An African Horse Sickness outbreak has ferociously killed hundreds of Thai horses in less than three weeks. How at risk is Australia?
In some languages, it’s called “The Equine Plague.” And given its rapid transmission, fierce symptoms, and haunting mortality rates, the “plague” might be a more apt description for the viral equine disease that’s currently sweeping across parts of Thailand.
African Horse Sickness (AHS), as it’s known in English, has ferociously killed hundreds of Thai horses in less than three weeks in an outbreak that purportedly started after zebras were imported from Africa. If not contained, it could spread southward to Australia, according to experts.
“There is a possibility of infection spreading as far as Australia if inadequately controlled, at least in theory, said Evan Sergeant, PhD, BVSc, MANZCVSc, veterinary epidemiologist at AusVet Animal Health Services in Canberry.
However, such a spread would take one or more years and a combination of “bad luck” and high numbers of midges—the biting insects that transmit the virus from equid to equid—transported by wind currents, Sergeant said.
Data provided by the World Organisation for Animal Health (OIE) indicate that these insect vectors could travel in the air distances of 700 km over water or 150 km over land, according to Sergeant. However, these figures are “probably extreme,” and spread over such distances “would be uncommon,” he said.
“Vector spread would obviously also depend on prevailing winds and/or monsoon activity,” he added.
AHS infects all equids but primarily creates serious disease in horses, which have a mortality rate ranging from 70 to 90% or more. The disease, which causes facial swelling, frothy nasal discharge, and internal edema and hemorrhaging, only kills 10% of donkeys and rarely leads to clinical signs in zebras. The current outbreak in Thailand is the first appearance of AHS in Asia (outside the Middle East) since 1961. While possible imports of zebras from Africa are widely suspected as the source of the outbreak, Thai officials have yet to report on zebra testing results or on any records of imports into the country.
Endemic in sub-Saharan Africa, AHS is a constant threat to South Africa, where scientists lead ongoing control measures including vaccination protocols and vector management. “The general rule is having horses indoors behind (efficient) screens from two hours before sunset until two hours after sunset,” said Camilla Weyer, BVSc, MSc, PhD, of South African Equine Health and Protocols, in Capetown. During times of outbreak, scientists recommend a four-hour margin around sunset and sunrise, or even full-time indoor management including walking the horses inside tightly protected enclosures, she explained.
The main vector species in AHS spread in Africa, the Culicoides imicola midge, is “not present in Australia,” according to Mark Schipp, Australia’s Chief Veterinary Officer and President of the OIE Assembly. However, other Culicoides species present in Australia could possibly act as vectors for AHS.
For Schipp, the risk, however small, is not to be ignored.
“Wind movement of infected insect vectors has the potential to spread AHS behind the current outbreak area (in Thailand),” Schipp said. “AHS is exotic to Australia. There would be devastating consequences to the Australian equine industry if it were to establish here due to the high morbidity and mortality in some strains.”
Australia’s current biosecurity requirements for imported horses already manage the risk of AHS, Schipp explained. “These include only permitting imports from countries that have been free from AHS for two years prior to export and which do not allow vaccination,” he said. Thailand is not approved to export horses directly to Australia.
While it might seem paradoxical, AHS vaccination itself presents a risk, according to Weyer. The live attenuated virus used in vaccines has been known to replicate and create its own, albeit less severe, form of outbreak.
With strong biosecurity measures in place and unfavorable conditions for natural spread via the wind, the risk is “still pretty low,” according to Sergeant. “Spread would probably take years, as infection would have to establish in equid populations along the way,” he said. “So, the spread would also depend on the presence of equid populations sufficient to support infection, in countries and/or islands along the way as well as in northern Australia.”
“Australia continues to monitor the outbreak of AHS in Thailand as part of our routine animal disease surveillance and is engaging with other countries to develop a regional response to this outbreak,” Schipp said.