As Australia faces a particularly bad year for Hendra virus with possible expansion of the range of this serious disease, there have been calls for a mass cull of flying foxes (fruit bats), the reservoir of the virus but also a protected species. The virus lives in bats and is spread mainly through their urine. Horses that are exposed to bat urine or feces (e.g. grazing under a tree where bats are roosting) can become infected and then serve as a source of human infection. Being a highly fatal disease with no vaccine, looking at ways to reduce exposure to the virus is critical, and when you have a wildlife-associated disease, questions about trying to eliminate the wildlife source often arise.
Any discussion of culling wildlife leads to intense debate, and this situation is no different. Some people support culling bats in areas around people and horses while others are opposed on various grounds, including a lack of evidence that it will be effective.
Can fruit bat numbers really be decreased? A lot of bats would have to be killed to have a significant impact on the population. Bats can reproduce quickly and migrate readily, therefore a single cull may have only a limited and short-term effect. Good understanding of the dynamics of the bat population is required to determine how many would need to be killed over a given area to have any significant impact. As Biosecurity Queensland's chief vet RIck Symons stated "Culling is against government policy. I believe in terms of biosecurity it's counterproductive, because it does stress flying foxes and they're more likely to excrete (the virus). It could be filled by another bat colony the next day and if you're moving them on, you're moving it on to somebody else and it's somebody else's problem, so that is not the solution."
Will a cull actually achieve anything? Even if effective at reducing bat numbers (probably just in the short term), culls don't necessarily have an impact on disease rates. All bats would not be eliminated, and it's unclear whether there is a critical mass of bats that is required to transmit infection or whether a small number of bats distributed across the same region would be as likely to result in infections. Small or temporary decreases in bat numbers may have no effect.
What unintended consequences might occur if a cull is effective at reducing bat numbers? Removing an animal from the ecosystem has an effect, and it's important to be confident that that effect isn't accompanied with problems of its own. I don't know enough about fruit bat ecology to say much here, but if this species is greatly reduces, are there other species that will come and occupy that ecological niche, and might they be associated with problems of their own? Good scientific study can help figure this out but you can never be certain.
Are other control measures, such as removing roosting sites from pastures and other bat avoidance measures, being adequately used? Culls should only be considered when other measures have failed. It's not necessarily an easy thing to address. Certainly, people in endemic areas should remove trees that bats roost in from pastures. However, not all cases are associated with identifiable roosting sites, such as one affected Queensland farm that does not have any fruit bats residing on the property but rather lies along a common flight path.
It's easy to talk about avoiding a cull when you're not in the heart of the Hendra epidemic, and I understand the reasoning behind the calls for a cull. It's a devastating disease that's a threat to both horse and human health, and it's unpredictable. People that have been exposed face an incredibly stressful period while they wait and see if they've been infected with a virus that kills in ~50% of cases. A vaccine is probably still a couple years away, leaving a period with continued risk and stress. With such a serious disease, considering culling is reasonable. However, it can't be a knee-jerk reaction to public outcry. It needs to be based on sound science to ensure that if it's used, it would be effective. The impact on this protected species also can't be ignored.
The Hendra Virus is gathering international attention
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