Thursday, 30 August 2012

African Swine Fever on its way, (though not from Africa)

Sometimes I think the numbers quoted in the introductions to papers and presentations etc. are a little inflated. Certain strains of Bluetongue virus, such as BTV-8 (Europe) or BTV-17 (US) can result in a high case fatality of around 40%, and this is what's often quoted, but more usually the fatality rates are much much lower. H5N1 'bird 'flu' is certainly dangerous, but Vincent Racaniello recently highlighted that this might not necessarily be the whole story. There are though exceptions where the fatality rate is said to be high, and is high; African horse sickness, for example, never seems to be much less than 50% fatal, which is not great if you're a horse owner. African swine fever is another virus which fits into this latter 'always nasty' category. 

I've heard about the devastation caused by ASFV from a Ghanaian vet who said he had been to farms in Africa where every pig was dead. If you're a pig farmer you've got to be worried; sheep farmers losing lambs to Schmallenberg virus have had a hard time, but if 100% of your stock die, what then? Imagine a barn full of dead pigs, it can't be great.
Dead pigs as a result of African Swine Fever Virus

ASFV is a big DNA virus that's transmitted by soft ticks. In Africa the virus persists in the environment with its wild host, wart hogs. In Europe though, it has the option of persisting in wild boar, of which there are many.

ASFV has historically been linked, not surprisingly given the name, with sub-Saharan Africa. Before 1957 that's where it stayed. Since then there have been occasional introductions into Europe, and more or less has always been eradicated. That is, until 2007 when there was an outbreak of ASFV in Georgia. Since then the virus seems to have been hanging around in the region, with the odd report here and there of a new outbreak in the Caucasus. Now though, for the first time ever, it's in Ukraine. It's on the move and, rightly, people are worried. Thus far, the Middle East, where there aren't many pigs, has provided a buffer to ASFV encroachment into Europe through Turkey. Now though there is the possibility of the virus coming in via a different route into areas where the pig density is much greater.

In a forest near you: will AFSV-infected wild boar soon be roaming Europe?

Infected meat has always been a threat for ASFV introduction to the UK, although regulations and the restrictions on swill feeding help curtail this threat. However, wild boar don't respect international boundaries, so if the virus manages to get into the tick population, and then wild boar, it could result in a tricky situation where it's difficult to weed out the virus. Wild boar were recently at blame for an outbreak of Foot and Mouth Disease virus in Bulgaria, hopefully they won't be responsible for introducing ASFV.   

Thursday, 16 August 2012

So what, the virus fluoresces; who cares?
The day after I infected the cells I went to the fluorescence microscope and looked for fluorescence. The uninfected control? Nothing, black. The culture with cells infected with BTV-mCherry had islands of red. It had worked. I had made a bluetongue virus which, when it infects cells, expresses a fluorescent protein, in this case mCherry (one of many options - a girl working with me wanted mPlum).

BTV-mCherry replicating in a Drosophila fruit fly. Green fluorescence represents autofluorescence of fly tissues.
Red =  fluorescence generated upon BTV-mCherry infection.

But why bother? What use is a virus which makes cells light up?

The reason it came in useful for us is that we wanted to see where in an insect the virus replicated. A more straightforward approach would be to find the virus using antibodies and then reveal the antibodies - immunolabelling. But this resulted in the insect muscles fluorescing. Using the fluorescent virus meant that we didn't need immunolabelling; instead we simply sliced up the insect and looked at which bits lit up when illuminated at the wavelength for mCherry.

Another reason is so that virus infections can be filmed - live cell imaging - to reveal dynamic processes. For example, virus protein 'X' is tagged with a green molecule. Where does it go? Fixing the cells means looking at just a point in time, rather than 'the virus protein travels from here (A) to here (B)'. A fluorescently tagged virus therefore lets you film the virus in real-time. I saw this in action with a fluorescently labelled Rinderpest virus (prior to its eradication). As the virus spread it formed large syncytia, swallowing up cells in an ever growing expanse of green. 

