Malaria might be the most widely recognized mosquito-borne disease, but it's hardly the only one. Dengue fever, transmitted by the Aedes aegypti mosquito, strikes 50 million people annually, and is endemic to over 100 countries. It's not often fatal but it does require medical intervention and, with that many infected individuals, even a low rate of mortality can add up. As with malaria, Dengue control generally focuses on its mosquito vectors, which can be challenging. Now, researchers have developed a way to keep the Dengue virus out of the mosquitoes: infect them with a bacterial parasite that protects its host from competing infections. The bacteria were even shown to be effective in a field test near Cairns, Australia.

Most approaches directed toward controlling the mosquitoes that carry viruses tend to run into problems because they simply kill the mosquitoes. Unless spraying is done continuously, however, mosquitoes will return from nearby areas, and the process creates a strong selective pressure that has driven the evolution of resistance to insecticides. There have been a few ideas about how mosquito control might be done through biological agents, but most of these would kill the mosquitoes too, and are likely to run into the same problems.

A better solution would be something that blocked the replication of the parasites but did nothing to harm the fitness of the mosquitos. And that's precisely what the new research has created.

Fighting a virus with a parasite

The work relies on a bacterial parasite called Wolbachia that is probably worth an article on its own. The bacteria can infect a wide range of hosts, including insects, where it will live inside the cells of various organs. Among its targets, however, is almost always the gonads, where it helps promote its spread by manipulating its hosts to ensure that infected females produce more infected eggs. It has several methods of doing this, including ensuring that all the male offspring die, causing infected males to develop as females, and allowing females to produce fertile offspring without ever mating.

The strain of Wolbachia used in the new work uses a somewhat different twist on this: when infected males mate with uninfected females, all their offspring die. When they mate with infected females, the offspring survive, but all carry a Wolbachia infection. This helps ensure that the bacteria spread quickly within a population.

The strain used here, which was originally isolated in the fruit fly, has another property that makes it especially useful for Dengue control: other infectious agents, including viruses, don't grow well when there's a Wolbachia infection. This, the authors reasoned, would make a Wolbachia-infected mosquito far less likely to spread Dengue virus in the wild. Thus, it has the makings of a great control system: it spreads on its own, allows the mosquitos to live, but keeps the Dengue virus from growing well within them.

There was an initial attempt at this a little while back, but the specific Wolbachia strain cut the lifetime of its hosts short. This limited Dengue reproduction, but also severely limited the spread of the protective bacteria among a wild population. This time, the authors identified a relative of the strain, native to the fruit fly Drosophila, that exacted much less of a fitness cost. To make sure the bacteria could find a home in mosquitos, the authors spent two years growing the Wolbachia on nothing but cultured mosquito cells.

The approach seemed to work well. Mosquitoes with the bacteria developed normally and lived nearly as long as their uninfected peers—their lifespan dropped by about 10 percent. (One of the containers in this experiment was a bit low on mosquitoes, a problem the authors explain as "most probably because of predation by two geckos found in this cage.") When a strain of the Dengue virus was introduced into the population, the Wolbachia did what is was predicted to do. Grinding up the female mosquitos showed the virus levels were reduced by 1,500-fold compared to a control mosquito strain.

Levels in the saliva, which is where it launches its infection of humans, appeared to be nil. The virus was only found in two of 48 pools of mosquito spit (yes, you can collect that), and both of those could be traced back to mosquitoes that escaped Wolbachia infection.

Real-world results

Normally, this is about far as most papers we see get, and indeed, this one ends here. But it's accompanied by another paper that goes considerably further, introducing the Wolbachia strain into a wild mosquito population. Dengue is present in Australia, and the authors managed to conduct a field experiment near Cairns in the northeast of the country. Two towns, both with a bit over 600 homes, were chosen for a trial that was "approved by the Australian Pesticides and Veterinary Medicines Authority (APVMA permit 12311) and were preceded by an extensive period of community engagement and subsequent strong community support."

It's not hugely surprising that they got community buy-in, given that an individual in the area became infected with Dengue during the trial. (This caused aerial spraying that also changed the dynamics of mosquito population in this study, much as the geckos had in the last.)

The trials involved the controlled release of over 300,000 Wolbachia-infected mosquitoes for a period of five weeks, with constant monitoring. If there were lots of uninfected mosquitoes in a specific area of one of the towns, more infected mosquitoes were set loose there. By the end of the release period, over half of the mosquitoes in the area carried Wolbachia. But, after the releases stopped, numbers continued to go up. By two months later, nearly 100 percent of the mosquitoes in one of the towns carried the bacteria. In the second, the infection rate topped out at over 90 percent. (Both numbers dropped a bit with the onset of the dry season, which changes the mosquito population dynamics.)

Mosquitoes aren't thought to travel much, but Wolbachia was detected nearly two kilometers away, and in a location that was across a major highway from any release sites. The bacteria didn't spread in sufficient numbers to take over the population there, but this does suggest a larger coordinated campaign could establish the infection over a much broader area. The authors also point out that, because the local population here reached over 90 percent infection, it provides a useful stock for transfer to other locales—no more need for the messy process of breeding them in the lab.

What the study did not do is determine if the Wolbachia could cut down on Dengue transmission; with only one case in the area (which was outside the two villages), the initial test was too small to really address that. The study was also too short to determine if, when the dry season was over, the infection would pick up where it left off. (Considering that getting the Wolbachia to grow in mosquitoes took two years, these shortcomings are pretty forgivable.) But the authors are clearly enthused about the work, and are undoubtedly looking to get back at it when the rains return later this year.

Nature, 2011. DOI: 10.1038/nature10355, 10.1038/nature10356 (About DOIs).