Australia works to keep out trio of nasty equine diseases

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Australian scientists and biosecurity experts are working to keep three emerging risks at bay, with the equine influenza (EI) outbreak in 2007 fresh in scientists’ minds.

The three main threats are Japanese encephalitis virus (JEV), Hendra virus variant (HeVg2) and African horse sickness virus (AHSV) and scientists with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Australia’s Department of Agriculture, Water and the Environment (DAWE) have spoken of how incursions may occur, and how the country needs to be prepared for such threat.

The EI outbreak infected 69,000 horses across 9600 properties and cost $571 million to eradicate, and the racing industry estimated it cost them $500,000 a day in lost income and containment costs.

There are about one million domestic horses in Australia, owned by about 400,000 people. The harness and thoroughbred racing and export industries are worth $1.6 billion a year to the Australian economy.

African horse sickness was detected in Thailand in March of 2020 associated with the import of zebras. There was also a small detection in Malaysia in September 2020, but only a small number of animals and likely vaccine-related. Culicoides biting midges are the main vector.

Dr Debbie Eagles, Deputy Director, Australian Animal Health Laboratory at the Australian Centre for Disease Preparedness at the CSIRO said there seemed to be subsidence of the Thailand outbreak over time.

“So, there’s been vaccination used for control and not significant further outbreaks since 2020. But still, this remains a risk for Australia and certainly has reminded us of the importance of being prepared for this threat.”

Eagles said that for African horse sickness to be introduced to any area requires vertebrae hosts, so equids needs adult insect vectors to be present both at source and the arrival site, temperatures for those vectors to survive and an introduction event.

“The most likely mode of introduction into Australia would be by the long distance wind-borne spread if infected vectors were to make it to particularly Timor-Leste, to Eastern Indonesia, or potentially to Papua New Guinea,” Eagles said.

“And we do know that novel Bluetongue viruses do get blown into Australia by infected Culicoides on a reasonably regular basis by this means.”

Dr Ed Annand, from DAWE’s Epidemiology and One Health Section, said it had been challenging for veterinarians and for horse owners to manage Hendra risks given that it spills over sporadically but consistently.

Annand was involved with the work that helped identify the Hendra virus variant which has also resulted in the development of a variant vaccine.

He said that since 1994, there have been 65 individual spillover events recognised where the virus, a paramyxovirus related to measles and canine distemper virus, has spilled over from flying foxes where it circulates to horses.

“Horses interact with a lot of biting insects. Also, they are very inquisitive in their grazing behaviour. And the thought is there that they will inhale a little bit of virus-laden neuron that’s coming from a bat and then get the virus that way. So, it’s very unpredictable where it can happen, and it really just takes one horse that’s unvaccinated to come in contact with that virus,” Annand said.

“The vaccine we’ve got that’s safe and effective is going to be reliable against this virus. And that’s matched up with the fact that the vaccine actually works for a far more distant related virus, the Nipah virus.”

Although Japanese encephalitis virus has infected pigs in Australia, there has not been an Australian horse reported as infected.

Dr Dan Edson, Manager of DAWE’s Pacific Engagement Program in the Office of the Chief Veterinary Officer, is involved with progressing the importation of a Japanese encephalitis virus vaccine for use in Australian horses.

Edson said JEV is endemic in South and East Asia, but in countries where the disease pattern is seasonal, it was not known exactly how the virus returns each year. “A variety of mechanisms have been proposed including persistence in enzootic foci within vertebrate hosts and mosquitoes, reintroduction of the virus through migratory birds and/or mosquitoes, and possibly other vertebrates including bats and over-wintering in mosquitoes.”

He said JEV can cause neurological disease in horses and should be considered as a differential diagnosis for any horses displaying neurological signs. “And additional complicating factors that the clinical signs are similar to horses infected with Hendra viruses, where a neurological presentation is also possible.”

Prevention measures include using a hooded rug, a fly mask and safe insect repellent. “While indoor spraying of surfaces with residual insecticides is an effective control option for some mosquito-borne diseases particularly malaria, this approach is largely ineffectual in reducing JEV transmission as the mosquito that transmits JEV tends to feed at night and is reluctant to enter dwellings,” he said.

Work was under way to allow the emergency use of a JEV horse vaccine in Australia, Edson said. As it was not considered eradicable, “it’s important for horse owners to remain vigilant year-round and particularly at those times when mosquito numbers are high such as seasonal flooding events”.

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