Hendra virus was first isolated in September 1994 from horses by the Australian Animal Health Laboratory (AAHL) and Biosecurity Queensland. Originally called 'equine morbillivirus', it was renamed Hendra virus when subsequently fully characterised, and became the first member of the new genus Henipavirus. 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 investigation conducted by the department.

During the incident, 13 horses died as a result of infection with the previously unknown virus. A further seven horses were later found to have been non-fatally infected, and were humanely destroyed to prevent relapsing infection and possible further transmission.

The first equine death attributed to Hendra virus infection was a mare named Drama Series, who died on 9 September 1994 after a short illness (first noticed to be ill on 7 September 1994). The virus caused severe damage to her lungs, with the accumulation of massive amounts of fluid in the lungs and airways.

The trainer and the strapper, who had close contact with the terminally ill mare, both became infected and suffered an acute febrile respiratory illness - with fatal consequences for the trainer.

Across Queensland, the department subsequently tested more than 4500 horses (both stabled and paddocked) and a range of other animal species for exposure to Hendra virus, and found no evidence of previous infection.

An earlier incident identified

In October 1995, a third (fatal) human infection with Hendra virus was identified. The man came from a Mackay property where it was believed that two horses had died from avocado poisoning and snakebite in August 1994. He had assisted in their necropsy.

New diagnostic tests on the preserved tissues of both horses established that Hendra virus was in fact the cause of their deaths.

The Mackay horses were actually infected about four weeks prior to Drama Series becoming ill in Brisbane, and are considered to be the earliest known equine cases. The 1994 Brisbane and Mackay incidents are closely clustered in time, but are geographically about 800 km apart. However, despite lengthy investigations, no direct link could be found between the two incidents.

Investigations

Experimental challenge of a range of animals with Hendra virus at AAHL showed that the disease could be reproduced artificially in cats and guinea pigs. A subsequent survey of 500 domestic cats from the Brisbane area showed that no cats tested positive for Hendra virus.

In addition, a retrospective study of possibly relevant archived equine samples submitted to the departments laboratories prior to 1994 found no earlier evidence of Hendra virus infection.

The potential exists for further Hendra virus cases in the future. Biosecurity Queensland workshops, fact sheets and online information have promoted risk management measures and a high level of horse owner and veterinarian awareness to decrease the likelihood of future incidents.

Experimental studies

Initial work at AAHL, which was partially funded by the department, showed that horses could be experimentally infected with Hendra virus by the nasal/oral route, and could excrete Hendra virus in the urine and saliva. These studies also showed that, on one occasion, an Hendra virus-infected cat likely transmitted infection to a horse in close contact. However, experimental transmission from horse to horse, or from horse to cat, could not be demonstrated. Experimentally infected flying foxes did not develop clinical disease (though some developed antibodies), and transmission from these bats to horses could not be demonstrated.

Since its detection in wild-caught flying fox foetal fluids and tissues, Hendra virus has also been identified in placenta and foetal tissues of experimentally infected pregnant guinea pigs and flying foxes.

Flying foxes: a natural host

Following the negative results of Hendra virus testing in a wide range of animal species, the search for the reservoir of Hendra virus focused on species present in both Mackay and Brisbane, and having possible contact with horses.

Flying foxes (Pteropus spp.) fitted these criteria and subsequent screening showed antibodies to Hendra virus in all four species on mainland Australia. Follow-up studies showed an antibody prevalence of between 20-60 per cent in flying fox populations across their mainland distribution. The virus was subsequently isolated from flying foxes, confirming that current infection existed in these populations. These findings (plus the absence of disease in experimentally infected flying foxes, and the detection of antibodies in archived flying fox blood samples) indicate that flying foxes are a natural host of Hendra virus.

Despite the recent cluster of cases in 2011, the spillover of Hendra virus to horses is a rare event. Flying foxes are protected native fauna and are widely distributed in Australia (as well as the Pacific, South-East Asia, the Indian subcontinent and Madagascar). To enable better management of the risk of spillover, current research seeks to understand how infection is maintained in flying fox populations and what factors are associated with the sporadic spillover to horses.

Flying foxes pose no identified risk of passing Hendra virus directly to people. All human cases have resulted from close contact with infected horses. Of 140 people with close contact with flying foxes (including bat carers, wildlife rangers and research scientists), none showed any evidence of Hendra virus infection. Nonetheless, because of the risk of contracting the invariably fatal Australian bat lyssavirus from any Australian bats, members of the general public should not handle flying foxes or any other bats.

If you see an injured or sick bat, you should report it to the Department of Environment and Heritage Protection on 1300 130 372, who have a network of skilled, vaccinated personnel.