Thailand scrambles to contain major outbreak of horse-killing virus

By Christa Lesté-Lasserre, Science Magazine, Apr. 16, 2020

Thailand, already battling the spread of coronavirus, is now contending with another deadly viral outbreak—in horses. With hundreds of horse deaths reported there in the last 3 weeks, horse owners are rushing to seal their animals indoors with netting, away from biting midges that spread the virus for African horse sickness (AHS). Some scientists suspect that zebras, imported from Africa, led to the outbreak.

The disease’s sudden appearance, far from its endemic home in sub-Saharan Africa, has surprised Thai veterinary authorities, who are ramping up testing for the disease and ordering the vaccination of thousands of horses, donkeys, and mules. It is the first major outbreak of the disease outside Africa in 30 years, and AHS experts are worried that it could spread to neighboring countries in Southeast Asia. “A sustained, persistent outbreak of [AHS] that spreads to other countries would be devastating, not only to the racing industry and companion animals, but also to some of the poorest workers in the region relying on working horses, donkeys, and mules,” says Simon Carpenter, an entomologist at the Pirbright Laboratory in the United Kingdom.

Without controls, the virus could even travel via wind-borne midges across seas to herds on island nations, gradually working its way to Australia, which has more than 1 million racing, sport, and feral horses. The nation is “engaging with other countries to develop a regional response to this outbreak,” says Australia’s Chief Veterinary Officer Mark Schipp.

The AHS virus infects horses, donkeys, and zebras, and is typically transmitted by Culicoides midges that live in warm, tropical climates. The virus causes severe heart and lung disease that kills at least 70% of infected horses, but spares zebras and most donkeys, which act as reservoirs for the virus, says Evan Sergeant, an epidemiologist at AusVet Animal Health Services in Canberra, Australia. Treatment options are mostly limited to palliative care, although euthanasia is sometimes recommended because of the brutality of the disease, which causes high fevers, swollen eyes, difficulty breathing, frothy nostrils, internal bleeding, and sudden death.

Aside from brief outbreaks in areas off the African coast, AHS has been contained in Africa since 1990, when veterinary authorities resolved a 3-year-long outbreak in Spain and Portugal caused by the importation of wild African zebras, Carpenter says. The virus hasn’t been reported in Asia since a major epidemic that ended in 1961. That epidemic spread from the Middle East to parts of India and led to hundreds of thousands of equine deaths.

The only commercially available AHS vaccine is based on a live, weakened version of the virus that sometimes produces mild symptoms and can even spread to other horses. Still, it has successfully eradicated previous outbreaks, according to Carpenter. “It’s not an ideal vaccine,” he says. “But it’s nowhere near as bad as the disease itself.”

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FEATURED IMAGE: A horse in Thailand is isolated behind netting that keeps out midges that spread African horse sickness. WIPAWAN PAWITAYALARP