural engineering school outside Paris, France, Sundermann flew to Cambodia and bought two local Khmer horses: First and his dam, Chanty. After some prep time they began a four-month, 1,500-kilometer (932-mile) ridden journey from Phnom Penh, Cambodia, to Chiang Mai, Thailand, passing over spectacular Asian landscapes.

Unfortunately, that’s where the journey ends. Not because of lameness, fatigue, or illness, but because of a virus.

It was April 2020. As humans battled the COVID-19 pandemic, equids in Thailand started battling a major African horse sickness (AHS) epidemic caused by a virus spread by flying insects. Suddenly, borders were closing everywhere—to horses as well as people—and Sundermann made the decision to head home to France. Because of AHS, which had already claimed hundreds of Thai horses’ lives, and the stringent movement restrictions to control it, however, she’d have to leave Chanty and First at a boarding facility in Chiang Mai.

Every day since, Sundermann’s been fighting a battle to get her healthy horses back to France. But there’s little she can do besides wait.

With mortality rates topping 90%, AHS has left a devastating wake of destruction across Thailand. And although it seems to finally be under control after implementation of a massive vaccination program and insect control measures, authorities are unlikely to give the gruesome disease any chance of spreading outside the endemic regions of sub-Saharan Africa, our sources say.

AHS: ‘Horrific’ and Deadly

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African horse sickness is definitely not the kind of illness you’d want to risk your horse getting, says Simon Carpenter, PhD, head of the Entomology Group at the U.K. Biotechnology and Biological Sciences Research Council Pirbright Institute, in Woking, Surrey.

“It’s a horrific disease,” he says. In its worst forms AHS causes pulmonary edema, myocarditis (inflammation of the heart muscle), pericardial hemorrhaging, hemorrhagic gastritis, and more. Horses die in agonizing pain, with high fevers and bloated fluid-filled chests, gasping for air as foam spews from their nostrils, sometimes with blood seeping from their inflamed eyes.

The disease is the ultimate horror movie for horse owners. And yet it’s real. It exists all across the countries of sub-Saharan Africa—from western Senegal to eastern Ethiopia and Somalia, extending into South Africa, where horse owners proactively battle the disease at their northern borders with massive vaccination campaigns and containment programs, says Camilla Weyer, BVSc, MSc, PhD, senior research officer and AHS control manager at South African Equine Health and Protocols in Somerset West, in the Western Cape.

The AHSV-Midge-Zebra Cycle

Primary culprits in the spread of AHS virus (AHSV) are tiny Culicoides midges, which are about a millimeter long. These swarming, blood-sucking, flying insects aren’t just a nuisance. They also serve as deadly vectors, picking up AHSV in one equid and delivering it to another following a period of replication. This results in massive spread, especially in horses that have never been exposed to or vaccinated against AHSV.

While the virus kills most horses it infects, depending on its serotype and strain, it usually spares zebras and most African donkeys. These equids host the virus for weeks at a time, generally without showing clinical signs. Entire herds of healthy wild zebras serve as reservoirs for AHSV, where it thrives, multiplies, and spreads to other zebra herds—and to any horses within the midges’ flight range—about 90 miles by land or 400 miles over water, says Evan S. Sergeant, PhD, of the AusVet Animal Health Services, in Canberra, Australia. Other animals, such as dogs, camels, and elephants, appear to develop antibodies against the virus, but their role in its propagation is currently unknown, he says.

Scientists believe it’s essentially impossible to eradicate AHSV in sub-Saharan Africa because the approximately 800,000 free-roaming plains, mountain, and Grévy zebras, as well as some local donkeys, keep the virus alive and circulating, says Carpenter. “Eradication is always hard where there’s a reservoir,” he says. But strict biosecurity and especially wildlife import controls should—in theory— keep AHS in the southern two-thirds of the African continent.

