Thailand Hit by African Horse Sickness, 42 Horses Dead

An outbreak of African Horse Sickness (AHS) in Thailand has caused at least 42 horse deaths out of 62 cases as of March 30, according to Thailand Equestrian Federation (TEF) reports. In response, the World Organization for Animal Health (OIE) has suspended the country’s AHS-free zone status.

Definitively identified by Thai veterinarians on March 27, this represents the first reported outbreak of AHS outside the African continent and Yemen in more than 30 years. In the late 1980s, the disease spread to Spain and Portugal, presumably via a herd of zebras imported into a Spanish zoo, and led to more than 3,000 deaths of primarily riding horses.

A fatal viral disease spread by various vectors including Culicoides–tiny, blood-sucking midges—and certain mosquitoes, AHS affects horses, mules, and donkeys and potentially dogs and camels. Horses are most susceptible to AHS, with a 75-90% mortality rate, said Polly Roy, MSc, PhD, FMedSci, professor of virology at the London School of Hygiene and Tropical Medicine, where her team is developing vaccines based on reverse genetics and proteins.

AHS’s sudden appearance in a country 4,000 miles across the Indian Ocean from the disease’s native endemic zone is “of great concern,” said Monique Eloit, DVM, director general of the OIE.

“(This) impacts the health and welfare of equine populations as well as international trade (or movements of horses for competition),” Eloit told The Horse. “The OIE Regional and Sub Regional Representations, in Tokyo and Bangkok respectively, are in close contact with the Veterinary Services of Thailand and neighboring countries to provide technical assistance, in the current context of spread of AHS outside its endemic areas in Africa. Regular updates on the situation from Thailand are expected to be communicated.”

The outbreak initiated in the Pak Chong province following a summer storm with heavy rain associated with an increase in flying insect populations, according to TEF. Initially, the federation reported spread of an unknown disease in a racing farm with clinical signs including high fever, pale gums, drooling, anorexia, and shortness of breath, leading to 30 equine fatalities. The disease spread to 11 farms in the area, killing 12% of the horses exposed to date. Once official veterinarians confirmed the diagnosis through laboratory testing, Thailand’s Department of Livestock Development of the Ministry of Agriculture and Coopératives and TEF set up immediate quarantine and travel restrictions as well as an online site for owner-based disease reporting.

On March 31, the department held a government meeting, in which dozens of masked delegates, veterinarians, and university professors convened in Bangkok and via videoconferencing, in the context of the human COVID-19 infectious disease crisis, to address the new equine infectious disease crisis plaguing their nation.

The source of the infection remains unknown at the current time, according to local authorities. However, the cause is likely to have come from an infected animal, “probably imported,” Roy explained. A vaccination program could help contain the epidemic. Her team’s research has led to “very good vaccine strains already for all serotypes based” with several trials in horses completed, she said. “These vaccines are highly protective against virulent virus challenge.” However, manufacturing has not yet begun. Existing vaccines, based on inactivated or modified live viruses, have had mixed results with regard to efficacy in outbreaks.