When horses suddenly started dying in Thailand as the nation locked down to stem the spread of COVID-19, researchers feared the cause was another deadly bat-borne virus that could kill humans.

"We had no idea what was causing it," said Nopadol Saropala, owner of a horse farm about 100 miles from the Thai capital, who lost 18 horses in nine days. "We found out later that it came from zebras that were apparently in transit to China."