It may be difficult for the most of us to find bats endearing, especially when we learn that not only do they harbour an extraordinary number of diseases but they do so without getting so much as a minor cold. How the bat immune system works so that they are able to carry these diseases without any harmful side effects is intriguing. Dr Michelle Baker, a senior research scientist at CSIRO, is currently carrying out some fascinating work on better understanding the immune responses of bats and their ability to coexist with deadly viruses. She visited Macquarie University last month to share the work that her team have been undertaking.
Bats are an intriguing mammal explained Dr Baker, they are the second most species rich and abundant group of mammals, they have a long lifespan in relation to body size and they are the only mammal with the means to fly. They have evolved to fill a range of important ecological niches and they harbour a large number of diseases. This high mobility combined with a wide geographical distribution puts them in a perfect position to spread pathogens. Despite this, research into bats is not widespread and until recently they were the least studied group of mammals.
The relatively recent emergence of zoonoses (infectious diseases which can pass from animals to humans) has raised interest in these animal and bats account for a large number of new and emerging pathogens (Ng and Baker 2013). Traditionally bats have been linked with viruses like Rabies, however, since 1994 there have been four new human pathogens emerge from bats. Specifically, these pathogens have emerged from the genus Pteropus, otherwise known as the fruit bat or flying fox (Iehle et al. 2007, Plowright et al. 2011)(figure 1). The most well known, or perhaps widely publicised was the Hendra virus, which in 1994 was responsible for the death of 14 horses and 1 human. Since then there have been two reported infections, or what Baker calls ‘spill-over’ events per year, a rather stressful statistic if you are a horse. While it is not certain what causes the spill-over from bats to horses, a recently developed Hendra vaccine means that horses can rest a little easier.
Hendra is not the only virus that comes from bats however, others include Ebola, Coronavirus (SARS), MERS, Menangle, Nelson Bay and Melaka, all of which can have huge human and economic impacts. The one thing that these viruses have in common is that they don’t make the bats sick, and this indicates that these Pteropus bats are indeed true reservoirs for some viruses (Plowright et al. 2011). Bats are social animals Baker explained, and so it is easy for viruses to circulate throughout the population. Occasionally, and for reasons not yet known, these viruses will spill-over into another species with infection via bat excrements such as faeces, saliva and urine (Ng & Baker 2013).
While the triggers for the virus transfer are important, it is the bat’s ability to coexist with these pathogens that intrigues Baker and she has been working on understanding this since 2008. Focusing on innate immunity rather than adaptive immunity in bats (figure 2), Baker has found that there are some differences in the genes that produce interferon (the proteins produced by cells in response to exposure to viruses or bacteria). Bats have three types of interferon and most work has been done on type 1 due to its importance in antiviral processes. Interferon can be quite toxic explained Baker, and so generally organisms keep it quite low until it needs to be stimulated to fight an incoming virus. In bats, however, type 1 interferon levels seemed to be stimulated all the time with no further increase when exposed to a virus (figure 3). It seems that rather than constantly fighting off the virus, bats are carrying around a package of infection, a process that is incredibly energetically expensive.
There is uncertainty as to why the bat’s immune system works as it does, however, there are 5 key things to take from this research to date. Bats are one of the most abundant and diversified mammals, they are true reservoirs for pathogens, there are some key differences their innate immune systems, there are a number of spill-over events from bats to other species but we do not yet understand what triggers these events.
There is a lot of work that needs to be undertaken to comprehend the bat immune system, which will in turn provide insights into the coevolution of pathogens and their hosts. Understanding the processes for increased disease prevalence within the population and the triggers for a spill-over to another species may help us understand when and where it is most likely to occur. Increasing urbanisation means that humans will increasingly sharing space with these animals and so understanding this unique relationship will be key in managing both existing and novel pathogens.