Bat cull 'will not stop white-nose syndrome spreading'
Culling will not halt the spread of a disease that has killed a million bats in the US since 2006, a study says.
Researchers reached their conclusion by modelling how white-nose syndrome (WNS) is passed from bat to bat.
Writing in Conservation Biology, they add that a cull would not work because the source of the fungal pathogen is believed to occur in the environment.
Earlier studies have warned that WNS could wipe out bat populations in the north-east of the US within 20 years.
Carrying out a cull of bats in areas where the disease is known to be present is one of the options available in an attempt to contain the spread of the killer fungus.
"We developed a model taking into account the complexity of the bat life history, looking at the roosts and the areas where there are large contacts between the bats," said co-author Thomas Hallam from the Department of Ecology and Evolutionary Biology at the University of Tennessee.
"Given the dispersal aspect of the problem and the complexity of hibernating bat ecology, it was a case that these things together certainly meant that culling would not work in the case of bats."
WNS, described by some as the worst wildlife health crisis in the US in living memory, is named after a white fungus that appears on the muzzle and/or wings of infected animals.
However, bats with WNS do not always have the characteristic visual symptoms, but may display abnormal behaviour around their hibernacula (caves and mines where bats hibernate during winter months).
These behaviours include flying outside during the day (when their insect prey is not available) in sub-zero temperatures, or clustering near the entrance to the hibernaculum.
Professor Hallam explained that there was a high degree of bat-to-bat interaction, which has been identified as the main way the disease is transmitted, during the course of a year.
In autumn, the mating season brings together large numbers of males and females.
This occurs shortly before colonies enter hibernacula, some of which are large enough to house in the region of half-a-million bats.
In the spring, females head to a maternity roost to have their young. Again, this brings bats into contact with members of different colonies.
Since WNS was first recorded in February 2006 in a commercial cave in New York, it has spread to at least 14 states. Cases have also been recorded in a number of Canadian provinces.
Researchers say the fungus associated with the disease, Geomyces destructans, thrives in the dark, damp conditions - such as caves and mines.
Out of control
In their paper, Professor Hallam and co-author Gary McCracken write: "Because the disease is highly virulent, our model results support the hypothesis that transmission occurs in all contact arenas."
They add: "Our simulations indicated culling will not control WNS in bats primarily because contact rates are high among colonial bats, contact occurs in multiple arenas, and periodic movement between arenas occurs."
Jeremy Coleman, the national white-nose syndrome co-ordinator for the US Fish and Wildlife Service (FWS), said that culling was a potential tool available to agencies attempting to curb the spread of the disease.
"The spread has been very rapid and very alarming," he told BBC News.
"The initial comment that spawned all of the ideas of culling was that if we had known what would happen, then we would have gone in and killed every bat and we would not be facing this problem.
"That had a real resonance among researchers and land managers," Dr Coleman recalled. But he added: "Most people, I would say, feel it is too late for any culling to be effective."
He explained that the final decision on whether to cull would rest with state or federal agencies.
It is believed that the fungus associated with WNS arrived in the US after it was somehow transported from Europe or possibly Asia.
"It was possibly brought over via 'human-assisted spread' of some sort - like on somebody's boots," Dr Coleman suggested.
"Another possibility is that a bat was somehow transported to North America, perhaps by a cargo plane or freight container, and mixed with bats in New York State."
A team of European researchers followed up unconfirmed reports in Europe that bats had white fungal growths appearing to match the symptoms of WNS.
In a paper in the Emerging Infectious Diseases journal, they suggested that the Geomyces destructans fungus was present throughout Europe.
However, they added, it seemed as if species of bats in Europe were possibly more immunologically or behaviourally resistant to the fungus than North American species, as it did not increase mortality.
No magic bullet
European bats may be resistant to the disease because they are generally bigger than comparable species in the US. Also, European colonies tend to be not as large as ones found on the other side of the Atlantic.
"What we hope to learn, through genetic means, is the similarities and differences between the North American strains and the European strains," explained Dr Coleman, who is overseeing the formation of a national management plan that hopes to bring together the efforts of state and federal agencies under one umbrella.
US researchers based at MIT recently sequenced the genome of the US strain of G. destructans and made the data publicly available in a hope that it would "jumpstart work on this problem, to help devise ways to track and combat this fungus".
"There are a lot of questions where some answers could potentially could give us some hope," observed Dr Coleman.
Professor Hallam said it was difficult to know if anything could be done to prevent the current outbreak from spreading further and wiping out millions more US bats.
"We have a lot of chemical agents that will get rid of the fungus," he told BBC News.
"The difficulty is the complexity of bats' life histories; it is almost impossible to treat enough bats to make it worthwhile.
"I don't see any easy solution on the horizon."