Friday, 3 January 2014

Bats vs. Rats
If I had to name one book that got me interested in viruses it would be 'Virus X: understanding the real threat of the new pandemic plagues', by Frank Ryan. The book largely concentrates upon virus emergence; why it happens, where the viruses come from, and what that might mean for the future. Whether or not I now agree fully with everything that's hypothesised is a different matter, although an honest evaluation is difficult considering the advances in science since its publication (1997). Nevertheless, it got me interested at the time.

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Flicking back through it last night I read a line regarding virus reservoirs that stood out. "The threat to humanity derives in particular from rodents". This was the logical conclusion derived from the fact that rodents are the most numerous mammal, which is fair enough. Nowadays it's almost all about bats. In the book Ryan does point out the suspicions of bats as reservoirs, but overall in this book the potential significance of bats is over shadowed by a focus on rodents.

With our current knowledge of virus natural reservoirs (a term itself worthy of debate), a suggestion that anything other than bats are the most important source of viruses as far as public health is concerned, is likely to be met with an element of scorn. Bats do indeed harbor a lot of viruses, as was published earlier this year. In this particular paper the authors also estimated the numbers of viruses still to be discovered (320,000), although wisely they also stated that such a calculation was based upon some rather large assumptions:

"Several important limitations must be considered in our extrapolations, including (i) the assumption that a mean of 58 viruses per species is a reasonable estimate and that host populations are panmictic with respect to viral transmission (such that expanded geographic sampling would not influence viral detections), (ii) the assumption that viruses are not shared by more than one host species, (iii) that only those viruses within the nine families are considered in this estimation, (iv) that the results are limited by the sensitivity and specificity of our tests, and (v) that a similar mean cost of sample collection is incurred across all species."  (Anthony et al. 2013, mBio) 

Nevertheless, it's a useful number to have.
Bats have long been suspected as reservoirs, and in the case of rabies it had been firmly established, but I'm not sure when exactly they became so popular for virus hunters. Perhaps around the time of Nipah and Hendra emergence. Nowadays everybody seems to be hunting for viruses in bats specifically.

Bats, it can safely be said, represent an important source of novel as well as known viruses. In terms of virus emergence and spread however, there is more to it. Yes, bats may harbor a lot of viruses. And perhaps yes for one reason or another those viruses may have a higher chance of being unpleasant. But there is more to epidemiology than simply the source. Spillovers in a forest/rural setting are inevitable, and in this case bats pose as much of a risk as rodents. However, the majority of people live in urban areas. And from this perspective, rodents are surely of greater importance for transmission as their populations are so intimately linked with humans. More contact means a greater likelihood of transmission. One of the worst epidemics in history, the Black Death (admittedly caused by a bacterium), was closely linked with rats. More recent, viral, examples include the Sin Nombre hantavirus in New Mexico, and the Arenaviruses (e.g. Lassa fever virus).

It could be argued that, because we've had so much interaction with rats over the years we're unlikely to find anything new. That doesn't mean they're of lesser importance; Lassa fever and Sin Nombre are responsible for the death of more people than those caused by more exotic viruses such as Ebola virus.

Global air travel: 'emergence hotspots' such as South East Asia experience more international travel than central Africa. Image: Max Planck Institute for Dynamics and Self-Organisation/ Dirk Brockmann
It is clear that the jungles and savannas of Central Africa harbor bats with viruses of great danger to humans. But is this more important than, say, the populations of rats in densely populated urban centers in South East Asia? Human traffic to and from such urban areas is higher, enhancing the probability of an infection spreading to other parts of the world; would SARS, for example, have spread so far if it had emerged in Uganda? In the future it may be that the world is equally connected. For now though, some places remain more connected than others, and this should be remembered when people decide what's more important: bats or rats.

Anthony SJ, Epstein JH, Murray KA, Navarrete-Macias I, Zambrana-Torrelio CM, Solovyov A, Ojeda-Flores R, Arrigo NC, Islam A, Ali Khan S, Hosseini P, Bogich TL, Olival KJ, Sanchez-Leon MD, Karesh WB, Goldstein T, Luby SP, Morse SS, Mazet JA, Daszak P, & Lipkin WI (2013). A strategy to estimate unknown viral diversity in mammals. mBio, 4 (5) PMID: 24003179

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