2020 Thailand Outbreak: Unexpected, Unrecognized, Unprepared

In March 2020, just as COVID-19 was breaking out across the globe, filling hospitals and shutting down schools and national economies, and just as monsoon rains started hitting Southeast Asia, dozens of horses about 100 miles northeast of Bangkok suddenly dropped dead following a short-lived but brutal illness. It was AHS—2,500 miles away from the Somalian coast, Africa’s easternmost shore—but 42 Thai horses would die before anyone recognized the disease, which had never before hit Southeast Asia. The virus hadn’t been reported in Asia since a major epidemic 60 years ago that spread from the Middle East to India and Pakistan.

By the time the Pirbright Institute—also an AHS reference laboratory recognized by the World Organisation for Animal Health (OIE)—confirmed the diagnosis of one treating veterinarian in Thailand, AHS was rampant across several Thai provinces. News spread as fast as the disease itself, but horse owners and stable managers were unprepared.

Rapid, Efficient International Coordination

Once local and international authorities realized what they were dealing with—the first AHS outbreak outside of Africa since an epidemic in Spain/Portugal 30 years earlier—a worldwide collaborative effort to address the Thai outbreak quickly came together, says Carpenter. The COVID-19 pandemic threatened to challenge unification, he says, but the consequent online conferencing might have actually been serendipitous. “Normally we would have been over there like we do routinely for outbreaks,” he says. “But there was good communication between the countries, and I think it worked surprisingly well.”

Hundreds of participants—scientists, veterinarians, government authorities, industry leaders—attended the five-part series of webinars the OIE, in Paris, organized to address the Thai outbreak and mitigate risks of further spread.

“Since the detection of the first outbreak in Asia, the OIE has mobilized its resources, as well as its network of experts and partners, to support Member Countries in their prevention and control efforts, in collaboration with local key stakeholders,” says Matthew Stone, BVSc, MVS, OIE deputy directorate general. The group developed communication programs and materials for local scientists and authorities as well as the public. They also created new guidelines to help Asian countries prepare for AHS should it spread further. The goal, he says, is to help those countries “establish an environment that will enable the implementation of emergency AHS vaccination.”

African Horse Sickness Update
Barn managers use insect fogger in an arena to kill any midges that might have made their way through the protective netting. | Photo: Courtesy Karen Wainwright

Vector Control and Vaccines

There’s no cure or treatment, other than palliative care, for AHS, our sources say. So the only plan of attack includes rigorous vector control—keeping midges away from horses—and vaccination.

Before selecting a vaccine, though, scientists must determine which of the nine AHSV serotypes is causing the outbreak, says Carpenter. Then manufacturers must ship the vaccine that will be most effective or produce it if supply is short, which was the case for Thailand. This was exacerbated by the fact that the Serotype 1 strain involved in the outbreaks had not previously caused detected epidemics in Africa, he says. COVID-19 restrictions slowed the process further, and Thailand didn’t receive its first vaccines until three weeks after the first positive AHSV test.

Without vaccines or the equipment to protect horses from midges, and with supply chains hampered by COVID-19 restrictions, the Thai equine industry scrambled to make do with what they could find to separate horses from insects and each other. “How we control vectors in this first stage of the process is critical in reducing transmission,” says Carpenter. That requires securing horses in stables, covering every window with tight-mesh netting, filling in the tiniest gaps with expandable foam, and soaking netting and cracks with insecticide daily.

African Horse Sickness Update
Karen Wainwright’s horse has turnout time inside a netted arena. | Photo: Courtesy Karen Wainwright

“In the Thai outbreak context, there was this whole scenario of just getting the initial response going with whatever netting and insecticide was available on their market,” Carpenter says.

Successful vector control isn’t just critical before the vaccine, he adds. Because they used the standard AHS vaccine based on a live, attenuated virus, there’s a risk that the vaccine virus could replicate, cause mild illness, and spread via midges. Horses would need to stay in tightly closed stables until 28 days after vaccination. “It’s not an ideal vaccine, but the alternative is a whole lot worse,” he says.

Fortunately, local suppliers seemed up to the task, Carpenter adds. Studying Thailand’s initial response has helped researchers develop guidelines for managing future outbreaks with regional market supplies.

