I was directed by the SGH microbiology blog‘s chief blogger to the Straits Times report yesterday detailing the National Environment Agency (NEA)’s tender for studying the effects of using Wolbachia-carrying male mosquitoes to combat dengue in Singapore.
But what is Wolbachia? It is a fascinating genus of bacteria that exists solely as a parasitic (or in some cases mutualistic) organism in insects and other arthropods. It belongs to the Rickettsia family of bacteria, which also includes organisms that cause human diseases such as typhus in Singapore and many parts of Asia, spotted fevers in other parts of the world (including Rocky Mountain Spotted Fever in the U.S.A.) , and ehrlichiosis in the U.S.A. Interestingly, all these other organisms are transmitted to humans via the bites of various arthropods (ticks, mites, lice, etc.). Unlike other Rickettsia (and most other bacterial species), however, Wolbachia cannot survive outside their arthropod hosts, and are passed vertically from female arthropods to their offspring. Wikipedia has an excellent entry, of course, but I prefer the review by Prof. John Werren from the University of Rochester (it is behind the Nature Microbiology Review pay-wall, but the whole pdf can actually be accessed here).
Because only females can pass on Wolbachia, these organisms have evolved to manipulate the reproductive capabilities of their hosts in four (rather terrible) ways:
- Feminisation – where infected genetic males develop as females.
- Parthenogenesis – where males are no longer required for reproduction (the most famous human example is the “virgin birth”).
- Male killing – where infected males are eliminated (so there are more females born in each generation) so as to give a survival and reproductive advantage to the infected females.
- “Male sterilisation” or cytoplasmic incompatibility – where infected males are unable to successfully reproduce with females that are uninfected, or infected with other types of Wolbachia.
It is estimated that up to 22% of all insect species are infected with Wolbachia.
I first learned about Wolbachia a decade ago, while reading up on the treatment of lymphatic filariasis during the famous Gorgas Course in Tropical Medicine in Peru. Like all medical students and infectious diseases trainees of that period, I had been taught that the drug of choice was the anti-helminthic (i.e. wormicide) drug diethylcarbamazine, with the most infamous side effect being a febrile reaction that was attributed at that time to “stuff like protein and toxins released from dying worms”. It was with a certain sense of wonder that I learned that clinical trials with the anti-bacterial drug doxycycline (most commonly prescribed for pimples in Singapore) was as effective in treating lymphatic (and other types of) filariasis, and such treatment did not elicit the febrile reaction seen with diethylcarbamazine. As it turns out, all the filarial worms that cause lymphatic filariasis have Wolbachia endosymbionts, and killing these Wolbachia with doxycycline rendered the worms sterile (thereby preventing further transmission of the disease), and also shortened the lifespan of the adult filarial worms considerably. And of course, the “stuff like protein and toxins” released by dying worms causing the febrile reaction to diethylcarbamazine turned out to be Wolbachia – almost no febrile reactions are seen when diethylcarbamazine is prescribed to patients with lymphatic filariasis who had already received a course of doxycycline.
Back to dengue and Wolbachia after that meandering discussion: will the NEA’s proposal work? The different strategies and possible outcomes have been discussed in this excellent article by James Bull and Michael Turelli. There are other wet laboratory and modelling/forecasting work that have been published since, and no doubt the experts at NEA are aware of all these, as can be seen from this earlier NEA information release at the end of 2014. Of the various Wolbachia-based strategies, it is also clear that the NEA has selected the option of male sterilization or cytoplasmic incompatibility – releasing loads of male mosquitoes that are unable to produce offspring with local female Aedes mosquitoes because they carry a different Wolbachia species. This particular strategy is least likely to cause the dengue virus itself to mutate, but the “mutant” male mosquitoes have to be repeatedly released in order to have a sustained effect (as they will die off rather quickly and the local “wild type” male mosquitoes will face no competition for the females after a while). Bull and Turelli have assessed this strategy as having a significant likelihood of short-term success, but long-term success will be challenging. I have to agree that that seems to be the most likely outcome.