A recent literature survey identified 1407 recognised species of human pathogen, 58% of which are zoonotic i.e. transmissible from animals to people. Of these 177 are regarded as emerging or re-emerging, with 73% of these being zoonotic (Woolhouse and Gowtage-Sequeria 2005). At first glance bats appear to be over represented as reservoirs of disease, being maintenance hosts for potentially fatal viruses like Australian Bat Lyssavirus, Hendra virus, Nipah virus, and Menangle virus. Bats, not civets, may also be the natural reservoir for SARS and the ever popular Ebola virus (not great apes as was initially thought). So, what is it about bats that make them such popular hosts for some of our scariest diseases?
Bats, in fact, are not over represented as ungulates (cattle, sheep, goats, etc) support the majority of the world’s zoonoses (over 250) and emerging diseases (over 50). Bats harbor less than 2% of human pathogens (Dobson 2005). Unfortunately the ones they do carry are associated with high human mortality rates, making them of much more interest to the media.
It is important to remember that there are a lot of bats, approximately 1240 different species. This represents more than 20% of all known mammal species (second only to the rodents). Bats are also the only mammal that can fly. Therefore, they have more opportunity to contact animals at different locations, enhancing opportunities for spread and transmission. Bats live in large groups, which also increases the potential for disease spread. Just look at how quickly the common cold rips through human populations.
One interesting theory postulates that there are good viruses as well as bad (this is starting to sound like an episode of Red Dwarf – http://en.wikipedia.org/wiki/Quarantine_(Red_Dwarf)). Hendra virus causes no disease in bats but does stimulate the bat’s innate immunity. This may help to protect it from other disease agents. Hendra virus normally persists at low levels in bat colonies. However, under conditions of stress, such as habitat destruction or potential predation, viral load and, therefore, viral shedding increases. As the virus is particularly deadly to other species, such as horses and humans, its release could act like a protective umbrella for the colony (Wang et al 2011).
Most of these disease agents evolved with bats over a long period of time and cause them no trouble. As habitat continues to be cleared, these viruses will contact new hosts (i.e. humans) with no prior immunological knowledge of them with ever increasing frequency. Viruses, like the rest of us, don’t enjoy being kicked out of their comfortable homes and forced to live somewhere new and strange. They tend to get a bit grumpy and anti-social and attempt to demolish their new house. Unfortunately for them, as the house is destroyed, so are they.
Our ability to diagnose diseases is always improving. Incredibly a recent paper reported that 57.2% of fatal encephalitis cases in Australia between 1993 and 2006 were undiagnosed (Huppatz et al 2009). Some of these cases could have been caused by previously unrecognised viruses. It is also a case of the more you look the more you find. Now that our antennae are up we will be searching harder than ever before and are almost certain to turn up new viruses (in fact a relative of the Ebola virus popped up recently in bats in Spain (Negredo et al 2011)).
This should not, however, be interpreted as open season on bats. Bats perform an incredibly important function as seed dispersers, plant pollinators and consumers of vast quantities of insects, many of which harm agricultural crops (Wibbelt et al 2010). Their demise would be catastrophic for agriculture and the planet as a whole. As we expand into new habitats novel diseases will continue to pop up, not just in bats, but likely in a range of species. We need to be aware of the risks, take steps to mitigate them and, in the words of John Howard, “be alert but not alarmed.”
Dr. F. Bunny
If, like me, you want to keep abreast of the situation and have all the latest disease information at your fingertips there are a few excellent websites worth consulting. The Centers of Disease Control and Prevention site contains a wealth of information about every disease you can think of, and probably quite a few you can’t: http://www.cdc.gov/. ProMED provide twice daily alerts about new diseases as they break. They cover not just human diseases but animal and plant ones too. Truly a hypochondriac’s delight: http://www.promedmail.org/. And if all that isn’t enough to make you live inside a plastic bubble for the rest of your life download the HealthMap app to your phone for disease alerts near you: http://www.healthmap.org/en/.
Dobson A.P. 2005. What links bats to emerging infectious diseases? Science 310:628-629.
Huppatz C., Kelly P.M., Levi C., Dalton C., Williams D. and Durrheim DN. 2009. Encephalitis in Australia, 1979-2006: trends and aetiologies. Communicable Diseases Intelligence 33:192-197.
Negredo A., Palacios G., Vazquez-Moron S., Gonzalez F., Dopazo H., Molero F., Juste J., Quetglas J., Savji N., de la Cruz Martinez M., Herrera J.E., Pizarro M., Hutchison S.K., Echevarria J.E., Lipkin W.I. and Tenorio A. 2011. Discovery of an ebola-like filovirus in Europe. PLoS Pathogens 7:10:e1002304.
Wang L.F., Walker P.J. and Poon L.L.M. 2011. Mass extinctions, biodiversity and mitochondrial function: are bats “special” as reservoirs for emerging viruses? Current Opinion in Virology 1:649-657.
Wibbelt G., Moore M.S., Schountz T. and Voigt C.C. 2010. Emerging diseases in Chiroptera: why bats? Biology Letters doi:101098/rsbl.2010.0267.
Woolhouse M.E.J. and Gowtage-Sequeria S. 2005. Host range and emerging and reemerging pathogens. Emerging Infectious Diseases 11:1842-1847.