Movement Restrictions: A Critical Control Measure

Thailand immediately established strict quarantine at a country level and halted all movement (transport) of equids, both domestically and abroad. The country lost its official “AHS-Free Zone” status with the OIE, meaning national authorities were required to stop any international movement of the affected species. Animal health officials can reapply for the status two years after the last detected case.

People don’t always respect movement restrictions, however, sometimes reacting in a panic, says Carpenter. “There’s this automatic response from owners to just move their horses as far away as they can, as well as moving animals for other reasons, and that’s always a nightmare in this situation,” he says. And that’s what happened within Thailand, with initial outbreak clusters followed by “seeding” across the country with horse transport. “Obviously that’s pretty lethal if you’re spreading the virus around,” he says. “Getting movement restrictions in place quickly is vital.”

Spread Into Malaysia

All summer, veterinarians swept through Thailand’s 12,000-strong equid population, made up mostly of sport and leisure horses and ponies as well as working equids, vaccinating animals by the thousands. By then, the virus had claimed the lives of 502 horses.

But Southeast Asia hadn’t seen its last case. By late August, five horses had tested positive for AHS in northeastern Malaysia, 500 miles from the nearest Thai outbreak. Fear resurged, with the recognition that AHS hadn’t been successfully contained within Thailand.

The situation took a strange turn, however. The Malaysian horses had extremely mild clinical signs—mainly lameness—compared to the Thai horses, and they all survived. (Even so, for the sake of outbreak management, they were euthanized.) And Malaysia hasn’t reported any cases since.

But, due to limited reporting and inconclusive testing efforts, “it’s hard to know what happened there,” Carpenter says.

Peter J. Timoney, MVB, MS, PhD, FRCVS, professor and Frederick Van Lennep Chair in Equine Veterinary Science at the University of Kentucky’s Gluck Equine Research Center, in Lexington, agrees. “Every week, countries in which this disease occurs are meant to provide follow-up reports to the OIE,” he says. “Certainly, there have been reports, but nothing of any further informational value in terms of that occurrence. It seems authorities are not willing to be forthcoming with respect to providing any more details about what has been happening there since August.”

Six months later, international scientists and authorities were still waiting on details—how AHS got to Malaysia, why signs were relatively mild, and which serotype it was. Serotype identity could help link the outbreak to the Thai outbreak or even resolve the issue of whether it was a “vaccine” outbreak. Timoney also wants to know why the virus didn’t spread elsewhere in the country, since its National Veterinary Authorities prohibited vaccination against the disease.

Answers remain scarce, however. “As reported by the national authorities to the OIE, epidemiological investigations to determine the source of infection in the Malaysian outbreak are still ongoing,” says Stone.

Is it Over? Lessons From Spain

With no new cases reported anywhere in Southeast Asia since the late August outbreak in the five Malaysian horses and a Sept. 2 outbreak in two Chapman’s zebras in Thailand, have we won this battle? Probably not, say our sources.

African horse sickness has “this horrible habit of disappearing and then coming back,” Carpenter says, referring to the outbreak that started in 1987 in the Iberian Peninsula after the arrival of 10 infected zebras from Namibia. Every year for three years, it seemed authorities had wiped the virus out by winter. But every spring through 1990, it reappeared and even spread into Morocco.

“How did it maintain itself over a period of more than three years in southern Spain?” says Timoney, adding that vaccination was broad and serotype-targeted. “Nobody has successfully answered that question.”

Therefore, international experts are waiting to see what happens in spring 2021 in Thailand. “We have a lot to learn from Spain and Portugal,” says Carpenter.

Take-Home Message

Sundermann’s horses will likely be stuck in Thailand for at least two years, despite her hopes of getting them home. It’s a huge sacrifice, but it’s a necessity for protecting horses across the globe. AHSV is a killer that spreads freely and abundantly through an essentially limitless supply of voracious vectors that thrive in climates throughout the world. With total isolation and less-than-ideal vaccines as their only weapons for the moment, horse owners and veterinarians must remain vigilant and in fighting AHS outbreaks, while hoping for reinforced government measures to control exotic animal movement. Meanwhile, veterinary authorities must be transparent in their reporting of the disease.


Written by:

Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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