An ecological perspective on bat viruses

Mammals hold a large reservoir of potential emerging viruses

Viruses universally infect the tree of life; all species ever known have most likely acted as hosts to viral pathogens. These species, harbouring many unknown viruses may therefore act as virus ‘reservoirs’ – storing potential emerging viruses that may one day infect humans or other species. This is illustrated by the recent emergences of swine and avian-origin Influenza, SARS-coronavirus and the Henipaviruses. Events just like these have probably contributed multiple viral pathogens to the human species over our evolutionary history, for example: HIV and the measles and mumps viruses.

Being able to predict when and where these events may take place is thus important to global health if we are to prevent further deadly pandemics. We therefore must be able to accurately judge zoonotic risks before they happen. Many processes, such as genetic factors, contact between species and the amount of virus present in the population will affect the emergence of viral pathogens; if we are to track these it may therefore be possible to predict these risks easily.

The European 'greater mouse-eared bat', Myotis myotis

Possibly due to them being relatively close relatives to us, mammals constitute the largest threat in terms of future viral pathogens that we know of. The most abundant mammals are the rodents and bats – making up well over 50% of all known mammal species with their large population sizes and geographic closeness to human populations favouring for the frequent ‘spill-over’ of viruses to us. It is therefore not surprising that many emerging viruses have originated in these species. Bats have gained much attention from the scientific community relating to virus emergence and have been shown to harbour multiple RNA and DNA viruses, although the extent to which each may spread to humans and other species is unknown (See recent metagenomic papers 1 and 2).. Despite the international awareness of bats as virus reservoirs, little work has been carried out investigating both the molecular biology of bat viruses and even the epidemiological and ecological dynamics of viruses within bat populations. If we are to fully understand how best to prevent virus emergence from bats and other species we will have to fully investigate these processes.

Drexler et al(2011) recently investigated the epidemiological dynamics of viruses in a single population of Myotis myotis bats in Germany in order to better understand the role these animals play in virus ecology. 

Abstract
Bats host noteworthy viral pathogens, including coronaviruses, astroviruses, and adenoviruses. Knowledge on the ecology of reservoir-borne viruses is critical for preventive approaches against zoonotic epidemics. We studied a maternity colony of Myotis myotis bats in the attic of a private house in a suburban neighborhood in Rhineland-Palatinate, Germany, during 2008, 2009, and 2010. One coronavirus, 6 astroviruses, and 1 novel adenovirus were identified and monitored quantitatively. Strong and specific amplification of RNA viruses, but not of DNA viruses, occurred during colony formation and after parturition. The breeding success of the colony was significantly better in 2010 than in 2008, in spite of stronger amplification of coronaviruses and astroviruses in 2010, suggesting that these viruses had little pathogenic influence on bats. However, the general correlation of virus and bat population dynamics suggests that bats control infections similar to other mammals and that they may well experience epidemics of viruses under certain circumstances.

Following the extraction of nucleic acids from bat droppings collected every 3 weeks in May, June and July over three years they were able to build up a generalised picture of how the number and diversity of specific virus genome sequences changed over time and how this may influence the bat population. Using this method, they detected a total of 7 separate viral sequences from a single coronavirus, 6 astroviruses (both RNA viruses) and one adenovirus (DNA). From this, they were able to track their abundance over the 3 months noting whether the amount of a virus increased or decreased over time. Despite there being no evidence that these viruses cause disease in humans or other animals, these may be used as a proxy for other deadly pathogens, including SARS-coronavirus and ebolavirus. A significant increase in bat numbers over the study period indictaed that these viruses had little pathogenic influence on populations.

Virus sequence abundance over the 3 years. A = coronavirus. B = Astrovirus and C = Adenovirus. Notice the cyclical dynamis with A and B yet not C

Two general patterns emerged from these studies:

• One where abundance initially increased only to decrease soon after and finally, in the last month increasing again – shown with the coronavirus and astroviruses. They attribute these different patterns to fundamental changes within the bat population. The initial increase in virus abundance (coronavirus and astrovirus) is possibly due to the increase in the number of bats living in the colony – as time goes on, the colony expands taking in new bats which are susceptible to virus infection just as is seen in human populations. As the bats become immune to the viruses and begin to give birth to pups of their own the transfer of maternal protective immunity to newborns causes an initial decrease in virus numbers; immunity to the virus is still present and hence abundance decreases. As maternal protection fades over time, the newborns become susceptible to infection, leading to rapid increases in virus abundance. These cyclical dynamics are completely the opposite of what is seen with adenovirus where no changes in abundance were observed.

• And another where virus abundance did not significantly change over time – seen with the adenovirus - the complete opposite to the corona- and astroviruses. This may demonstrate basic differences in virus transmission, with adenoviruses persisting long-term in hosts and do not therefore rely on a readily susceptible population of hosts. These viruses have mechanisms of evading host immunity meaning that increases or decreases in protection do not lead to direct changes in adenovirus numbers.

This paper illustrates the potential importance that bat populations play in the ecology of viruses within general ecosystems shared with humans. This work, although not focussing on proven human pathogens, allows us to infer general principles of virus dynamics; these bat colonies may be able to signifcantly amplify RNA virus abundance across time, indicating potential periods of increased risk. The ability to accurately predict the chances of virus emergence may allow us to prevent future epidemics of seirous disease. But, the major questions to ask are whether this does accurately predict the behaviour of other viruses and whether even if virus numbers increase, will this even represent a signifcantly greater risk to other species? More work should be carried out to explore the role of not just bats but other animals in the spread of viruses through an entire ecosystem.


Drexler JF, Corman VM, Wegner T, Tateno AF, Zerbinati RM, Gloza-Rausch F, et al. (2011). Amplification of Emerging Viruses in a Bat Colony
Emerg Infect Dis, 17 (3) : 10.3201/eid1703.100526