Sleep despite fungal attack: Researchers have uncovered why European bats survive white-nosed disease while millions of their North American cousins perish from it. According to this, the immune defense of Europeans can keep the pathogen in check even during hibernation. The American bats, on the other hand, are roused from their sleep by the infestation, whereby they use up their winter energy reserves and die. Since the fungal pathogen originates from Europe, the bat species there were apparently able to develop a tolerance, the researchers explain.
During hibernation, infected bats get a “moldy nose” – this symptom has given the fungal infection the name white nose syndrome. The pathogen Pseudogymnoascus destructans can develop particularly well when the bats of the north are slumbering cool and motionless in their winter quarters. The infestation in North America has become particularly bad in the last 20 years: The white-nose disease has killed millions of bats and brought some species to the brink of extinction. Apparently it is an introduced pathogen: As an earlier study has shown, P. destructans originally comes from Europe. This could explain the greater resistance of European bat species to this disease: they mostly survive an infection with fungus. The mechanisms on which this lower susceptibility is based, however, has so far been unclear.
How do European bats survive?
The researchers around Marcus Fritze from the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin have now investigated this question. To this end, they examined 61 great mouse ears (Myotis myotis) infected with the fungus in winter roosts in Germany. The animals were divided into three groups according to the severity of the infestation: asymptomatic, mild, or severe symptoms. The scientists recorded the body characteristics of the test animals, carried out blood tests and, above all, analyzed behavior during hibernation. Because it is already known from the American species that the fungal infection leads to more frequent awakening from the twilight state.
“We were now able to show that there is no demonstrable connection between the degree of infection and the frequency of waking up in the mouse-eared bats,” says Fritze. “This fits in with the idea that the fungus does not trigger an acute immune reaction in European hibernating bats, but is kept under control by the basic immunity of the bats,” says the scientist. In contrast, North American bats such as Myotis lucifugus often wake up when infected with the fungus so that the immune system can get going. However, this waking up and the immune reaction require a lot of energy and force the premature breakdown of fat stores, the researchers explain.
No need to wake up
Their investigations also uncovered details of the resilience of European bats: the protein haptoglobin plays a central role in the immune reaction of the eared mouse bats during hibernation. This is a defense substance that can also be formed during hibernation without a lot of energy. “Our results confirm the central role of haptoglobin in the immune defense against the fungus. Interestingly, the basic concentration of this protein is enough to protect European bats from the fungus. Accordingly, there is no need for the bats to actively synthesize the protein during hibernation, ”explains co-author Gábor Á. Czirják from the IZW. In contrast, when fighting the pathogen, the American species are evidently forced to ramp up their metabolism – which they apparently cannot afford for the most part.
The results of the study thus clarify the differences in the defense reactions to the pathogen causing the white nose syndrome in European and North American bat species, the scientists sum up. “Tolerance strategies can represent an efficient defense against fungal diseases, as our results with the mouse-eared bat show,” says Voigt. “In North American bats, however, this ability is not sufficiently available, possibly because the pathogen originally comes from Europe and the European species have a head start in developing efficient defense mechanisms,” says the researcher.
So it remains to be hoped that in America too, natural adaptation mechanisms will one day lead to the bats there no longer being roused from their sleep by the fungus.