Single introduction was likely behind African Horse Sickness outbreak in Thailand – study

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Last year's outbreak of African Horse Sickness in Thailand probably arose from a single introduction of the virus into the country, the findings of molecular-based testing suggest.
A volunteer policeman patrols a beach in Hua Hin, Thailand. Image by DigitalDDay

Last year’s outbreak of African Horse Sickness in Thailand probably arose from a single introduction of the virus into the country, the findings of molecular-based testing suggest.

Dr Napawan Bunpapong and her colleagues at the Chulalongkorn University in Bangkok have reported their findings on the virus that wreaked havoc on the nation’s equestrian industry.

It was the first outbreak of the dangerous disease in the country’s history.

The study team, writing in the journal Emerging Infectious Diseases, also described the circumstances surrounding the outbreak, first reported in Nakhon Ratchasima Province, in northeastern Thailand.

The virus, spread by biting midges, ultimately spread to multiple provinces and killed hundreds of horses before being contained.

The researchers tested the virus, finding it was serotype 1 (there are nine known serotypes of the virus).

Of note, all virus samples tested in Thailand contained unique amino acids at positions 95V, 166S, and 660I, suggesting a single introduction from the same virus ancestor into Thailand.

They speculated that the virus potentially spread outside Africa from imported subclinically infected animals, such as zebras.

The Thailand government implemented control measures to prevent further spread, including movement restrictions, quarantine, disinfection, and vector control.

To prevent spread in Thailand and neighboring countries, mass vaccination of equids with a live-attenuated vaccine was conducted.

The study team said the Veterinary Diagnostic Laboratory at Chulalongkorn University was notified of unusual horse deaths on a recreational horse farm in March last year.

A total of 49 horses (two thoroughbreds, 21 miniatures and 26 native horses) were kept on pasture at the property.

Clinical signs included depression, fever, shortness of breath, and swelling near the eyes, followed by sudden death within 48 hours.

Samples were taken for testing, with African Horse Sickness identified. The outbreak lasted three weeks and affected 30 horses.

On April 26, horses on the farm were vaccinated; no horses showed clinical signs after vaccination and implementation of insect-proof housing.

In total, during the three weeks of the outbreak, 30 of the 49 horses on the farm died.

In their testing of the virus, the team extracted viral RNA from eight blood and tissue samples. All eight samples were positive for the virus.

Gene-specific testing followed, to learn more about the particular form of the virus involved in the outbreak.

Bunpapong N, Charoenkul K, Nasamran C, Chamsai E, Udom K, Boonyapisitsopa S, et al. African Horse Sickness Virus Serotype 1 on Horse Farm, Thailand, 2020. Emerg Infect Dis. 2021;27(8):2208-2211. https://doi.org/10.3201/eid2708.210004

The study, published under a Creative Commons License, can be read here

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