Volume 195 - Issue 5
Hendra virus
Authors: Jeannette R Young, Christine E Selvey and Rick Symons
Med J Aust 2011; 195 (5): 250-251. || doi: 10.5694/mja11.10967
Published online: 5 September 2011
Published online: 5 September 2011
Low infectivity but high mortality: strategies to minimise spread until the vaccine arrives are the key
Hendra virus (HeV) infection in humans is an emerging zoonotic disease that has a high mortality rate, but low infectivity. In all cases to date, the infection has been transmitted to humans from bats of the genus Pteropus (flying foxes) via an intermediate equine host.
HeV and Nipah virus are the only known members of a new genus, Henipavirus, within the family Paramyxoviridae. HeV was first described after an outbreak of severe respiratory disease in horses that led to the deaths of 14 of 20 infected horses and the death of a horse trainer — one of two humans infected — in Brisbane in 1994.2
There have been seven cases of HeV infection producing pneumonic or encephalitic illnesses in humans. Four of these people died, three soon after exposure and the fourth from fatal encephalitis caused by HeV, which developed 13 months after full recovery from an initial aseptic meningitis.3 Of the three survivors, two made complete recoveries while the third experienced ongoing complications of the initial encephalitis.4,5 Subsequent serological testing for HeV in a large number of human contacts of the first three cases of HeV infection was completely negative.6
Before 2011, there had been 14 events of spillover of HeV infection from flying foxes to horses, and subsequent transmission to humans in five of these events. All events occurred in coastal Queensland except for one in northern New South Wales. Forty-four horses were infected; 34 of these (75%) died and the remaining 10 survived but were later euthanased. Seroepidemiological studies of more than 2000 horses and more than 5000 samples from 46 other animal species in Queensland did not identify HeV infection.7
Spillover events have occurred through most months of the year, but with increased frequency from June to September. The virus can survive under ideal cool and moist environmental conditions (eg, in bat urine at 22°C in the laboratory) for up to 4 days, but is generally thought to survive for only hours. Horses are thought to be infected by ingesting food or water contaminated by urine, saliva or birthing products of infected flying foxes. The virus amplifies within the horse, and humans who are exposed to a large amount of the secretions or blood from an infected horse can become infected. Laboratory studies have shown that horses may excrete virus for up to 72 hours before showing clinical signs.8 All seven humans infected with HeV to date had high levels of exposure to body fluids of infected horses, such as during unprotected autopsy or by direct contact with respiratory secretions or aerosols. Not all people with high-level exposure have contracted the disease or seroconverted. There is no evidence that prolonged close contact with flying foxes engenders a risk of HeV infection in humans.9
A wide range of mammals carry the appropriate receptor enabling them to be infected with HeV experimentally.10 Yet, although HeV infects the endothelium of blood vessels in many species in the laboratory, resulting in systemic vasculitis, outside the laboratory setting, only flying foxes, horses and humans are known to have been infected. One dog is known to have seroconverted without any clinical illness or detection of virus.
Flying foxes were identified as the natural host in 1996, and antibodies to HeV have been found in archived samples of flying fox serum dating back to 1982.11 Flying foxes do not develop overt disease. All four species of flying fox in Australia, from as far north as Madang in Papua New Guinea to as far south as Melbourne, have been found to carry the virus.12 HeV is genetically stable, and there is no evidence that the virus has changed significantly since it was first isolated in 1994.8
This year has been a major year for the detection of spillovers of HeV into horses, with 14 events being notified by 17 August. Eight of these occurred in Queensland, with one being the first event notified west of the Great Dividing Range, in Chinchilla. Ten horses had been infected in Queensland and, for the first time, a dog has seroconverted, probably through contact with an infected horse. Six spillover events had occurred in northern NSW, with seven horses becoming infected. No humans have been infected this year to date, although not all of those potentially exposed have reached the end of their incubation periods; no one had a high degree of exposure to infected secretions.
The incubation period in humans is 5 to 21 days. The clinical presentation has been variable, with both respiratory and encephalitic symptoms. There has been no transmission of HeV between people, but routine droplet precautions are advised.
There is no known effective treatment for Hendra virus infection, and clinical management is based on treating symptoms as they arise. When a person has had high-level exposure to body fluids of infected horses, an experimental human monoclonal antibody (mAb) can be made available for postexposure prophylaxis. Henipavirus mAb has been shown to be effective in preventing infection in ferrets if administered within 12 hours of intrathecal injection of a high dose of HeV.13 However, this product has not yet been trialled for safety or efficacy in humans. It has been administered to three humans. The first person was late into the progression of severe encephalitis and subsequently died. The second two had moderately high-risk exposure, but no evidence of infection, and they did not develop illness or seroconvert.
Currently, despite its unknown safety or efficacy, mAb is offered to people who have had high-level exposure to infected horse blood or secretions, after obtaining ethics approval and appropriate consent for each individual.
Preventive measures are essential. Horse owners are advised to keep horses away from flowering and fruiting trees, and to remove feed and water troughs from under trees. Vets are advised to wear appropriate personal protective equipment when attending a sick horse or when performing invasive or aerosol-generating procedures on any horse. Horse owners and the public are advised to isolate sick horses from people, horses and other animals.
The most promising prospect for controlling HeV outbreaks in humans is the vaccine for horses that is expected to be marketed in 2013.14
Competing interests
No relevant disclosures.
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