Hendra virus: the initial research
This information sheet reports the emergence of Hendra virus in horses, experimental studies in horses and other animals, and identification of a natural host of the virus.
On this page:
- A new disease
- An earlier outbreak identified
- Subsequent cases
- Experimental studies
- Fruit bats: a natural host
The virus was first isolated in September 1994 from horses by the Australian Animal Health Laboratory, the Department of Primary Industries and Fisheries' (DPI&F's) Animal Research Institute, and Queensland Health. Although originally classified as a Morbillivirus, the virus has now been reclassified and named Hendra virus (HeV), a member of a new genus (Henipavirus) in the family Paramyxoviridae. The name Hendra reflects the name of the Brisbane suburb where the disease was first detected.
The isolation of this virus was achieved as part of an emergency disease outbreak investigation conducted by the Department of Primary Industries and Fisheries, Queensland (DPI&F).
In the outbreak, in September 1994 in Brisbane, 13 horses died as a result of infection with the previously unknown HeV. A further seven horses were shown to have been infected by the virus and, to avoid possible relapse and further transmission, these horses were humanely destroyed.
The first death that was attributed to the infection was a mare, 'Drama Series', who died on 9 September 1994 after a short illness (first noticed to be ill on 7 September 1994). The virus produced very severe damage to the lungs with the accumulation of massive amounts of fluid.
Two people who had close contact with 'Drama Series' when she was ill also became infected with the virus and suffered respiratory illness, in one case with fatal consequences.
The DPI&F tested over 4500 stabled and paddocked horses and a wide range of other animal species for exposure to HeV with negative results. This testing initially concentrated on the site where 'Drama Series' was thought to have become sick, but later extended across the state.
An earlier outbreak identified
During October 1995, a third, fatal human infection with HeV was diagnosed. The man came from a Mackay property where two horses had been diagnosed as dying from avocado poisoning and snakebite during August 1994.
By the end of 1995, new tests had been developed allowing tissues preserved in formaldehyde to be tested for evidence of the HeV agent. Using these tests, both horses were shown to have been infected by the virus, and enough material had been saved from one of these horses to show that HeV was indeed the cause of death.
The diagnosed man had been ill at about the same time as his horses. He had close contact with the horses before and after their deaths.
The Mackay horses died about four weeks prior to 'Drama Series' becoming ill in Brisbane, and are considered to be the earliest known cases in horses. The two episodes (Hendra and Mackay) are closely clustered in time, but geographically are about 800 km apart. Despite lengthy investigation, no direct link has been made between the two occurrences.
In January 1999, a thoroughbred mare in north Queensland, used for polocrosse, became ill with facial swellings and died suddenly. Profuse quantities of yellowish froth poured from the mare's nostrils after death. A series of tests performed at AAHL confirmed HeV to be present in formalised samples.
In late 2004, in two separate incidents in north Queensland, a further two horses were fatally infected and a human case non-fatally infected. The diagnosis was confirmed in one horse and presumptive in the second, based on history, clinical signs, gross pathology, and on the detection of a rising antibody titre to HeV in the attending veterinarian.
Experimental challenge of a range of animals with HeV showed that the disease could be reproduced artificially in cats and guinea pigs. A subsequent survey of 500 cats from the Brisbane metropolitan area showed that none tested positive for HeV.
A retrospective study of specimens submitted to laboratories in Queensland going back as many years as possible has looked for cases that may have been caused by HeV but which were not diagnosed as such at the time. No cases have been found.
Sporadic HeV infection in horses is expected to continue. An ongoing watch for possible future cases will be maintained.
Work at the Australian Animal Health Laboratory (AAHL), which was partially funded by DPI&F, has shown that horses can be experimentally infected by the nasal/oral route and can excrete HeV in the urine and saliva. It is possible to transmit HeV from experimentally infected cats to horses, but experimental transmission from horse to horse, or from horses to cats, could not be demonstrated. Although grey-headed fruit bats develop subclinical disease when experimentally infected with HeV, transmission from these bats to horses could not be demonstrated.
Virus has also been identified in placenta and foetal tissues of experimentally infected pregnant guinea pigs and flying foxes.
Fruit bats: a natural host
Following the negative results of testing a wide range of animal species for infection with HeV, the search was focused on animals present in both Mackay and Brisbane and with possible contact with horses. Fruit bats (Pteropus spp.), commonly known as flying foxes, fitted these criteria and subsequent screening showed antibodies to HeV in all four species of flying fox occurring on mainland Australia. Follow-up studies showed an antibody prevalence of between 20-50 % in flying fox populations across their mainland distribution. The virus has also been isolated from three species of flying foxes. These findings indicate that flying foxes are a natural host of Hendra virus.
Spillover of HeV to horses is a rare event. Flying foxes are protected native fauna, widely distributed in Australia (and south-east Asia and India). Current research seeks to understand how infection is maintained in flying fox populations and to identify factors associated with the sporadic spillover to horses, to enable better management of the risk of spillover.
It appears that flying foxes do not pose a significant risk in respect to passing this virus directly to people. This contention is based on the negative results of testing of 130 people, all with close contact with flying foxes and the fact that all human cases of infection with HeV appear to have been contracted from acutely ill horses. Nonetheless, because of the risk of contracting another (fatal) infection, Australian bat lyssavirus, from flying foxes (and other bats), members of the general public should not handle flying foxes or any other bats. Injured or sick bats should be reported to Queensland Parks and Wildlife Service, who have a network of skilled, vaccinated personnel.