The immune system of mammal species has evolved in reponse to the specific selective forces that act on their life-history. For example, parasites and pathogens are known to shape the genetic immune constitution of animals. Furthermore, enviornmental factors such as feeding habits and habitat choice may as well influence the immune system of mammals. In light of the recent emergence of potentially zoonotic diseases, we try to shed light on the factors that influence the immune system of bat species. The basic question is: Are certain species more susceptible to pathogens because of their lower immune competence? Tropical bats are ideal candidates to investigate this question because they exhibit a large variety of social and feeding habits. Currently, we quantify the adaptive and innate immune system of various bat species to give answers to the above question. White blood cell counts, for example, vary largely among species (see graph from Schinnerl et al. 2011).
Our recent survey of neotropical bat species suggested that the cellular domain of the immune system is influenced by the species-specific feeding habits of bats. Bats that include vertebrates at least partially in their diet have higher white blood cell counts than e.g. insectivorous species. Phytophagous species exhibit intermediate values for white blood cell counts (see picture from Schneeberger et al. 2013 PLOSONE). Presumably, carnivorous bats may catch pathogens from their prey that may affect them as well, whereas insectivorous bats feed on food items that do not host bat relevant pathogens. Fruit may be contamined with pathogens when faeces or urine spill over it. Therefore, frugivorous bats may show higher white blood cell counts than insectivours bats, yet lower values than carnivorous bats. The humoral domaine of the immune system (measured as the bacterial killing ability of the blood plasma) is not related to the feeding habits of bats (see picture).
Recently, a fungal disease has caused unprecedented fatalities in North American hibernacula. The psychrophylic (cold-loving) fungus Geomyces destructans grows on the skin of hibernating bats, particularly on the wing membrane, the ears and the muzzle (therefore the disease has been called white-nose syndrome). The same fungus has also been detected in European caves (see picture of Myotis myotis with G. destructans; picture courtesy A. Niedling), yet no one in Europe has witnessed fatalities associated with the fungus. Currently, we test the idea that the immune system of European bats is sufficiently competent to fight off an invasion of the fungus during hibernation. This could explain the contrasting pattern between Europe and North America with respect to bat fatalities in hibernacula. This work is part of a collaboraiton with Dr. Gabor Czirjak at the IZW.
Viral spill-over from wildlife into human populations has been increasingly observed over the past decades, probably as a result of the increasing encroachment of humans in natural ecosystems. Changes in anthropogenic land use frequently increase the contact zone between wildlife and humans (ecological driver), yet it is unknown whether habitat disturbances promote virus prevalence in wildlife species, and if it does so, whether chronic stress caused by habitat degradation is increasing virus shedding and transmission risk (physiological driver). In the proposed work, we ask if habitat disturbance causes stress-induced immune-suppression and an increasing virus prevalence in insectivorous bats according to their roosting habit and sociality. We will use sites that reflect the usual sequence of human encroachment in forest habitats (pristine forest, logged forest, fragments) and use a comparative approach in two congeneric pairs of bat species, where pairs differ either in roost choice or sociality. Specifically, we will ask: Does roost ecology affect the virus prevalence in bats in habitats with increasing level of disturbance? Does chronic stress caused by habitat disturbance affect the viral shedding in bats via reduced immune-competence? Does bat sociality affect the virus prevalence in three habitats with increasing level of disturbance? This study will elucidate the role of physiological drivers in promoting the spill-over of viral diseases, and will determine whether physiological drivers act on wildlife species according to their physiological resilience. This work is supported by the DFG under the frame of SPP 1596