One month after the U.S. Fish & Wildlife Service asked people to voluntarily stay out of caves in the Northeast to hopefully prevent the further spread of the deadly white-nose syndrome (WNS) that has already killed 500,000 bats, the U.S. Forest Service has taken things a step further, closing thousands of caves in 20 states to human activity.

The emergency order, issued last week, covers caves in national forests stretching from Maine to Minnesota. The Forest Service, which has jurisdiction over national forests and grasslands, says it plans to close caves and old mines in 13 more states later this month.

The fungus that causes WNS grows on bats' facial skin and flight membranes, possibly causing them to starve. In caves where it has been observed, bats have suffered morality rates ranging from 75 to 100 percent.

The condition was first observed just two years ago and has spread rapidly from cave to cave, and from state to state. WNS appears to be transmitted from bat to bat, but it seems that something else may be helping the fungus move from cave to cave. "We suspect that white-nose syndrome may be transmitted by humans inadvertently carrying WNS from cave to cave where bats hibernate," FWS Northeast Regional Director Marvin Moriarty said last month in a statement.

Local agencies are also taking WNS seriously. Last week, officials in New Jersey closed caves in the Delaware Water Gap National Recreation Area to explorers, where WNS has recently been observed.

WNS was also found in Virginia for the first time two weeks ago.

One slight advantage bats have right now is that we are heading into warmer months. The fungus that causes only grows at temperatures below 20°C (68°F), but biologists recently warned that the arrival of Spring doesn't mean that deaths will immediately cease. "Many of these bats are so physically compromised that they will be exhibiting abnormal behaviors such as flying or roosting out in the open during the day or flopping on the ground as they die," Vermont state wildlife biologist Scott Darling said in a statement.

Image of little brown bats (Myotis lucifugus) affected by white-nose syndrome at a graphite mine by Al Hicks, New York Department of Conservation. Used with permission.