Cells infected with GFP-expressing Rinderpest virus from Banyard et al., (2010). A; individual cells.  B; syncitia.

The ability to film dynamic processes of an infection has other advantages. Fluorescence can be measured, and can be used to reflect the extent or rate of replication: in addition to its use in virus research, this can obviouly be used as a measure of therapeutic value.

And in the animal?
The simplicity of looking for these sorts of viruses in animal tissue means it's possible to sample from more tissues of the infected animal, increasing our knowledge of the virus in the host. Even during infection, a machine can sort a blood sample so that we can find which blood cells the virus is replicating in. The insect infected with bluetongue is one example, but the most impressive I've seen so far was at the SGM in Dublin (now published in J. Virology). A canine distemper virus expressing GFP (green) or dTomato (red) was used to infect ferrets. This time it wasn't just a case of looking at dissected tissues, the authors looked at entire organs to determine where the virus replicated: virus infection at the macroscopic level - something impossible without such a virus.

Ferret tissues infected with green or red expressing Canine distemper virus (from Ludlow et al, 2012); A, infection around the eye and mouth, but not the nose; B, gingiva; C, skin epidermis; D, tongue and tonsils; E, salivary gland and lymph nodes; F, lungs; G, liver; H, spleen; I, stomach/gastrointestinal tract; J, infected B cell follicles in Peyer's patches; K, absence of fluorescence in the leptomeninges (redCDV); L,  fluorescence in the leptomeninges (greenCDV).

So, there you have it, not so useless after all. As freaky as they may be, fluorescently tagged viruses certainly have their place.

Shaw AE, Veronesi E, Maurin G, Ftaich N, Guiguen F, Rixon F, Ratinier M, Mertens P, Carpenter S, Palmarini M, Terzian C, & Arnaud F (2012). Drosophila melanogaster as a Model Organism for Bluetongue Virus Replication and Tropism. Journal of virology, 86 (17), 9015-24 PMID: 22674991

Banyard AC, Simpson J, Monaghan P, & Barrett T (2010). Rinderpest virus expressing enhanced green fluorescent protein as a separate transcription unit retains pathogenicity for cattle. The Journal of general virology, 91 (Pt 12), 2918-27 PMID: 20719989

M. Ludlow, D. T. Nguyen, D. Silin, O. Lyubomska, R. D. de Vries, V. von Messling, S. McQuaid, R. L. De Swart and W. P. Duprex (2012). Recombinant Canine Distemper Virus Strain Snyder Hill Expressing Green or Red Fluorescent Proteins Causes Meningoencephalitis in the Ferret Journal of virology, 88 (14) DOI: 10.1128/JVI.06725-11

Wednesday, 8 August 2012

Schmallenberg....where are we at?

There's increasing chat about the circulation of Schmallenberg virus in the UK this summer. Is that surprising? Maybe, it was more or less impossible to know as the virus was new. Likewise, new virus, new test. Serology (looking for antibodies as evidence that an animal has previously been infected) is the way to find where it's been and there's a commercial ELISA to do this serology. But how can you compare it to a gold standard when there isn't a gold standard? There's always the chance that the test doesn't detect as many as it should; the bTB skin test is the classic case of a test being hopeless. 

An ELISA showing cattle sera positive for SBV antibodies

Is the circulation important? The worrying thing is that we only have a rough idea about prevalence. The situation is reminiscent of the situation in Germany in 2007; seeded with bluetongue virus in late 2006, BTV exploded in Germany the following summer, with 1000s farms being infected. The main indication of SBV infection seems to be congenital deformities, so, unlike BTV, we'll have to wait; it could still be the horror everyone was worried about.

Re-circulation does bring it onto the radar of vaccine companies though, so vaccines might be available soon, but when exactly nobody knows. One of the ideas with Akabane virus (one of SBV's close relatives) is that protection is achieved young and thus the animals may be more resistant to infection when pregnant. 
So is Schmallenberg important? We still don't know. If it affects fertility it could be massive. Right now  though it's 'watch this space'. Certainly in the south west of England, bTB remains the major scourge.