northern long-eared bat
December 4, 2014

Bat Disease White-Nose Syndrome More Deadly In The Winter

April Flowers for redOrbit.com - Your Universe Online

Over the last seven years, a deadly fungal disease known as white-nose syndrome has spread throughout bat populations in North America, and the disease has left several species at risk of extinction. A new study, published in the journal Proceedings of the Royal Society B, reveals that the impact of this disease is due, in a large part, to the seasonal dynamics of infection and transmission.

The team, led by UC Santa Cruz, was surprised to find that the fungus could infect nearly every bat in a colony over the space of one winter when the bats were hibernating. In the summer, bats that survive are able to clear the infection as their body temperatures rise above the growth limit of the fungus. During hibernation, however, the infection rates are remarkably high, leading to a very high mortality rate at the time of year when the bat populations are naturally at their lowest.

"It hits when the population is at its smallest, and by the end of winter nearly 100 percent of the bats in a cave can be infected, which helps explain why it has such large impacts," Kate Langwig, a graduate student at UC Santa Cruz, told Tim Stephens of UC Santa Cruz.

This study represents the first description of the dynamics of fungal infection and transmission in bats. According to Marm Kilpatrick, associate professor of ecology and evolutionary biology at UC Santa Cruz, the findings were in stark contrast to other infectious diseases.

UC Santa Cruz graduate student Joseph Hoyt examines a little brown bat for signs of white-nose syndrome outside a mine New York. Credit: K. E. Langwig

"For many diseases, population density and social interactions play a big role in transmission, but these bats are actually very social and live in dense colonies in both summer and winter," he said. "In this case, transmission is tied to hibernation and body temperature. When the bats start hibernating, it's almost like they become petri dishes for this fungus to grow on."

White-nose syndrome (Pseudogymnoascus destructans) thrives in cold environments. It grows on exposed skin, such as the bats' noses, ears and wings. Bats' body temperatures drop significantly during hibernation to match the ambient temperatures of their hibernacula — the caves and abandoned mines where bats hibernate for the winter. At these temperatures, between 35 and 45 degrees Fahrenheit, the fungus grows at its best. Winter hibernation for bats can last up to five months, giving the fungus time to spread throughout the colony and on individual bats. The highest mortality occurs in late winter, when both infection prevalence (the number of infected bats) and infection loads (the amount of fungus on individual bats) are at their highest.

"The peak load in late winter is probably what's driving mortality," Langwig said. "Bats that survive the winter can clear the fungal infection once they warm up, so that by mid-summer, when the young of the year are born, almost no bats are infected anymore."

Six species of bats were tested for infection during three key periods of their annual cycle. The bats were first tested in fall, when they mate outside the hibernacula. Next, they were tested in winter, as they began their hibernation; and summer, as they migrated to maternity sites where the females give birth.

In the fall, low levels of infection were detected. The researchers believe this is as a result of bats coming into contact with fungal spores inside the hibernacula. "The bats are still active then, so the infections are at a very low level and don't appear to be growing. But as soon as they go into hibernation and their body temperature drops, the infections ramp up really quickly," Langwig said.

It is important to note that the bats are not transmitting the infection during migratory periods, which limits the rate at which it is spreading geographically. In contrast, West Nile Virus (also first discovered in New York State like white-nose syndrome) spread rapidly throughout most of North America because birds are infected during migration.

The study has valuable implications for creating planning strategies to manage white-nose syndrome. If an effective treatment can be developed, the study findings indicate that the best time to apply it would be early winter. Kilpatrick is working with Winifred Frick, an adjunct professor of biology at UC Santa Cruz, to develop a "probiotic" treatment using bacteria to suppress the growth of the fungus. They have isolated several bacteria species that are naturally occurring on bats, and are attempting to measure their ability to suppress the white-nose fungus in laboratory tests.

"The idea would be to apply the bacteria on bats during hibernation and see if it suppresses the growth of the fungus," Kilpatrick said.

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