Of bats and their viruses
Background: (check this podcast out: http://www.twiv.tv/2012/05/13/twiv-183-bats-out-of-hell/) and this opinion/review article (it’s very fun!) http://www.ncbi.nlm.nih.gov/pubmed/22440923
We live in a sea of microbial life, some of who like to live at the expense of others. Those that do this have the capability of switching their host either temporarily or long-term, depending on certain evolutionary/ecological factors. People have become interested in Bats as a major source of emerging, zoonotic diseases – particularly viruses – since the often fatal ebola outbreaks in the ’70′s were linked to them. Since then there have been numerous outbreaks of viruses originating from these winged-mammals: Ebola, Marburg, SARS virus, Hendra, Nipah and of course the always present threat of rabies. When they happen, these outbreaks have cost the lives of humans, thousands of animals and at the expense of millions of pounds/dollars/Euros/ Renminbi to the world’s economy. However, to date none of these viruses has become permanently adapted to humans or domestic wildlife. Whether they have done in the past is open to question.
The factors that influence virus emergence/spillover/adaptation to new hosts (humans, horses, pigs etc.) are of paramount concern. Nature is the best bioterrorist so we need to stay one step ahead of her. The questions that need addressed are: What microbes do bats harbour? Why do they harbour such apparently deadly viruses? What causes these viruses to spill over into new host populations? What factors influence whether they will establish themselves in the new host? What can we do about it? These all require co-ordinated ecological, epidemiological and molecular projects.
This paper (see below) out earlier this year tries to answer some of the above questions by focusing on one particular group of viruses, the paramyxoviruses (negative sense RNA viruses). This group of viruses includes the human mumps, measles and respiratory synyctial virus (RSV) as well as the emerging Hendra and Nipah viruses. They undertook a large survey of rodent/bat samples from around the world and looked specifically for paramyxovirus-like sequences. Firsty, bats seem to host much for diverse viruses. They found some interesting viral sequences (also isolated virus) including what appear to be viruses that are very(!) closely related to human viruses like mumps. They also did some phylogenetic analysis to test the liklihood of bats being the major host species for this group of viruses.
- Why look for viruses? What about bacteria or parasites?
- Why bats? What other kinds of species would you look at? Or are bats special?
- Are their sample sizes enough to capture true diversity of bats/rodents?
- Why did they specifically look for paramyxoviruses? Why not utilize deep sequencing approaches? (They did but there were issues with it) – compare with this paper http://www.ncbi.nlm.nih.gov/pubmed/22855479
- What does having a viral sequence tell us? Of most of these viruses they never actually found the virus, only sequences. What problems are associated with this?
- Does their data really back up the title of the paper? To show that a species is the reservoir host, what kind of evidence do you need?
- What’s the deal their phylogenetic methods?
- What would you do next with this data? Should we kill all the bats?
The large virus family Paramyxoviridae includes some of the most significant human and livestock viruses, such as measles-, distemper-, mumps-, parainfluenza-, Newcastle disease-, respiratory syncytial virus and metapneumoviruses. Here we identify an estimated 66 new paramyxoviruses in a worldwide sample of 119 bat and rodent species (9,278 individuals). Major discoveries include evidence of an origin of Hendra- and Nipah virus in Africa, identification of a bat virus conspecific with the human mumps virus, detection of close relatives of respiratory syncytial virus, mouse pneumonia- and canine distemper virus in bats, as well as direct evidence of Sendai virus in rodents. Phylogenetic reconstruction of host associations suggests a predominance of host switches from bats to other mammals and birds. Hypothesis tests in a maximum likelihood framework permit the phylogenetic placement of bats as tentative hosts at ancestral nodes to both the majorParamyxoviridae subfamilies (Paramyxovirinae and Pneumovirinae). Future attempts to predict the emergence of novel paramyxoviruses in humans and livestock will have to rely fundamentally on these data.