Tuesday, 17 July 2012

The Wolbachia story rolls on

For the last few years one of the hot topics in arbovirology has been Wolbachia, an endosymbiotic bacterium of insects. One aspect of Wolbachia is that it can affect insect reproduction, but in the case of viruses the interesting fact is that infection with Wolbachia can lead to resistance to infection by viruses.

Initially the observations were made in the fruit fly Drosophila melanogaster and the induction of resistance to a variety of RNA viruses upon infection with Wolbachia. This was quickly seized upon as a potential tool for altering the transmission potential of mosquito populations; infection with Wolbachia would effectively reduce the vector competence of mosquitoes to transmit pathogens. In turn, the last couple of years have seen the successful infection of notorious mosquito species such as Aedes aegypti and A. albopictus with Wolbachia. As was predicted, the Wolbachia infection led to the induction of resistance to infection of the mosquito by some medically important viruses such as dengue virus (Bian et al., 2010) and Chikungunya virus (Mousson et al., 2010). We recently showed that fruit flies infected with Wolbachia are resistant to infection by Bluetongue virus (Shaw et al., 2012).

This work has got to the stage where field release of the mosquitoes are a real possibility, it will be interesting to see whether there are similar successes to some of the well known sterile male releases. 

It was interesting therefore that a paper came out recently describing the infection of army worms (Spodoptera exempta) in Tanzania in which infection with Wolbachia resulted in increased susceptibility to infection with a baculovirus, S. exempta nucleopolyhedrovirus (Graham et al., 2012). At present there's not really much idea why this is. Clearly it's a different system; the insects are Lepidopteran rather than Dipteran, and the virus is a dsDNA virus as opposed to the RNA viruses which have been studied so far. Different dynamics are clearly at work and it will be interesting to dissect the interactions between the insect, Wolbachia and the virus to determine the reason behind the differing outcome. 

There is, however, another important message. The fact that this system differs from that observed with mosquitoes/Drosophila and RNA viruses should be a clear signal that things are often more complicated than it may appear, and it would be dangerous to assume otherwise. It's reminiscent of the situation with   Bluetongue virus. Until the BTV-8 outbreak in Europe a few years ago, where it was found that the virus could pass the placenta and infect the fetus, the ability of BTV to cross the placenta was thought to be strictly a property of cell culture adapted live vaccine strains. BTV-8 proved this not to be the case. Similarly, the Culicoides midges which transmit BTV were thought not to go inside buildings, so one of the recommendations for protecting your animals was to keep them indoors when midges were active outdoors. When the outbreak happened in northern Europe, people started looking at the local midge species and found that, actually, they do go into buildings. 

Clearly there's not always such a thing as 'one size fits all'.

Bian G, Xu Y, Lu P, Xie Y, & Xi Z (2010). The endosymbiotic bacterium Wolbachia induces resistance to dengue virus in Aedes aegypti. PLoS pathogens, 6 (4) PMID: 20368968

Mousson L, Martin E, Zouache K, Madec Y, Mavingui P, & Failloux AB (2010). Wolbachia modulates Chikungunya replication in Aedes albopictus. Molecular ecology, 19 (9), 1953-64 PMID: 20345686

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 PMID: 22674991

Graham,R.I., Grzywacz,D., Mushobozi,W.L. and Wilson,K. (2012). Wolbachia in a major African crop pest increases susceptibility to viral disease rather than protects Ecology Letters DOI: 10.1111/j.1461-0248.2012.01820.x