The science and mystery of hendra virus
Hendra virus was first observed in 1994 but Australia's leading researchers are still quick to list what they do not know about it.
Scientists identified the previously unknown virus after the death of Queensland horse trainer Vic Rail and 13 of his horses at Hendra, a Brisbane suburb, in 1994. The virus was later named for the suburb.
After extensive testing of a range of animal species, the CSIRO identified flying foxes (fruit bats) as its natural host.
A CSIRO-developed vaccine for horses was released in November 2012.
Despite their progress, scientists remain puzzled by several key elements in the story of Hendra:
- Why was 2011 the worst year for cases of Hendra?
- What is the trigger for the movement of the virus from fruit bats to horses?
- Is it one or more of Australia's four flying fox species that are responsible for its spread?
- How are fruit bats immune to the effects of such a virulent virus?
What is Hendra?
Hendra is a zoonotic disease, meaning it is able to move from animals to humans.
It was originally called 'equine morbillivirus' but was renamed Hendra and became the first of a new genus, henipavirus, within the paramyxoviridae family. It is closely related to the Nipah virus, which does not exist in Australia.
CSIRO scientists have recently (mid-2012) identified a third related virus, now called Cedar virus.
The discovery is described as a ‘lightbulb moment’ in Hendra research, with scientists keen to discover the genetic difference between the two.
While Hendra virus has proven to be deadly to both horses and humans, Cedar virus is harmless.
The evidence to date shows Hendra can be transmitted from flying fox to horse, from horse to horse and from horse to human.
In 2011, a dog tested positive to Hendra for the first time.
Scientists say it is known that other species may be susceptible to Hendra, but until now a case had not been seen beyond the laboratory.
Further research is now under way into how the virus affects dogs and whether they can transmit it to humans.
Fruit bats show no sign of illness when infected with Hendra, although when they shed the virus, it is highly virulent.
In fact, Hendra has been described as one of the most virulent viruses in the world.
It causes a range of symptoms in horses; they are typically fast acting and death comes rapidly from either respiratory or neurological symptoms.
Of horses infected with Hendra, 75 per cent die as a result of the virus. Horses found to have Hendra are typically killed to minimise any further spread of the virus.
While the transfer of the virus from horses to humans is rare, four of the seven human cases since 1994 have resulted in death.
Hendra virus symptomsSymptoms in horses typically include:
- a sudden fever;
- laboured breathing;
- frothy or blood-stained nasal discharge; and
- neurological changes such as loss of vision, muscle twitching or loss of balance.
Symptoms in humans include:
- a flu-like illness that can develop into pneunomia or encephalitis.
CSIRO scientists suggest Hendra is a stable virus, quite unlike strains of the human flu, which change and adapt.
It is also known to be fragile in the environment, has little chance of living in a decomposing animal and is not easily transmitted. It is easily killed by soap or detergents, heat or in a dry environment.
Close direct contact is needed for transmission to occur and it is not spread by droplets in the air like human flu or the highly contagious equine influenza.
Hendra is thought to be a very ancient virus within fruit-bat populations. Since research into Hendra began, the virus has been found in all four mainland species of fruit bats.
Recent research shows Hendra virus present in 30 per cent to 60 per cent of bat urine samples collected. It is thought that only some fruit bats carry high enough levels of the virus to allow it to move between species, a phenomenon known as 'spillover'.
How spillover occurs is unknown but bat urine, saliva, aborted bat fetuses or reproductive fluids from bats may be involved.
While the majority of Hendra cases have been recorded east of the Great Dividing Range in Queensland and northern NSW, Hendra has the potential to be found wherever fruit bats occur.
Hendra was first observed in September 1994 when Vic Rail and some of his horses were affected by a sudden and unknown illness.
Several days later, Mr Rail and 13 horses had died. A second person became infected with the virus, but survived.
Another seven horses were infected with the virus. They did not die from Hendra but were put down to prevent a relapse of the infection.
The Queensland Department of Primary Industries collected samples from the property in Hendra on the outskirts of Brisbane and sent them to the CSIRO in Victoria for analysis.
The CSIRO's Australian Animal Health Laboratory isolated and identified a virus in what is thought to be record time. The virus was unknown anywhere else in the world.
The second human death from Hendra came in October 1995, 13 months after the first case. A 35-year-old Mackay farmer was the third person to be infected with the virus.
It was only after his death from a relapse that an investigation uncovered the death of two horses on his property in August 1994, a month earlier than the Hendra outbreak near Brisbane. The horses were found to have died from Hendra.
Two Queensland vets died from the virus in separate incidents in 2008 and 2009.
Dr Ben Cunneen was infected after treating an infected horse at a veterinary clinic in Redlands, south-east of the Brisbane CBD.
An outbreak at a Cawarral property near Rockhampton killed four horses and 55-year-old vet Dr Alister Rodgers.
There have been seven human infections since the virus was first detected 17 years ago; four people have died as a result.
Hendra outbreaks in 2011 and 2012
There was an unprecedented spike in the number of Hendra cases in 2011. Twenty-one horses died and many hundreds were tested in NSW and Queensland.
