White-nose syndrome is a fungal disease that infects bats, and since its first detection in North America in 2006, it has devastated bat populations across most of the continent. But Asian bats – which have coexisted with the fungus for a long time – seem to be resistant to the disease, according to a new study published in Proceedings of the Royal Society B. The findings suggest that resistance might evolve over time.
The pathogen that causes white-nose syndrome, Pseudogymnoascus destructans, is a cold-growing fungus that infects the skin of bats as they hibernate. It can even persist on cave walls for long periods of time without bats. The fungus has been documented on multiple species living across Europe and Asia, yet widespread declines like that of North America haven’t been observed. Genetic studies suggest that the same pathogen has been present in the Old World for millennia. Previous work revealed that isolates of P. destructans from Europe are at least as virulent for North American bats as the North American strain, but the virulence of P. destructans from Asia for North American bats is still unknown.
To see why Asian bats can persist with white-nose syndrome, a team led by Joseph Hoyt from the University of California, Santa Cruz, wanted to compare patterns of bat fungal infections. They rubbed sterile swabs on the wings and muzzles of bats at five sites in northeastern Asia and five sites in midwestern North America between 2012 and 2015. The team also swabbed the caves and mines where the bats were hibernating and took temperature readings. The North American sites were sampled for P. destructans during the first three years of pathogen invasion, and the sites on both continents had similar climate and colony sizes.
"Uniformly, across all the species we sampled in China, we found much lower levels of infection – both the fraction of bats infected and the amount of fungus on infected bats were lower than in North America," Hoyt says in a statement. Lower transmission intensity and pathogen growth suggest that Asian bats are resistant to the disease, and this is likely thanks to host resistance – the defenses that reduce pathogen growth. The other hypotheses the team considered (but found no support for) were host tolerance, lower transmission due to smaller populations, and lower pathogen growth rate due to environmental factors.
The team also found variation in infection intensity within most of the North American species. If this variation is the result of genetic differences, that means that species might evolve resistance over time.
However, the mechanisms underlying this resistance are still unknown. "It doesn't have to be the same strategy for every species. It could be differences in the skin microbiome in one and hibernation behavior in another," study co-author Kate Langwig of UCSC adds. "But we just don't have those details yet."
On the left: a cluster of greater horseshoe bats (Rhinolophus ferrumequinum) hibernating in a cave outside of Changchun, China. On the right: northern long-eared bat with white patches of fungus in Illinois. Joseph Hoyt
Image in the text: Hoyt swabs a cluster of greater tube-nosed bats (Murina leucogaster) in northeast China. Guanjun Lu