White-Nose Syndrome: Why Bats?
This week’s blog is on an emerging infection in bats called White-Nose Syndrome (WNS). Since its first recognition in upstate New York in 2007, an epidemic of this devastating fungal infection has swept across the United States. The disease is characterized by a fuzzy white deposit around the bat’s nose, thus its name.
You may be asking, what does this bat infection have to do with Homo sapiens? Before describing the WNS epidemic, this is a good time to introduce two relatively new concepts in the field of infectious diseases that will help answer this question. First, is the concept of emerging infections. The original definition of “emerging infections” was provided in a 1992 publication by the National Academy of Sciences entitled Emerging Infections: Microbial Threats to Health in the United States. In this landmark publication, emerging infections were referred to as “infections that have recently appeared within a population or those whose incidence or geographic range is rapidly increasing or threatens to increase in the near future.”
Since 1967, at least 140 infections have emerged in humans that fit this definition. But far more common are infections that have emerged in other animals. WNS is one of them. While the fungus that causes WNS, Pseudogymnoascus destructans, or Pd for short, appears to target only bats, about 60% of the emerging infections in humans are transmitted to humans by animals (so-called zoonoses). Nipah virus infection is an example of a bat-transmitted zoonosis.
The second concept of infectious disease that is relevant to our understanding of why an infection in bats makes a difference for humans is that of One Health. Simply stated, One Health recognizes that the health of humans, animals, and plants are interconnected. That is, “If one goes down, we all go down.” In the case of WNS, even though Pd doesn’t appear capable of infecting humans, the cost of the bat die-off to humans is enormous.
Most bat species are insectivores, and they have voracious appetites. A single bat can eat up to 1,200 mosquito-sized insects every hour, and each bat usually eats 6,000 to 8,000 insects per night. On average, a bat eats its weight in insects every night!
While their appetite for mosquitoes certainly makes a backyard more comfortable, it is their contribution to our food supply that’s more important. Their ability to clear farmlands of insect pests, such as mosquitoes, moths, and beetles is estimated to be worth $3.7 billion each year to American farmers. Bats also provide crucial pollination and seed dispersal services. Finally, eradication of mosquitoes is also of benefit in controlling the zoonosis, West Nile virus infection.
What is WNS? The fungus Pd thrives in cold, damp places such as caves where it infects unsuspecting bats. It attacks their bare skin while they are hibernating and in a relatively inactive state. But as it grows, Pd causes changes that make bats more active than usual resulting in increased metabolism of fat that they need to survive winter. Pd also causes detrimental behavioral changes such as flying outside in the daytime during the winter. Infected bats also wake up more frequently. All of these metabolic and behavioral changes add up to dehydration, starvation, and death before spring arrives.
As of 2019 WNS is estimated to have killed more than 6 million cave bats in 33 states and Canada. And the spread shows no signs of slowing. Several species are affected. Hardest-hit, and threatened with extinction, are the northern long-eared bat, little brown bat, tricolored bat, and big brown bat. In Minnesota, the bat population has become so shrunken and fragile that our state biologists are considering giving up their annual December bat count.
While there is no evidence that Pd can be transmitted from bats to humans or that humans can directly transmit the fungus to bats, Pd spores can last a long time on surfaces such as clothes, shoes, and outdoor gear. Thus we play an important indirect role in infecting bats (this kind of human to animal infection is called anthroponosis). To help stop such an anthroponotic spread of Pd, the United States Fish and Wildlife Service has declared a moratorium on caving activities in affected areas.
Pd was unknown to science until it was discovered in the United States in 2007, and much remains to be learned about the fungus and WNS. How it got here is unknown. Researchers have been looking for it elsewhere and have found it on bats in Europe and Asia, where bats don’t appear to get as sick. No one knows for sure if it can infect other mammals, or whether some day it will develop the capacity to jump from bats to other animals, such as Homo sapiens.
In addition to implementing measures that clear clothing and gear of fungal spores to control the spread of Pd, researchers are working on a vaccine and developing habitats suitable for hibernating bats but unfavorable for the fungus. Nothing, however, appears to be on the immediate horizon to stop the loss of additional bat species. Sadly, within the larger context of the current animal extinction crisis—scientists estimate we're losing species at up to 1,000 times the usual background rate—the extinction of bats isn’t very high on the list of most animal lovers, with the exception perhaps of the farmers among them.