Hendra virus (HeV) is an emerging virus disease that was first identified in a large outbreak of the highly lethal disease in racehorses in 1994 in two adjoining Brisbane racing stables where 13 horses either died or were humanely euthanised, and 7 horses recovered and were subsequently euthanised. A trainer died from multi-organ failure and his stable hand became ill but recovered.
HeV is a single-stranded, negative-sense RNA virus within the family Paramyxoviridae and the genus Henipavirus. There are two other known members of this genus - Nipah virus (NiV) which, like Hendra, is also highly pathogenic, and Cedar virus (CedV) which does not appear to be highly pathogenic.
Four species of Australian flying foxes are the natural reservoir of HeV and the virus does not appear to cause clinical disease in them. HeV and NiV have also been detected in flying foxes in Papua New Guinea.
Hendra virus is regarded as an endemic disease in flying foxes in Australia. All known cases in horses have occurred in coastal areas of Queensland and NSW and research indicates that all infections have occurred within the range of the black flying fox (Pteropus alecto) and the spectacled flying fox (Pteropus conspiculatus) species. The virus transmission dynamics in these 2 species and in the other 2 species, the grey-headed (Pteropus poliocephalus) and little red (Pteropus scapulatus), are complex and incompletely understood, so the transmission risk to horses from the latter 2 species is still undetermined.
The clinical signs of Hendra virus in horses are mainly respiratory and/or neurological usually with a rapid progression to death and with a 79% mortality rate. Some horses diagnosed with Hendra have recovered from the disease, and to date, they have all been euthanased.
Research has now established there is no evidence that recovered horses or humans shed infectious virus and the previous policy regarding euthanasia of seropositive recovered horses has recently been reviewed in 2016 by Australia’s Animal Health Committee and compulsory euthanasia is no longer mandatory. Management of seropositive non-vaccinated horses is at the discretion of State Chief Veterinary Officers.
Hendra virus is a rare disease in humans. There have been seven known cases, with four deaths, a case fatality rate of 57%. The human cases have all occurred by close contact with the secretions or body fluids of infected horses which act as amplifiers of the virus. One person became infected from a horse in the late incubation stage before it showed clinical signs.
There has been no evidence of direct flying fox-to-human transmission, or of human-to-human transmission as has occurred overseas with the closely related Nipah virus. Horse to horse transmission is more likely to occur when horses are in close contact with each other.
There are 2 recorded incidents in 2011 and 2013 where dogs have become infected, plausibly by contact with blood or excretions from infected horses on the properties. The clinical signs in dogs are mild and likely to go unnoticed, however, dogs can represent a transmission risk to humans from saliva soon after they become infected.
How is Hendra virus spread?
A recent study confirmed that flying fox urine is the most significant route of HeV excretion in wild-caught black flying foxes. Transmission pathways facilitating spillover to horses via urinary contamination of pasture, feed and water sources is highly plausible. Routine observation of the abundance of flying-fox urine, faeces, and food debris underneath trees in which they are feeding suggests that horses have a high likelihood of direct physical contamination with the potentially infectious material at these sites. As horses are more likely to be resting than grazing or browsing when flying-foxes are most actively foraging, direct inoculation of infectious flying-fox urine with the mucosal surfaces of horses is another plausible transmission pathway in addition to ingestion of contaminated material.
The AVA believes that the Hendra vaccine is the single most effective way of reducing the risk of Hendra virus infection in horses and therefore of preventing disease transmission to people.
What animals are at risk of contracting Hendra virus?
Hendra virus infections have been confirmed in flying foxes, horses, and dogs.
Other animals known to be susceptible include cats, ferrets, pigs, guinea pigs, hamsters, and non-human primates.
Horses suspected of being infected with HeV should be isolated from all other animals to avoid spreading disease. People should not kiss horses on the muzzle, and personal protective equipment should be worn when dealing with sick horses. Good personal hygiene should always be practiced around horses, dogs, and other animals - it's not a good idea to let dogs lick people's faces at any time, and washing of hands after touching any animal should always be practised. Children of any age should always be supervised by an adult around dogs.