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Culling Wildlife to Prevent the Spread of Disease Makes Everything Worse

Those anti-cull demonstrators were right actually.
December 5, 2013, 11:39pm
Virginia McKenna at an anti-badger-cull demonstration in London, June 2013 via Wikimedia Commons

Trying to prevent the spread of a disease by culling a wildlife population is not only viscerally upsetting for some—to the point where everyone from lay people up to rock and roll royalty will call on you to stop—it can actually be counterproductive and cause disease to spread even faster than it would naturally.

This week, cull opponents got two big boosts.

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The unpopular British badger cull came to an end this week. The program, designed to stop the spread of bovine tuberculosis (bTB) by reducing the badger population by 70 percent in Somerset and Gloucestershire, was a potent combination of unpopular and ineffective. The first six weeks of culling didn’t produce nearly enough dead badgers, so the program was extended by another eight weeks. But with three weeks still to go on the extension, officials just called the whole thing off, having killed less than 40 percent of Gloucestershire’s badger population.

While an independent panel will evaluate the results and issue a report next year, anti-cull activists are saying that the program was a total failure. Queen guitarist and celebrity anti-culler Brian May called it "a failure" and "shambles." It didn’t achieve its goal in terms of badger pelts nor, experts say, in terms of preventing the spread of bTB.

“It's very likely that so far this cull will have increased the TB risk for cattle inside the Gloucestershire cull zone rather than reducing it," Rosie Woodroffe, the UK’s leading badger expert, told The Guardian. The cull causes badgers to flee, potentially spreading TB more widely through effect called perturbation.

While the effects of the British badger war will be sorted out in the coming months, there's evidence in other mammals that culls aren't as effective as thought. Monday, University of Michigan researchers published research finding that five decades of culling vampire bats in South America in order to prevent the spread of rabies hasn’t been effective. Computer models by the researchers found that the culling bat populations could cause infected bats to flee to other, healthy bat colonies nearby.

Common vampire bat via Wikimedia Commons

Vampire bats transmit rabies throughout South America, contributing to thousands of livestock deaths and even the occasional human fatality. In the past, bat populations were reduced through poisons and explosives, but since the culls weren’t targeted at infected bats, they didn’t actually prevent the spread of rabies. Using data collected by tagging bats, the researchers ran computer models that demonstrated why indiscriminate culling wasn’t helping.

"In the current paper, we do a number of things. First, we fit models that encompass alternative assumptions regarding this system and we identify an important role of movement between colonies,” said the paper’s senior author, Pejman Rohani, a population ecologist and epidemiologist. “We then use the best-fitting model to examine what happens under culling, especially if the cull is indiscriminate, rather than targeting infected bats specifically. Again, culling is shown to be ineffective, but now the model helps us understand why that is."

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Thousands of computer simulations were run, and the most successful models demonstrated that a single, isolated vampire bat colony cannot maintain the rabies virus over time. Frequent movement of infectious bats between colonies is needed to keep the rabies virus at levels consistent with the field observations.

"While programs targeting specific colonies may limit local spillover from bats to humans or domestic animals, regional viral persistence will likely remain unaffected due to high connectivity between bat colonies," Rohani said. "Moreover, if culling increases movement due to freeing up space or disturbance-mediated dispersal, culling could, perversely, have the opposite of the intended effect on rabies transmission."

In spite of this, culls still surface as a viable wildlife management option. The Long Island Farm Bureau and the Wildlife Services division of the United States Department of Agriculture proposedcutting the population of deer in East Hampton in order to prevent the spread of Lyme disease and other tick-borne diseases. Laura Simon, a wildlife ecologist with the Humane Society, said that culling the population isn’t going to make it safe to go back into the woods.

“The science, and there is an abundance of it, proves that you can’t control the human risk of Lyme disease by killing deer.” Simon told the East Hampton Star.

Of course sometimes culls are called for to prevent things other than the spread of disease. That East Hampton Star article notes that the cull proponents think it will prevent car-and-deer collisions and slow the harmful spread of deer fences that are “changing the face of the village.”

In Australia, Western Australian Premier Colin Barnett talked about culling the population of large sharks to prevent shark attacks, but experts from the University of Western Australia took to the internet to point out that the cull might be emotionally resonent, but that it wasn’t going to prevent shark bites. “When shark culling was carried out in Hawaii, between 1959 to 1976, over 4,500 sharks were killed and yet there was no significant decrease in the number of shark bites recorded,” the Sydney Morning Herald reported the professors as saying.

Not all killing in the name of wildlife management is ineffective or wrongheaded. Hunting licenses are arguably a type of culling that, when handled responsibly, help keep wildlife populations healthy. A scrutinized cull of deer in Illinois was able to keep the rate of chronic wasting disease, the deer-equivalent of mad cow disease, in the wild population down at one percent from 2002-2012. Over the same period in non-culling, neighboring Wisconsin the rate rose to 5 percent.

As the British showed through negative example and the University of Michigan showed through computer modeling, culling depends on a lot of data—on the disease, on how it spreads and how the animals react. The most consistent trait that culls share seems to be that a vocal portion of the public utterly reviles them.