zoological category. There are kangaroo carers, bird carers, possum carers, and bat carers. Hume Field knew of them from his years of veterinary practice; he had virtually been one of them, during his student days at the animal refuge. Now he sampled some of the animals in their care.
But damn it: still no trace of Hendra.
In January 1996, with the search for a reservoir host at impasse, Field took part in a brainstorming session of agency officials and researchers, called by his supervisor at DPI. What were they doing wrong? How could they better target their efforts? Where would Hendra strike next? Queensland’s racing industry stood in jeopardy of multi-million-dollar losses, and human lives were at risk. It was an urgent problem of governance and public relations, not just a medical riddle. One useful line of thought was explored at the meeting: biogeography. It seemed obvious that the reservoir host (or hosts), whatever type of animal it was (or they were), must exist both at Mackay and at Cannon Hill—exist there for at least part of each year, anyway, including August and September. This pointed toward animals that were either broadly distributed in Queensland or else traveled broadly across the state. The brainstormers (partly guided by genetic evidence suggesting there was no localization of distinct viral strains—that is, the virus was moving and mixing) leaned toward the second of those two possibilities: that the reservoir host was quite mobile, an animal capable of traveling hundreds of miles up and down the Queensland coast. That in turn directed suspicion at birds and . . . at bats.
Provisionally, Field and his colleagues dismissed the bird hypothesis, on two counts. First, they were unaware of any other paramyxovirus that spills over from birds into humans. Second, a mammalian reservoir simply seemed more likely, given that the virus infects humans and horses. Similarity of one kind of host animal to another is a significant indicator of the likelihood that a pathogen can make the leap. Bats are mammals, of course. And bats get around. Furthermore, bats famously harbor at least one fearful virus, rabies, although Australia at that time was considered rabies-free. (Many other bat-virus-human connections would be discovered soon afterward, including some in Australia; but at this time, 1996, the link seemed less obvious.) From the meeting, Field took away a new mandate: Look at bats.
Easily said. But catching bats on the wing, or even at their roosting sites, isn’t so simple as trapping rodents or possums in a meadow. The most conspicuous and far-ranging bats native to Queensland are the so-called flying foxes, which belong to four different species within the genus Pteropus, each one a magnificent, fruit-eating megabat with a wingspan of three feet or more. Flying foxes customarily roost in mangroves, in paperbark swamps, or high in the limbs of rainforest trees. Special trapping tools and methods would be required. Short of gearing up immediately, Field returned first to the “carer” network. These people already had bats in captivity. At a facility in Rockhampton, up the coast toward Mackay, he found that the wounded animals under care included black flying foxes (Pteropus alecto). Bingo: Blood drawn from a black flying fox had antibodies to Hendra.
But one bingo moment wasn’t sufficient for a scientist so fastidious as Hume Field. That datum proved that black flying foxes could be infected with Hendra, yes, but not necessarily that they were a reservoir—let alone the reservoir—from which horses became infected. He and his colleagues kept looking. Within a few weeks, Hendra antibodies turned up in all three other kinds, the grey-headed flying fox, the spectacled flying fox, and the little red flying fox. The DPI team also tested old samples from flying foxes, which had been archived for more than a dozen years. Again, they found telltale molecular tracks of Hendra. This showed that the bat population had been exposed to Hendra virus long before it struck Vic Rail’s horses. And then, in September 1996, two years after the Rail outbreak, a pregnant grey-headed flying fox got herself snagged on a wire fence.
She miscarried twin fetuses and was euthanized. Not only did she test positive for antibodies; she also made possible the first isolation of Hendra virus from a bat. A sample of her uterine fluids yielded live virus, and that virus proved indistinguishable from Hendra as found in horses and humans. It was enough, even within scientific bounds of caution, to identify flying foxes as the “probable” reservoir hosts of Hendra.