Wolobachia- one Parasite to Kill all the bugs!

Wolobachia is a bacterial genus which infects arthopod species, which includes a high proportion of insect i.e. more than 60% are insects. Mosquitoes are incredibly successful parasites and cause millions of human deaths every year through the infections they spread. But they are no match for the most successful parasite of all – a bacterium called Wolbachia. It infects around 60% of the world’s insect species and it could be our newest recruit in the fight against malaria, dengue fever and other mosquito-borne infections.

Can this actually outperform that?

However, The bacterium was first identified in 1924 by Marshall Hertig and S. Burt Wolbach in Culex pipiens, a species of mosquito and Hertig formally described the genus in 1936 as Wolbachia pipientis, The parasite doesn’t generally infect Mosquitos But Scott O’Neill from the University of Queensland is leading a team of researchers who had developed a strain that not only infects mozzies, but halves the lifespans of infected females. Now, as the year comes to an end, they’re back with another piece of good news – their life-shortening bacteria also guard the mosquitoes from other infections. Infected insects are less likely to carry parasites that cause human disease, and those that do won’t live long enough to spread them. It’s a significant double-whammy that could have a lot of potential in controlling mosquito-borne diseases.

Using Wolbachia for biological control makes sense because the bacterium uses many self-serving strategies that allow it to spread like wildfire. And because it’s transmitted in the eggs of infected females, all of its strategies involve screwing over male insects, whose sperm are useless to it. Sometimes it kills males outright before they’re even born. Sometimes, it turns them into females. Captain Skellett says,”Wolbachia IS cool! They do some crazy things in their female-boosting game. In some insects they even cause the males to turn into females, or encourage the females to reproduce by parthenogenesis (virgin birth). Amazing little microbes.”

In other subtler cases, it ensures that infected males can only mate successfully with infected females. If they try to breed with uninfected ones, the embryos die at an early stage of development. This strategy is known as “cytoplasmic incompatibility“. It gives infected females (who can produce living young with any male they like) a competitive advantage over uninfected females, who can only start the next generation with uninfected males. Once Wolbachia gets a foothold in a population, massive swathes of it eventually become carriers. This is exactly what O’Neill’s strain does – it induces complete cytoplasmic incompatibility. Once introduced into a natural population, it should invade with tremendous zest.

This might seem like a flimsy victory, but it’s an important one. Once an individual sucks up a mouthful of infected blood, it takes two weeks for any parasites or viruses they’ve drink to reproduce in their gut and travel back to their salivary glands. Only then do they become infectious. This means that mosquitoes only really pose a threat to human health once they’re old and they are fairly short-lived insects anyway. Any technique that slashes their already limited lifespan will have a huge impact on controlling the diseases they carry.

Even though Wolbachia lowers a female’s lifespan, they don’t hurt her egg-laying ability, or kill her off before she gets a chance to breed. So a Wolbachia-based approach would never drive a mosquito to extinction – it would just kill older individuals before they become capable of spreading disease.

So far, so promising, but O’Neill acknowledges that there’s a lot of work to be done before his strain of Wolbachia can prove its worth for human health. So far, he has only shown that it interferes with infection by P.gallinaceum and not the Plasmodium species that causes human malaria.  Likewise, he has only shown that Wolbachia does the job in the A.aegypti mosquito, rather than Anopheles gambiae, the main vector for human malaria.

Then there are the practical aspects. It’s likely that Wolbachia‘s anti-male strategies will allow lab-infected mosquitoes to rapidly spread the bacterium. Certainly, earlier studies have found thatWolbachia can infect an entire laboratory population of mosquitoes within just a few generations. But that still needs to be tested, first in contained greenhouse settings and then later in field experiments. Computer models will also help to understand whether releasing infected mosquitoes will make a sizeable dent in the wild population, how long that would take, and whether resistant strains would eventually evolve.

This certainly sounds promising, but I’m reminded of the myxomatosis used to kill off feral rabbits in Australia- they eventually developed a resistant rabbit population. Then they brought in another virus- I think it was a parvovirus (?)- and now, not many years later, the rabbits are appearing again. What makes the rabbits so adapted to our “foreign” environment, that they can survive in sufficiently resistant numbers to save their species from two fullblown biological attacks? Mosquitoes are rather ubiquitous and so I thought they might become resistant to Wolbachia fairly swiftly as well… Hmm…thoughts?On this, Jay Says: ”The other concern with biological control is what the control agent will do to *other* species. I’m not an arthropod, so I guess I don’t have to worry about it getting into *my* gametes – at least, not unless it makes quite a few evolutionary leaps – but what about every other prey animal, insectivore, or blood-drinking insect (e.g. horseflies?)”

Reference: Moreira et al. 2009. A Wolbachia Symbiont in Aedes aegypti Limits Infection with Dengue, Chikungunya, and Plasmodiu. Cell DOI: 10.1016/j.cell.2009.11.042 and here is one nature article which I had no access of!