On Tuesday it was announced that a dog from the Brisbane suburb of Beaudesert tested positive to Hendra, adding to the list of possible carriers of this deadly virus.

Hendra virus is named after the northern Brisbane suburb where in 1994 it killed the horse trainer Vic Rail, saw his stable hand become seriously ill and infected another 13 horses that were destroyed. Fruit bats tested from around the property carried the virus.

There have been 14 outbreaks of this virus up to 2010, killing a farmer, a horse trainer and two vets.  This year there have already been 11 more outbreaks, with Hendra virus now found into northern NSW with many more horses destroyed.

The public and political reaction to these outbreaks to date has been to blame fruit bats. Though fruit bats of most species have been identified as carriers, what is missing is a direct causal link. How did the disease actually get from fruit bats to horses and now dogs?  Fruit bats are flying mammals that are very different from the many species tiny insect-eating bats that often roost in house ceilings, sheds, etc. They are very much larger than these “typical bats”, have excellent eyesight and only a few rare species use echo location. Fruit bats do not live in sheds or house ceilings. Typically, they form colonies or camps, most often on or adjacent to waterways where they roost during the day but they travel many kilometres to feed at night, raiding even suburban fruit trees.

Before Europeans arrived in Australia, fruit bats fed on wide variety of native fruits and blossom found in forests. Since the development of agriculture they have been seen as a pest, particularly in tropical fruit orchards.  Their love of mangoes is legend.  This has made them particularly unpopular with farmers and they have long been shot, especially in the north — though more recently netting orchards can be equally as effective.  Apart from Hendra virus, fruit bats also are known carriers of lyssavirus and rabies. To be seen in perspective, dogs, waterbirds, parrots, cats and many other animals also carry diseases fatal to humans — and still people are more often killed by diseases passed on to each other. The macadamia nut is a native to  northern NSW and Queensland and the recent rapid growth of this industry has, in effect, restored elements of the original rainforest. This has brought back fruit bats onto farmland where they feed on the macadamia blossom, for them an original food source, and cause no harm while pollinating the trees.

Their obvious link to the Hendra outbreak is not helping their popularity. Queensland politician Bob Katter has predictably called for a fruit bat cull and to be fair, he is far from alone in this view.  The most recent and rapid expansion of the disease this year also corresponds to major rainfall events and it is hard to link this with increased interaction with fruit bats — although it is claimed that the Hendra virus or antibodies have been found in “pooled” blood collected from mosquitoes in the Hendra case paddock in 1995 (A.R. Gould: from a thesis by Hume Field).

If fruit bats have always carried this disease, why was the first recorded outbreak in 1994? It is possible the deaths have gone undiagnosed? Have the fruit bats become more infectious for some reason? The macadamia industry has also recently been in the news for its use of agricultural chemicals that have been blamed for fish deformities in the Noosa River — could these chemicals impact fruit bats.

Another factor not yet considered is the secondary impact of agricultural chemicals recommended for use by state agriculture departments and declared safe to use, if the instructions on labels are followed, by the Australian Pesticides and Veterinary Medicine Authority (APVMA). However, it is very hard to keep chemicals out of waterways, for instance; in a wet year and we may need to pay more for our nuts if we want them without chemicals used to grow them.

The deaths in 2009 of dolphins in the Swan River that flows through Perth has been attributed to a morbillivirus, from the same “family of viruses” as Hendra. Their suppressed immune system was noted but not linked to anything beyond the obvious disease.  Similar dolphin deaths in the Lakes Entrance system of Victoria were reported with very similar symptoms but no cause was found. The link between these deaths by a similar disease may be found in catchments for estuaries that contain intensive agriculture and heavy industry and chemical waste that may have compromised these animals’ immune systems.

There is now a growing body of research linking immune suppression and and increased vulnerability to disease to agricultural chemicals in honey bees, amphibians (frogs, etc) and shrimps. The same rainfall event could also cause agricultural chemicals to be flushed into the sea, which is also co-incident with the deaths of green turtles off north Queensland beaches.

To deal with the Hendra virus, our politicians, the public and scientists need to keep an open mind.  A vaccine for horses has been developed but it has to go through trials with the APVMA before it can be used. This is not just red tape if we want proper assessment of chemicals generally — but trials should not be delayed from lack of money or unnecessary bureaucracy. A vaccine for humans is equally important — especially for vets.

If we chose to tackle Hendra virus by just moving fruit bat colonies and trying to shoot out all the bats, we could make the problem worse. Stressing fruit bats and likely weakening their immune systems is something you would not want to do to a carrier of a deadly virus.