New strain of horse-killing Hendra virus identified

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A grey-headed flying fox.
A grey-headed flying fox. © Mike Lehmann, CC BY-SA 3.0, via Wikimedia Commons

Australian scientists have identified a new strain of the deadly Hendra virus that was the cause of a previously unexplained horse death in September 2015.

The newly recognised variant, identified by the Australian veterinarian-led research project, ‘Horses as Sentinels’, had not been detected previously by routine biosecurity testing in horses.

The new strain shares ~99% sequence identity with the 2015 horse case strain, and has been detected in grey-headed flying fox samples from Adelaide, South Australia, in 2013. Partial sequences of the variant have also been detected in flying foxes in other states. Grey-headed flying foxes migrate and their range includes parts of southern Australia, which previous advice classed as low risk – with some interpreting this to mean negligible risk of Hendra virus spillover. Up until now, the original strain of Hendra virus has been known to occur only within the range of black flying foxes and spectacled flying foxes.

Hendra virus is highly lethal in both horses and humans, with mortality rates about 79% and 60% respectively. The originally recognised strain of Hendra virus has resulted in the deaths of four humans and over 100 horses in Australia, since 1994.

The grey-headed flying fox (Pteropus poliocephalus) is classed as a megabat and at its last classification by the IUCN Red List in 2008, the population was classified as vulnerable to extinction. In the face of threats to its survival, roost sites have been legally protected since 1986 in New South Wales and since 1994 in Queensland. The species is protected under Australian federal law and its status is outlined in The Action Plan for Australian Bats.

Flying foxes play a critical role in the Australian environment through the pollination of native trees and the spreading of seeds. Without flying foxes, eucalypt forests and rainforests would cease to exist. Interaction between domestic animals and flying foxes can be exacerbated by pressure on flying fox populations, such as through loss of natural forest habitat, forcing flying foxes to seek urban and peri-urban food sources.

The ‘Horses as Sentinels’ research team has developed updated diagnostic laboratory techniques capable of identifying the new strain. They have also established that the current Hendra virus horse vaccine is expected to be equally effective against the new strain.

The researchers say the finding indicates that HeV should be considered as a differential diagnosis in unvaccinated horses anywhere in Australia that flying foxes are present, and that unwell, suspect horses who return an initial negative Hendra virus test should continue to be treated with the same caution as a Hendra virus-positive case, until testing for the new variant is performed.

There are several measures that people who work closely with horses can take to reduce the risk of infection with HeV and other viruses, such as vaccination, good biosecurity, use of personal protective equipment and good hygiene. “Now is the time for Veterinarians, horse owners and handlers to review their Hendra virus management plans,” the researchers said, adding that biosecurity protocols are explained on State Government and the Australian Veterinary Association websites.

Hendra virus was first identified in 1994 when racehorse trainer Vic Rail died after suffering a mysterious pneumonia-like illness when 20 racehorses in Hendra, Queensland, also died. Subsequently, a previously unknown virus was identified as the cause of both the trainer’s and the horses’ deaths. The attending veterinarian was Dr Peter Reid.

Nineteen years later, Reid reached out to Dr Ed Annand – a veterinarian involved in the discovery of Australian bat lyssavirus in horses in 2013. The pair shared an interest in previously undetected zoonotic disease spillover, and teamed up with Dr Ina Smith of CSIRO’s Risk Evaluation and Preparedness Program to launch the ‘Horses as Sentinels’ project.

The research team also includes collaborators from The Westmead Institute for Medical Research, The University of Sydney; Sydney Medical School; The Sydney School of Veterinary Science; CSIRO’s Health and Biosecurity; Queensland Department of Agriculture and Fisheries – Biosecurity Sciences Laboratory; Department of Agriculture Water and the Environment; and leading international scientists in this field in the United States.

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