I’ve mentioned previously the role, or potential role, that bats play in disease transmission. They have long been suspected, and recently identified, as hosts for the Ebola virus. (Whether they’re the main reservoir species and what–if any–role they play in transmission of the virus to humans remains to be determined). They’ve also been implicated in the emergence of SARS and Nipah virus, and of course, have long been associated with the maintenance of rabies virus. A new paper reviews the role of bats in the maintenance and emergence of novel viruses.
Bats represent a huge portion of mammal species: approximately 20% of the 4,600 mammalian species are bats, which range in size from a 130 mm wingspan up to 2 m and are found on all continents with the exception of Antarctica. Within these species, at least 66 different species of viruses have been isolated from bats or detected within their tissues, and there is serological evidence for many others. Rabies is by far the most important, as far as human health goes. However, even though 55,000 human deaths occur from rabies every year, only a small portion are from viruses associated with bats. Viruses that have been isolated include influenza virus, Nipah and Hendra viruses, SARS coronavirus, Chikungunya virus, Japanese and St. Louis encephalitis viruses, Hantaan virus (a relative of the Sin Nombre hantavirus), and Rift Valley fever virus, among others. Despite this incredible diversity of human-pathogenic viruses that have been associated with bats, there are giant gaps in our knowledge of both bat ecology and immunology, further discussed below.
A number of factors have been suggested to play a role in the maintenance and spread of viruses within and between species of bats (and from bats to other species of animals, what the authors call “spill over”). One the authors mention is the long lifespan of bats, which can live 25 years or longer. Therefore, if a bat becomes chronically infected early in their life, and can shed virus for months or years, the potential for spread of the virus is large. Add into this the fact that bats are widely distributed, can be present in high densities in massive populations (up to several million individuals, packed into areas with up to 300 bats per square foot), it’s easy to see how virus can quickly spread through and be maintained within a population. Additionally, many species of bats are migratory, potentially spreading virus hundreds of miles during their travels, and infecting other species of bats (as well as other animals).
Despite all this, very little is actually known about bats, their native viruses, and even their basic immunology. Bats are one of the ancient mammalian lineages; molecular analysis has dated their origin to approximately 50-52 million years ago. It’s not known whether their immune responses differ in important ways from our own, or from those of animals which have been more intensively studied, such as rodents. Other important unanswered questions abound as well. We’ve seen an increase in emergence of viruses from bat species in recent decades. However, it’s not known if this is simply an artifact of increased detection and awareness of such emergent viruses, increased contact of bat species with humans (due, possibly, to encroachment into the ecological niches of bats, or destruction of these niches with radiation of bats into human habitats), changes in the viruses themselves, or a combination of some or all of these factors.
Where to start filling these knowledge gaps? The authors suggest a number of avenues. They note that from around 1930 to 1970, there was an emphasis on field studies and collections in order to harvest and identify novel viruses. This type of “fishing” expedition has fallen out of favor, but as they note, we still know very little about what kind of viruses lurk not only in bats, but in many other animal species, and this type of survey is invaluable in the event of an outbreak of a previously unidentified virus. Therefore, they suggest that “some survey efforts are better than none.” They also suggest additional studies into the immunology of bats, in order to obtain even a basic understanding of their immune system.
In the end, they pose more questions than they answer in their review:
What role, for example, does a fruit-eating bat play in the life cycle of a human or livestock pathogen? If human and livestock infections from bats simply are host-switching phenomena, why have these viruses not been recognized previously, and why have they emerged now? Are these events the results of ecologic alterations, such as impingement of human activities on heretofore virgin areas, consequences of global climate change, or the product of improved surveillance activities coincident with the technical advances in diagnostic capabilities required to identify heretofore undescribed zoonotic viruses?
Are viruses of bats symbionts, parasites, or commensals? Is pathogenicity for humans and livestock simply a freak occurrence? …Might fruit-eating bats transmit viruses to or from plants? How? Are insectivorous bats intermediate hosts between insects and vertebrates (or plants)? Are fruiting events part of periodic amplification cycles of viruses from frugivorous bats to wildlife and humans, as suggested by Dobson?
…Are transmissions between bats and other vertebrates infrequent, incidental spillover events? Do many of the 66 viruses listed in Table 2 represent fortuitous, irrelevant events, or have we detected only the tip of the iceberg? Do bats differ from other mammals in their ability to clear viral infections? Does the persistence of asymptomatic viral infections in bats indicate that bats are an important reservoir for the wide variety of viruses in nature? Is the prevalence of RNA viruses in persistent infections in bats indicative of a defect in host resistance or viral clearance mechanisms, such as interferon or interferon-responsive genes that lead to clearance of RNA viruses from other vertebrates?
Again, this is what I love about science…there’s so much we don’t know, but each of the questions they answer is certainly addressable, given the time and resources to investigate them.
Calisher et al. 2006. Bats: Important Reservoir Hosts of Emerging Viruses. Clinical Microbiology Reviews. 19:531-445. Link.
Image from http://www.purdue.edu/UNS/images/loven.bats.jpeg