Friday, 28 September 2012

Badger Herpes
For anyone reading this outside of the UK a huge uproar is currently occurring over the imminent start of a trial whereby it will be legal to intensively cull badgers. The hope is that this will reduce bovine TB (bTB) which is rife in 'hotspots' in the south west of England and Wales. The controversy is that a previous culling trial, the Randomised Badger Culling Trial (RBCT), suggested that reactive culling (that is, culling badgers in an area where a bTB breakdown has occurred) disrupted badger communities with the result that bTB incidence increased (incidentally, this itself adds further weight to the fact that badgers are important in bTB epidemiology). However, in areas where culling was proactive (whereby all badgers in an area were culled up front), bTB levels did indeed drop by a significant level (Vial and Donnelly, 2012), so culling works, right? The current trial is aiming to create regions which have been culled to a sufficient extent to reduce bTB incidence by culling within boundaries to badger movement (rivers, busy motorways etc.). Interestingly, the results of culling in Ireland are somewhat more positive, and in previous periods of culling in England (intensively using gassing) bTB levels were low. 

The large proportion of the UK population like badgers and, combined with the contrasting results, there are plenty of people who are less than keen for the culling to go ahead (hopefully they won't cause the level of culling to be insufficient and thereby cause an increase in bTB!).

Brian May is a big fan of badgers.
So what about viruses? A paper by Banks et al (2002) described the discovery of a gammaherpesvirus in a badger from Cornwall in England. The paper reports bits of sequence from the virus; nowadays the complete genome would, in theory, be relatively simple to obtain and report. The badger was negative for bTB (based on histological and immunological examinations) but was in a poor state of health and had pathology in the liver, kidneys and lungs. Of course this doesn't mean it was the virus that caused these effects. It would be interesting, and potentially important, if this virus inadvertently interferes with bTB epidemiology.
According to the phylogeny, badger herpesvirus appears to fit nicely among other herpesviruses, most closely with equine herpesvirus 2.

Bager herpresvirus (BadHV) most closely related to equine herpesvirus in either the gb  (a) or  DpoI (b) genes. From  Banks et al 2002

The virus can be propagated in mink cells so in theory could be manipulated, though my current knowledge of herpesviruses isn't that great. Perhaps a virological-fantasy would be that the virus could be modified in such a way that it induces immunity/resistance to bTB in badgers - release the virus into the wild badgers and let it spread through the population. Simple and effective. 

But releasing a genetically modified virus into the wild......would that really be any less controversial than the culling?

Banks M, King DP, Daniells C, Stagg DA, & Gavier-Widen D (2002). Partial characterization of a novel gammaherpesvirus isolated from a European badger (Meles meles). The Journal of general virology, 83 (Pt 6), 1325-30 PMID: 12029147
Vial F, & Donnelly CA (2012). Localized reactive badger culling increases risk of bovine tuberculosis in nearby cattle herds. Biology letters, 8 (1), 50-3 PMID: 21752812

Sunday, 16 September 2012

Virus evolution: hitting the bottle(neck) in mosquitoes
When a virus population transmits between hosts, regardless of how this is achieved, not all of the progeny viruses make it, most likely the majority are left behind to contemplate a life of non-existence and the impeding destruction. At the animal level this is fairly straightforward; an animal becomes infected, produces virus, some of which will infect a new animal, which then becomes infected etc. etc. But transmission events impose bottlenecks. Imagine a mosquito taking a bloodmeal from a human. The virus population within that person will, more or less, be spread throughout the body. An adult human contains approximately 5 litres of blood; a mosquito will only take a few microlitres, i.e. a few millionths, of the blood, and therefore only a tiny fraction of the virus in the body. Taken to a more subtle level, of the virus which makes it to a new host, only a fraction of that virus will infect cells within the new host. In the case of the mosquito, there are many 'barriers' which are familiar to virologists studying arboviruses. The first step towards infecting a mosquito (or other biting insect such as midges) is to infect the midgut. At this point there's another bottleneck, known as the midgut-infection barrier; only a few of the viruses which were taken up in the blood-meal will be able to infect the gut cells. In the case of bluetongue virus in midges there are other described barriers, including a midgut escape barrier, a disemmination barrier, and a salivary gland infection barrier. All of these must be overcome in order to allow transmission to another mammalian host.

An array of barriers must be overcome by a virus in order to continue the virus lifecycle. Image from Black et al, 2002. 

The significance of this becomes more apparent when the virus population itself is considered. Viral RNA polymerases are notoriously error prone, resulting in a population of viruses with differences in their genomes, in other words the so-called quasispecies (itself something worthy of discussion). What happens to the population structure as the transmission cycle progresses through the various stages and bottlenecks?

An interesting talk at the SGM in Dublin this year has now been published in PLoS Pathogens. In this case, the authors made an artificial virus population of Venezuelan equine encephalitis virus (VEEV) using reverse genetics. This then allowed particular clones to be followed through the various steps of infection as the virus passes from a mouse containing the clones through to how many are injected into a fresh mouse at the end of the mosquito steps of the virus lifecycle.
If they allowed mosquitoes to feed on a mouse containing the different 'clones' of VEEV, and then looked to see how many they could find in the mosquito, they found between 6 and 8 of the clones (i.e. ~70% of all clones). After a few days however, once the infection process is under way, the number of clones had decreased, implying that not all of the clones had made it through the midgut infection process.

By the time the virus has disseminated to the legs/wings, and salivary glands, there were only 1-4 of the clones present, suggesting that yet more had been lost along the way. In the saliva, which represents the population of virus that will be passed on to the new host, they only found on average 1-3 of the clones. This contrasts to the number of clones found in the legs/wings, showing that only some of the clones from the haemocoel managed to successfully infect the salivary glands.

Interestingly, there didn't appear to be a strong bottleneck in transmission to an uninfected mouse; the same clones that were in the saliva were the same clones which infected the mice, although considering the low numbers of clones in the saliva there wasn't a massive choice.

All of this leads to the puzzling question of how does the virus survive? If at each point viruses are removed, then by chance the surviving population may be less fit. Looking for marked viruses as this study has done is an elegant way of doing it, but it is limited to signatures within the genome. A further study using deep sequencing to look at the whole population more comprehensively may offer even more detail about what's going on. Perhaps this all relates to the observation of purifying selection being popular among arboviruses, time will tell, but to begin with this study has shown that such barriers exist. How the severity of the barriers vary, and how the virus deals with these barriers, will be interesting to see.

William C. Black IVa, , , Kristine E. Bennetta, Norma Gorrochótegui-Escalantea, Carolina V. Barillas-Murya, Ildefonso Fernández-Salasb, Marı́a de Lourdes Muñozc, José A. Farfán-Aléd, Ken E. Olsona, Barry J. Beaty (2002). Flavivirus Susceptibility in Aedes aegypti Archives of Medical Research DOI: 10.1016/S0188-4409(02)00373-9