Before 2011, NSW had recorded only one horse death from Hendra, in Murwillumbah in 2006.
During 2011, there were 10 confirmed cases of Hendra in NSW on eight properties. Hendra was found in Wollongbar, Macksville, Lismore, Mullumbimby, Ballina, and South Ballina.
There have been no new cases of Hendra in NSW in 2012.
- NSW Primary Industries: Latest Hendra information
In Queensland in 2011, Hendra was confirmed in nine locations, resulting in the death of 11 horses.
Hendra has been found in Beaudesert, Boonah, Park Ridge, Kuranda, Hervey Bay, Boondall, Logan Reserve, Chinchilla and the Gold Coast.
So far this year (2012), Hendra outbreaks have been confirmed in six locations in Queensland including in Cairns, Rockhampton and Mackay. A number of people who’ve come into contact with sick horses are being monitors for signs of the virus.
- Queensland Primary Industries: Latest Hendra information
Scientists are unsure why 2011 was a particularly bad year for Hendra outbreaks.
One theory is that Cyclone Yasi and the summer season flooding in Queensland over 2010/2011 may have destroyed food sources and fruit-bat habitats in northern Queensland, forcing bats south.
CSIRO scientists believe stressed fruit bats may shed more of the virus. They may have spread the virus to southern fruit-bat populations. It is also thought infected animals shed higher levels of a virus when they are first exposed.
The equine industry and Government authorities are on high alert in 2012 after the deaths of two horses in Rockhampton and Townsville in May.
The transfer of Hendra virus from bats to horses and from horses to humans is considered rare.
There is no evidence to suggest that Hendra can be transferred directly from fruit bats to humans.
The virus is not highly contagious. Close direct contact with an infected horse is required for transmission to other horses or carers.
Hendra has an incubation period of five days to 16 days, and horses can shed the virus about two to three days before showing symptoms.
In humans, Hendra usually comes to light as a flu-like illness that can develop into pneunomia or encephalitis.
People who have recovered from Hendra virus may relapse, sometimes many months after the first infection.
If you are a horse owner, you are encouraged to assess your property for trees that may attract bats, observe the seasonal changes in those trees, and isolate horses from any area where bats may roost during the day.
- Australian Horse Industry Council: Managing bats and trees
Trees to be aware of include, but are not restricted to, a range of fig trees, melaleucas, eucalypts, wattles and passion fruit vines. Flying foxes are also attracted to flowering or fruiting trees with soft fruits, like paw-paw, mangoes, lillypillies, grevillias and some palms.
Evidence from June and July 2011 indicates that all cases of Hendra occurred in paddocks with access to flowering or blossoming trees attractive to bats.
Horses should be fenced away from fruiting or flowering trees where fruit bats may be present. They should be removed from areas where bats roost during the day.
Food and water for horses should be kept well clear of the bat habitat, and should be under cover where possible.
Horse feed should not include food like apples, carrots or molasses, which may attract bats.
Horse owners are encouraged to change boundary fencing to avoid nose-to-nose contact with neighbouring horses.
Research from the 2011 Hendra outbreaks indicates that infected horses are unlikely to pass on Hendra to stable or paddock mates, unless there has been close contact.
Vets or horse owners should follow strict quarantine procedures if a horse is suspected of being ill. Ideally, avoid all contact until the illness has been investigated. Take great care with hygiene and personal protection, including the use of full protective clothing.
If you notice symptoms of Hendra virus in your horse, quarantine the animal and contact your vet immediately. Contact the Emergency Animal Disease Hotline on 1800 675 888.
- Equine Veterinarians Australia: 12-step-guide to minimising Hendra risks
The science on Hendra
In November 2012, a Hendra vaccine for horses was released. CSIRO tests showed the vaccine had prevented infection in horses.
The horse vaccine is seen as a tool in breaking the cycle of Hendra transmission from horses to humans.
It is thought that a vaccine for humans is still many years away, although work is underway to develop better treatments for people who do contract Hendra virus.
In mid-2012 CSIRO scientists identified a previously unknown related virus, now called Cedar virus after its discovery in Cedar Grove south of Brisbane.
The discovery is described as a 'lightbulb moment' in Hendra research with scientists keen to discover the genetic difference between the two.
While Hendra virus has proven to be deadly to both horses and humans, Cedar virus is harmless. It is hoped a comparison of the two viruses will determine what makes Hendra so deadly.
Scientists are also focusing on a better understanding fruit bats.
They are studying bat ecology and bat immune systems to understand how Hendra maintains itself, how the virus emerges from bats and what causes it to switch hosts.
They are looking for biological signs in bats that will help to predict potential Hendra 'spillover' events. This information could be used to offer an early warning to horse owners about likely outbreaks of Hendra.
The CSIRO is looking particularly at the black flying fox. Its habitat mirrors the geography of outbreaks in Queensland and NSW.
A study of bat urine from all bat species is also under way to determine if one species naturally carries higher rates of the virus than others.
Hendra is a notifiable disease in Australia. Report suspicions to Biosecurity Queensland on 13 25 23 or the Emergency Animal Disease Watch Hotline on 1800 675 888.