Posts Tagged Hendra virus
MERS (Middle East Respiratory Syndrome) is the latest disease in nature’s little bag of tricks, which could turn into the next pandemic and solve our population issues for us. It is closely related to SARS (Severe Acute Respiratory Syndrome), both belonging to the same virus family. But, while SARS appeared in Asia, MERS first popped up in Saudi Arabia in 2012. Since then all cases have been seen either in the Middle East or in people who have travelled to the Middle East. As of June 2014 we have had 697 cases of which 210 people have died. As usual symptoms have been flu-like.
This is quite a high case fatality rate but, like bird flu, the virus has not raced around the world killing a third of the world’s population because it is not easily transferred from person to person, yet.
Any time one of these new diseases pops up it requires quite a bit of detective work to try and determine where it came from and how it works. It will probably come as no surprise that bats are once again implicated, the virus having been found in Egyptian tomb bats, a name guaranteed to generate sympathy with a nervous public.
Intriguingly the virus has also been found in camels, a situation which has some similarities with Hendra virus in Australia. Hendra virus lives happily in bats, infects horses (and kills them) and then spreads from horses to humans. However, as yet, there are no recorded cases of MERS causing disease in camels (although a number of camels have died in the UAE recently of undetermined cause, so this situation may change) and we have only recently seen the first confirmed case of a human catching the disease from a camel. Most of the other cases have been in people who have had very close contact with other sufferers. Where they caught it originally is still open to speculation.
It makes me think that the practice of veterinary medicine may be more dangerous than first thought. While we are all aware of the wonderful things we can catch from our primate neighbours no one really believed there was anything worth catching from our more distant cousins, like horses. Twenty years ago you would not have thought twice about examining a snuffly horse. Two years ago you would not have thought twice about examining a camel.
Australia has the largest feral camel population in the world, but no human cases of MERS. Preliminary testing of 25 camels has failed to find any evidence of MERS, implying that the Middle East camels may have been infected from the tomb bats or that the virus appeared in Middle East camels after the Australian population was established.
The upshot of all this is that we have absolutely no idea where the next fun plague might be coming from. As I get older and more paranoid I am becoming increasingly more nervous about travelling on crowded trains, planes and buses, full of sneezing and coughing people. And I am starting to think that sport is much more enjoyable when viewed from the comfort of my living room than from one of those packed sporting stadiums. I can see the day coming when I refuse to venture outside without my biohazard suit on.
Paranoia aside, most viruses are transferred between us via the things we touch, such as door handles, coffee cups, pens, etc. The best way to prevent this is by washing your hands frequently, with soap and water. To do it properly, sing the Happy Birthday song through twice while you are soaping up your hands. Oh, and don’t kiss bats, camels or any other non-human life forms. And maybe not even the human life forms.
Dr. F. Bunny
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.