Special Issue "Emerging Tropical Pathogens of Bats"
A special issue of Tropical Medicine and Infectious Disease (ISSN 2414-6366).
Deadline for manuscript submissions: closed (31 May 2019).
During recent decades, bats (Order Chiroptera) have been increasingly recognized as reservoirs of emerging zoonotic infections, important for veterinary and public health. The majority constitute RNA-viruses including lyssaviruses, causing rabies; coronaviruses, including but not limited to the acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) related viruses; paramyxoviruses such as Hendra and Nipah, with recently recognized related paramyxoviruses (Cedar and Achimota viruses) for which zoonotic potential has to be investigated; and filoviruses. For Marburg virus, the role of the Egyptian fruit bats (Rousettus aegyptiacus) as virus reservoirs has been demonstrated via field studies and experimental sampling. However, no direct evidence supports the role of bats as reservoirs of Ebola virus. Nevertheless, a number of indirect findings (e.g., filovirus nucleic acids and filovirus-neutralizing antibodies) documented in a variety of bats in Europe and Asia support the suggestions that bats are filovirus reservoir hosts. Many other viruses from different families have been documented in bats during recent years, but their zoonotic potential and significance for veterinary service remains unclear. In addition, several bacterial pathogens have been described in bats, including bartonella, Brucella, and leptospira, which have clear zoonotic potential, but the potential role of bats as their transmitters to “terrestrial” mammals and humans needs to be elucidated. It is remarkable that the majority of these pathogens—which cause severe (frequently fatal) diseases in humans and other mammals—do not cause apparent clinical signs in bats. There may be something unique in the bat immune system in general, or in its interaction with the specific pathogens, that might evolve with other unique features of bats such as their ability of flight (associated with substantial changes in metabolism, body temperature, and DNA damage), longevity (20–30 years as documented at least for several bat species), the ability to develop torpor—either seasonal or diurnal—with the decreasing of all metabolic processes from several hours to several months.
Bats are very diverse, second only to rodents per species number among mammals (about 1200 species described to date). They are abundant in subtropical and tropical areas around the globe, occupying different ecological niches and including frugivorous, nectarivorous, insectivorous, carnivorous, and omnivorous representatives. They are social animals, forming colonies from several tens to millions of individuals, and frequently roost in close proximity to humans or agricultural animals, which provides ample opportunities for pathogen exchange. Moreover, many species of bats are migratory, and annually travel hundreds or thousands of kilometers, depending on food abundance, environmental temperatures, and other parameters, which leads to long-distance pathogen dispersion.
Despite these features that frequently bring bats in close contact with humans and agricultural animals, these mammals are very difficult to study. They are nocturnal, roost during the day high in tree canopies (such as pteropodid bats) or occupying small and narrow crevices in rocks, trees, buildings, or caves, and as such are frequently not readily available for direct observations, ecologic studies, and sampling. Not many bat species are tolerant to captivity, which would make them a reliable subject for laboratory experiments.
Nevertheless, as the attention to bats as infection reservoirs is increasing, more and more studies are being done which help to find pieces of the puzzle which, in time, will help us to understand the ecology of pathogen–bat interactions, their significance for veterinary and public health, and to develop appropriate prevention strategies which would protect humans and their animals, and at the same time, will not be severely harmful for bats themselves, as many of them are vulnerable to environmental changes, suffer from direct human interventions (e.g., hunting for food or extermination as an attempt to destroy disease reservoir, or simply “clean up” the dwellings occupied by bats due to the various inconveniences), and decline in numbers.
Studies of bat pathogens constitute several layers which partly overlap and complement each other. One layer consists of a random surveillance (usually performed as pilot pathogen discovery studies) which further usually focuses on specific hypotheses, is extended to longitudinal and latitudinal studies, frequently complemented with a modeling component. With certain flexibility, these efforts can be classified into the category of ecological studies of bat–pathogen interactions.
Another layer includes laboratory studies of bat–pathogen interactions. Part of these are done in vitro using bat cell lines (with very limited numbers by both, bat species and tissue types, available to date), which frequently requires the additional development of specific reagents that would react with bat proteins or nucleic acids. Another part is done in experimentally-infected bats, but only for a few pathogens is the relevant reservoir bat species known or can be successfully kept in captivity.
Further, a significant effort is dedicated to public health outreach, which includes surveys, analysis of the obtained data, and generation of strategies which would help to avoid bat exposure or implement prophylactic and treatment solutions if exposure occurs. An additional element frequently incorporated in such studies and based on the One Health concept is dedicated to bat protection—an ability to coexist near each other and not suffer from such a neighboring.
This Special Issue is dedicated to different studies of bat-borne pathogens that collectively contribute to better understanding of the existing complexities and outline further research and intervention strategies.
Dr. Ivan V. Kuzmin
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- emerging infectious diseases
- One Health