For humans, it’s flu season. We should take care to wash our hands and cover our coughs. But we aren’t the only species with seasonal illnesses. Most mammals tend to be troubled by diseases in the spring, summer and fall, when they are active and spending time with other members of their species. Bats have surges in rabies during the summer, when new bats are born and populations increase.
White-nose syndrome, the fungal disease Pseudogymnoascus destructans that affects bats, is a little different. A large survey shows that white-nose syndrome hits bats hardest in the winter during their hibernation. The findings give researchers a clue as to why white-nose syndrome might be hitting U.S. bat populations so hard.
“We knew that more of the bats die during the winter, but we didn’t know whether there are other times during the year when they are carrying the disease without dying,” says study coauthor Kate Langiwg of the University of California, Santa Cruz. “When they carry the disease has implications for how fast it can spread.”
To find the prevalence of white-nose syndrome at different times of year, Langwig and her colleagues scanned 30 different sites across North America where six species of bats like to hang out. Some were hibernacula — places such as caves where bats spend the winter. Others were maternity sites where bats might roost while breeding in the summer. At each site, the researchers carefully swabbed bats’ wings and faces. They then examined the swabs for signs of the white-nose fungus.
In results published December 3 in the Proceedings of the Royal Society B, Langwig and her group showed that bats can successfully fight off the fungus from about May to mid-October, when infection rates drop to nearly zero in two species. But between November and May, when bats head off to their hibernacula for the winter, white-nose syndrome levels skyrocket. Paul Cryan, a research biologist at the U.S. Geological Survey’s Fort Collins Science Center in Colorado calls the study “a remarkable effort that for the first time provides a big picture of patterns and timing and amounts of fungus affecting the species.”
The winter surge is due to two major factors: the hibernacula sites and the bats’ body temperature. Bats return to the same locations to hibernate every year. Once these sites become infected with the white-nose fungus, they stay infected. “The fungus can survive for five years in a dish without needing a bat,” says Langwig. Bats may fight off the infection during the summer, only to be reinfected when they come home to roost.
White-nose infections also get a boost in winter from the bats’ body temperature. When they hibernate, bats drastically decrease their metabolism and temperature down to near the ambient temperature of their winter hideout. This low body temperature is a boon to the fungus, which grows best at lower temperatures.
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If the bats can make it through the winter, their summer activity raises their body temperature to around 35° Celsius (95° Fahrenheit), a good 15 to 20 degrees Celsius higher than the fungus prefers. In such heated conditions, the bats can fight off the fungus entirely and recover. Until winter comes again.
The findings also help explain why white-nose syndrome is not spreading as fast as might be expected. “West Nile is carried by migrating birds, and the disease can be spread really rapidly,” Langwig explains. But because the bats are infected with white-nose when they are less active, the fungus has spread much more slowly. “It’s still only halfway across the country, which is relatively slow.”
Researchers had clues that white-nose syndrome was a disease of hibernating bats, Cryan says. “But [this study] showed the fungus is on these bats at really high loads mostly during the winter. And then it disappears during the summer. It’s a neat illustration of the reverse seasonality of the disease.” He also notes that attacking during winter is especially tragic for the bats. “The timing couldn’t be worse,” he explains. “No bat is safe.”
Currently, there is no treatment or cure for white-nose syndrome. But understanding the seasonality of the disease could help scientists time treatments for when they would be most effective. Langwig says many efforts aim to treat bats during the fall, when they are active and scientists don’t have to worry about disturbing the delicate hibernation cycle. “But our data suggest that’s a bad time to do any treatment that’s not very long-lasting,” she explains. “The bats just aren’t infected at that time. You have to wait until early winter to begin treatments.”
But without any treatments available, the best scientists and bat enthusiasts can do, Cryan says, is to make sure they don’t accidentally transfer the fungus from one cave to another and hasten its spread. “So far there isn’t a ‘fungus-be-gone’ we can spray on bats,” Cryan notes. “But if there ever is, this kind of seasonal information will tell us when to apply it.”
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