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.”

The outbreak in Thailand may have begun in late February, with the unexplained death of a racehorse in the Pak Chong district near Bangkok. By late March, after rains that might have helped midge populations flourish, more than 40 additional Pak Chong horses were suddenly reported dead, says Nuttavadee Pamaroon, a veterinary officer in Thailand’s Department of Livestock Development (DLD). Thai veterinary authorities ordered AHS testing and immediately froze all horse movement. “It’s not only us who have been locked down because of COVID,” Pamaroon says. “The horses are right now locked down as well.”

However, some infected horses had already moved out of the outbreak zone. On 10 April, its last official update, DLD reported 192 horse deaths across 37 racing, sports, and leisure riding farms. But according to a source working closely with DLD who spoke on condition of anonymity, a total of 302 deaths had been declared to officials by 14 April and numbers are still rising.

Veterinary authorities are ordering testing and the vaccination of disease-free horses in a zone 50 kilometers around the initial outbreak site, Pamaroon says. Because the vaccine can create outbreaks of its own, each vaccinated horse must be held under “strict individual nettings,” says Siraya Chunekamari, a Bangkok-based equine veterinarian who is working with DLD to manage the outbreak.

The first batch of approximately 4000 doses of vaccine was scheduled to arrive last Monday in Bangkok, authorities stated last week. However, local sources say they are still waiting for the vaccine, with delivery expected for Thursday or Friday.

The government is now offering subsidies for AHS testing and vaccines, alleviating financial burdens for owners already hit hard by the economic effects of the coronavirus pandemic, says Nopadol Saropala, a physician who also runs a business offering guided horse rides in Pak Chong. Saropala, who has lost 17 horses to AHS so far, says he joined DLD’s task force last week, representing industry owners. “Many of us already had mosquito netting, but midges are only a millimeter long so we had to put up netting so tightly woven that even light barely gets through,” he says.

Owners want the government to address how the outbreak started, Saropala says. Zebra importers may have benefited from biosecurity loopholes that allowed them to bring the animals into Thailand relatively liberally—a stark contrast to the strict quarantines and inspections required for horse imports. “We know zebras were imported from Africa recently,” he says. “I’m asking the DLD for official data, but they keep dragging their feet.”

Legislation passed 2 weeks ago conspicuously places zebras under DLD jurisdiction for the control of disease outbreaks, but the government remains tight-lipped with regard to zebra import records and test results. “We are testing zebra populations, and for the moment, the investigation is ongoing,” Pamaroon says.

Imported zebras are a plausible source for the outbreak. Midges don’t transmit the virus through cadavers, meat, or hides, Sergeant says, and they haven’t been documented carrying the virus by air farther than 150 kilometers over land or 700 kilometers over water.

Thailand has now lost its AHS disease-free status with the World Organisation for Animal Health, which means it must halt its imports and exports of equine species, wild and domestic. It will take at least 2 years to apply for disease-free status again.