African Horse Sickness: OIE Webcast Offers Sobering Reality
Insecticides. Insect repellants. Quarantines. Double doorways. Nets across every barn window and silicone stuffed into every nook and cranny to stop midges and mosquitoes from carrying African horse sickness (AHS) from one equid to another. And a freshly arrived live-attenuated vaccine that might or might not start a chain of viral spread of its own.

All this, in a context of social distancing, closed businesses, and anxiety as horse owners and stable managers face the human COVID-19 pandemic.

Such is the grim reality facing horse owners across Thailand, particularly in the affected provinces where AHS has been killing horses at a rate of 94% in those showing clinical signs. Thai authorities have reported that 192 horses in six provinces have died as of last week, an increase of nearly 40 from the week before. The outbreak apparently began February 24 with the death of a racehorse in the Pak Chong district, but 42 horses had died before scientists confirmed a diagnosis of AHS in the previously AHS-free country one month later.

With no effective treatment in existence, the only strategy authorities can implement is one of prevention, said leading global experts during a webinar dedicated to Thailand’s AHS outbreak hosted April 10 by the World Organisation for Animal Health (OIE). Limited stocks of vaccines arrived from South Africa on April 13, with a priority given to horses in the most affected zones.

Vaccine use alone is insufficient, however, said Camilla Weyer, BVSC, MSC, PhD, senior research officer and AHS control manager at South African Equine Health and Protocols in Somerset West area. Owners of both vaccinated and unvaccinated horses throughout the country must adhere to stringent confinement procedures to prevent viral spread and further deaths. That starts, she said, with very well-closed stables.

“Your protective measure is only as good as how well closed your stable is,” Weyer said. “The ideal form of stabling would be a solid structure with something like brick walls and concrete floors with minimal external openings and windows … with the smallest gaps possible between the solid structures.”

Those gaps can be filled with expandable foam, silicone, or tight-mesh shade cloth impregnated daily with insecticide “so that even if those midges then do try to wiggle their way through the little gaps that are present, they are likely to come into contact with an insecticide and will probably die before they can bite one of the animals inside,” she said.

Weyer also recommended a double door system for entering stables, where horses and handlers can douse themselves with insecticides in a closed “in-between” zone before entering the protected area within the stable itself.

African horse sickness spreads from equid to equid through the bites of tiny, blood-sucking midges called Culicoides and occasionally mosquitoes, explained 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 toward the virus, but their role in its propagation is currently unknown.

“Insecticides or repellants alone aren’t enough; they have to work together,” Weyer continued, addressing Thai veterinarians, agricultural leaders, and about 200 other global participants in Friday’s online conference. “Insecticides are important for killing the midges before they can spread disease to other horses, but they won’t stop them from biting a horse before coming in contact with the insecticide. Repellant will help keep the midges off the horses, but it won’t kill them. So during this outbreak, you need to use both.”

Since the Pirbright Institute in the U.K. identified the Thai outbreak’s virus as Type 1 AHS, scientists can now pinpoint a vaccine strategy geared for that strain, 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 best option at this stage is a live attenuated vaccine because no other kind of vaccine is currently available commercially (although several other vaccines are still in the testing stages), he said. But this comes with the real, albeit low, risk of the vaccine setting off its own version of AHS in some horses, creating disease—though usually with less serious clinical signs—and viral spread through midges.

Around 12,000 horses, ponies, mules, donkeys, and zebras in Thailand face the threat of AHS—which might have entered the country through affected zebras, according to local sources. However, Thai officials, including Nuttavadee Pamaroon of Thailand’s Department of Livestock Development in Bangkok, continue to state that the investigations into those reports are ongoing. Donkeys and mules show less serious signs of disease, and zebras generally show no signs of disease, making these species “healthy reservoirs,” Sergeant said.

Containing the outbreak is critical not only for Thailand but also elsewhere in Asia, Sergeant said. While movement restrictions can prevent animals themselves from crossing borders, they won’t have any effect on the free movement of midges.

Midges generally stay within a “home zone” of about a 1-mile radius, especially if they have ample hosts to feed on, said Carpenter. However, if they multiply significantly or run out of hosts, they could travel significant distances.

“AHS can spread in midge migration up to about 150 km (about 93 miles) across land or 700 km (about 435 miles) across water—and I think there were outbreaks in Yemen that were probably associated with the movement of vectors on prevailing winds rather than by the movement of infected horses or other livestock,” Sergeant said. “It can cover fairly large distances and could infect other countries in Southeast Asia, either across land borders or even potentially to some of the island nations.”

That said, infection rate within midges themselves is low, he added. “So you’d either need a fair number of vectors or very bad luck for that to actually happen,” he said.