Joseph Hoyt and his team first showed up to the abandoned mineshaft in Wisconsin during the late summer of 2017, personal protective equipment in hand. Long before Covid-19, the supplies were to protect them from the chlorine dioxide gas they had brought along. Their aim was to use the disinfectant gas to kill the Pseudogymnoascus destructans fungus lining the walls of the mineshaft, which had already killed millions of bats across North America.
“You’re talking about essentially an entire taxonomic group that has been reduced by over 90% – it’s like the equivalent of losing all birds or something like that,” says Hoyt, an assistant professor in disease ecology at Virginia Tech University. This isn’t just bad for the victims – bats play an essential role in ecosystems by consuming large numbers of insects, dispersing seeds and pollinating.
Rather than solely relying on host-to-host transmission, the fungus persists in the environment. Once infected, the bats develop white-nose syndrome, where the fungus compels them to burn through their fat reserves while hibernating, depleting them of the energy they need to survive. Other diseases, such as snake fungal disease and avian influenza, have also been shown to persist in the environment and scientists are becoming increasingly aware of the need to act on environmental reservoirs of disease.
Hoyt first became aware of white-nose syndrome when it emerged in a town next to where he was working as an undergraduate. “It was brought here by human movement,” he says. In Europe and Asia, where the fungus is also active, bat populations are not being decimated. Hoyt says this is because the fungus dies off in the summer. An effort to recreate that dynamic in North America is what inspired the disinfection idea.
“We didn’t completely eliminate the pathogen from the environment and so there’s still some fungus in there,” says Hoyt. Although they didn’t test how other microbial communities were affected by the disinfection, Hoyt reasons that as the fungus wasn’t completely eradicated, it’s unlikely that other microbes were.
During the initial field test in 2017, the concentration of chlorine dioxide was too weak and no change in transmission was recorded. After doubling the concentration in the following two years, they started getting results. For the little brown bat (Myotis lucifugus), overwinter survival rates from white-nose syndrome increased from 14% to 76%, and between-winter survival rose from 7% to 59%.
Hoyt and his team aren’t the only ones to have successfully slowed transmission through disinfecting wildlife habitats. A team led by Dr Jaime Bosch, senior scientist at the MNCN-CSIC institute in Spain, managed to eradicate the Batrachochytrium dendrobatidis fungus from artificial ponds in Mallorca for two years. The fungus causes amphibian chytridiomycosis and, much like white-nose syndrome, is pushing some amphibians towards extinction.
“The only solution we found is to apply chemicals in a particular population where the fungus is killing the animals,” says Bosch, whose research was published in 2015. They removed the animals from the ponds, treated them with an antifungal, and then treated the ponds with a fungicide.
“This was a very expensive and demanding process,” says Bosch. In the years since, he has been working with toads in Andalusia, in southern Spain, and their strategy has changed. Now, they add a fungicide directly to the pond, without removing the animals. He says, however, that it’s a fungicide commonly used in agriculture and, citing yet to be published data, says that it is undetectable after 24 hours and has no impact on the microbiota of the ponds.
“We’re trying to do something now because we’re seeing many populations disappear and we have no time to wait,” says Bosch.
In some senses, Hoyt’s work is a lot easier, as it is possible to disinfect the mineshafts in summer while the bats are not hibernating. There are, however, other limitations. The treatment must be applied every year, and so requires long-term commitment. Otherwise, the sites they treated were quite small, ranging from about 20 to 40 metres long. This was largely, but not only, for practical reasons.
“We’ve never really proposed to do this in caves, given the sensitivity of those environments,” says Hoyt. “There’s likely unique microbiota and stuff that are present in these sites, and so you wouldn’t want to go in and just try to sterilise the environment with the potential of having some kind of adverse impact on other things that exist in those sites.” He suggests, however, that larger sites could be treated, for example by identifying sites that bats tend to flock to and limiting treatment to those areas.
“We have to think very carefully about the collateral damage that would result from some of these strategies,” says Pejman Rohani, a professor in infectious disease ecology at the University of Georgia, who has previously considered using chlorination to target avian influenza. “The unintended consequences may be hard to quantify.”
He uses Delaware Bay as an example, a place he calls a “hotspot for avian influenza”. In such a location, he says, the wildlife effects of a chlorination strategy “would be just disastrous”. In other situations, he is not entirely opposed. “If we’re dealing with the last population of a species at a particular location, then it’s possible that a draconian control strategy – despite its unintended consequences or collateral damage – might be worth considering,” he says.
Dr Demetra Andreou, principal academic in environmental science at Bournemouth University, is equally cautious. “This is like a last resort, and one that has to be applied really carefully,” she says. “It should only be used when there is a huge problem, such as in cases where the species would become locally effectively extinct if you didn’t take any measures.”
Andreou says that there should be more modelling using empirical data when deploying a strategy such as chlorination, as there is a chance that disinfection could select for a more harmful strain. She adds that solutions such as vaccines should be prioritised, as there is also a limit to where you can disinfect.
“I think there needs to be public awareness that prevention is much more important to reduce the risk of transferring these diseases between new places and then putting new species at risk,” Andreou says.
Hoyt agrees that prevention is the most effective strategy, but argues that white-nose syndrome is so widespread it requires a more active approach. Citing research from 1999 and 2001, he also suggests that vaccines in wildlife populations aren’t very effective, and in some cases can even make things worse. He agrees it is possible that disinfection could create a more harmful strain, but points to a meta-analysis which found no evidence that more persistent pathogens – those that may be more likely to survive disinfection – are more harmful.
While under no illusions that disinfection is a perfect strategy, Hoyt says globalisation is spreading disease and disrupting natural systems. “In those instances, I think there is some obligation to try to intervene, and certainly when it’s the case of potentially losing an entire species,” he says.
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