In September 1994, a mystery virus killed horses and humans in Brisbane, Australia. Scientists later identified it as Hendra virus, which had jumped from bats to horses to humans, marking the beginning of our coronavirus era. The outbreak occurred in the Brisbane suburb of Hendra, where a pregnant mare named Drama Series fell ill and died, followed by seven other thoroughbreds within 12 hours. The horses exhibited severe symptoms: fever, facial swelling, respiratory distress, and bloody froth from nostrils. Veterinarians ruled out known exotic diseases and toxins, but the cause remained unknown.
The first human victims
Two people who treated the horses also fell ill. Trainer Vic Rail died after a week in intensive care, with organ failure and fluid-filled lungs. A later examination confirmed he had the same virus as the horses. The virus was named Hendra virus, and epidemiologists suspected a zoonotic disease that had jumped from horses to humans, possibly from an unknown third species.
Search for the natural host
Ecologist Hume Field joined the search for the natural host. In October 1995, a second case emerged in Mackay, 1,000 kilometres away, when horse owner Mark Preston died from similar symptoms. Blood tests revealed Hendra antibodies in Preston and his horses, indicating the virus could travel vast distances. In April 1996, the first Hendra antibodies were found in black flying foxes, and within weeks, in three other flying fox species. Archival blood samples showed the virus had been circulating in bats for at least a decade before spilling over.
Why bats?
Bats are unique: they fly, have high metabolism and body temperature, and live long lives. Their viruses are forged in bodies that reach fever temperatures daily, making them resilient. Flying foxes, in particular, are nomadic, flying dozens of kilometres nightly to feed. Scientists deduced that horses acted as an amplifying host, replicating the virus abundantly before it jumped to humans. Field noted, “Disease emergence is all about increased opportunity for contact. It’s all about human impact on the natural system.”
The ecological trigger
Why did Hendra emerge only then? The answer lay in habitat loss. Australia has lost half its woodlands since European settlement, and Queensland has been the land-clearing capital, responsible for 80% of annual tree felling. This destroyed the bats' winter food sources, pushing them into urban areas where they encountered horses and humans. By 2003, urban roosts tripled, and spillovers increased. In 2011, 18 spillovers occurred, prompting a new response.
Predicting and preventing spillover
Wildlife ecologist Peggy Eby led a 25-year study, published in Nature in 2022, showing that spillover can be predicted and prevented. The team found that bats moved closer to people to survive, but didn't return to forests when food shortages ended. However, in 2019, despite drought and wildfire, only one infection occurred. The bats were drawn to a massive bloom of forest red gums near Gympie, providing nectar. This suggested that planting winter-flowering eucalyptus trees could draw bats away from humans, preventing spillover. Dr. Raina Plowright of Cornell University argued that restoring winter-flowering trees could act as a broad-scale inoculation.
The broader implications
Hendra virus led to the discovery of Nipah virus, another bat-borne pathogen, and highlighted the role of ecological disruption in disease emergence. Bats are a major reservoir for pandemic-potential pathogens. The work underscores the urgent need for ecological solutions to prevent future spillovers, as deforestation and habitat loss continue worldwide. This is an edited extract from Sentinels: When Diseases Spread Between Humans and Animals by Michael Delaney, available 4 August through Scribe Publications.
