Microbial assassin brings down a killer

A tiny microbe employed as a living ‘guided missile’ has raised hopes for halting in its tracks one of the world’s most debilitating pandemics – the dengue virus.

Over the past two years an international scientific team led by Monash University’s Professor Scott O’Neill has achieved a series of remarkable breakthroughs in the war on the dengue virus, a mosquito borne scourge that infects 50 to 100 million people a year and threatens the health of 2.5 billion citizens of the tropics.

The microbial ‘missile’ is a bacterium that, in effect, intercepts the virus between its transmission from mosquito to human. Researchers say the breakthrough may have opened up a fresh avenue for the control of other insect-borne diseases including malaria, which claims around two million lives a year.

Without an effective control, the impact of these diseases is likely to only get worse as many parts of the world get warmer and wetter. The World Health Organization (WHO) has already warned that the emergence of giant tropical megacities with huge populations, combined with inadequate sanitation and water management have provided the perfect breeding ground and opportunity for bloodthirsty female Aedes aegypti mosquitoes to spread their deadly cargo of viruses and parasites at ever-increasing rates.

There are four strains of dengue virus, and their incidence has risen dramatically in recent decades making it the world’s most prevalent mosquito-borne viral infection. In its extreme form it produces the condition dengue haemorrhagic fever (DHF), in which patients literally leak blood and can die. DHF epidemics first appeared in the Philippines and Thailand in the 1950s and are now regularly seen in about 40 countries. Dengue virus has become endemic in more than 100 countries, with explosive outbreaks occurring in new areas every year as insecticides fail to check the carriers.

The WHO attributes the spread of dengue to the expanding geographical distribution of the four dengue viruses and their main mosquito vectors, the most important of which is the predominantly urban pest Aedes aegypti. Poor household water storage, open drains and inadequate waste disposal are key factors in the spread of the mosquitoes – while modern air travel helps disseminate the virus from one city to another, as infected travellers pass it on to local mosquitoes. Of even greater concern is the potential exposure of hundreds of millions more people as climate change enlarges the mosquitoes’ range around the world.

Against this background, international teams of scientists have been labouring with rising urgency to find fresh approaches to controlling a pandemic disease for which there is, at present, neither vaccine nor anti-viral treatment. In the end it comes down to either controlling the mosquitoes better or impairing their ability to spread disease.

From their results to date, a consortium of more than 60 researchers worldwide, led by the Monash team, are hopeful they may have cracked the latter challenge.

In 2009, members of the consortium reported finding that a common bacterium called Wolbachia had the effect of blocking the transmission of dengue virus from mosquito to human.

Wolbachia is rather an enigmatic organism that is thought to infect the cells of around 60 per cent of the insect species on Earth. It has a range of effects on its host, some beneficial and some negative. What was needed was a strain that could block the virus – but not cause such a strong detrimental effect that its spread through the mosquito population would be impaired.

Professor O’Neill and his colleagues have been working with just such a strain of Wolbachia known as wMel, which occurs naturally in fruit flies, and is of low virulence when transferred to mosquitoes. Initial trials on caged ‘mozzies’ in the laboratory showed it would spread rapidly and effectively from mother to offspring, infecting many different parts of the insect including salivary glands and ovaries. This, in particular, means Wolbachia could be transmitted to future generations of mosquitoes in their eggs without risk of being passed to any other insect, animal or human.

The researchers fed their Wolbachia infected mosquitoes a blood meal laced with dengue virus – and counted the viruses. They found that levels of dengue virus carried by the mosquitoes infected by wMel Wolbachia dropped by an astonishing 1500 to 2600-fold, compared to mosquitoes not infected with the bacterium. In fact, 87 per cent of the infected mosquitoes were found soon after to be free of the dengue virus.

By comparison, in the mosquitoes not infected with this bacterial blocking agent the team found dengue in the saliva of 80 per cent. The virus was found in only four per cent of the Wolbachia-infected mosquitoes – and these proved to be individuals that had somehow escaped infection. In the lab at least, the deadly dengue was completely blocked by the bacterium.

The unanswered question is ‘how’. Professor O’Neill believes the bacterium may prime the mosquito’s immune system and compete with the virus for key intracellular molecules. He says the eventual answer may open up fresh avenues of research into ways to block viruses generally.

In its report on the development, the scientific journal Nature recounted: “Many studies would stop there. But the investigators took the astonishing next step of attempting a release of Wolbachia-infected mosquitoes into the wild, and, even more astonishingly, they did it in their own backyard, Queensland, Australia.” This region has a recurring dengue problem, and was seen as an ideal location for an initial test release.

The Queensland trials began after extensive public consultation, which resulted in strong community support among the 1300 households of Yorkeys Knob and Gordonvale, outlying suburbs of Cairns. During the early weeks of 2011, the team released some 300,000 adult male and female mosquitoes at numerous locations in these suburban areas. The mosquitoes and their offspring were monitored in an elaborate trapping program, which revealed that Wolbachia had successfully invaded the local mosquito populations at these two locations until nearly every mosquito carried the dengue-blocking Wolbachia.

Nature hailed the success of the trial: “These studies mark the first time that a deliberate Wolbachia-mediated population-replacement strategy has been attempted in nature, and herald the beginning of a new era in the control of mosquito-borne diseases,” it wrote.

Further trials will be conducted in Cairns in the 2012-13 wet season (December to February) using an even more protective strain of Wolbachia, as the team seeks the ideal balance between preventing dengue transmission and preserving the fitness of female mosquitoes so they can spread the control agent.

To answer the all-important question of whether infecting mosquitoes with Wolbachia will cut dengue rates in humans, major new field trials are planned for cities in Vietnam, Brazil and Indonesia, where major dengue outbreaks are common. “We’re in the final stages of obtaining government regulatory approval to run a trial in central Vietnam, and we also hope to begin work shortly in Yogyakarta, Indonesia,” Professor O’Neill says. “We’re also gearing up for future work in Rio de Janeiro, contingent on government approval and community support.”

He estimates it will take around three years to determine whether the use of Wolbachia-infected mosquitoes can significantly suppress the rate of dengue fever infections in humans.

“So far we’ve demonstrated that we can transfer Wolbachia from the fruit fly into the Aedes aegyptimosquito,” he says. “We have also demonstrated that when Wolbachia is present in the mosquito it reduces the ability of the mosquito to support and transmit dengue viruses, and it can be deployed into field settings. We have also shown that Wolbachia infections in mosquitoes reduce the ability of mosquitoes to transmit other human pathogens that cause malaria, chikungunya and yellow fever.”

Late in 2011, the research received welcome encouragement in the form of a $6 million grant from the Australian Federal Government. “This grant will allow the development of new Wolbachia-A. aegypti strain combinations, new knowledge regarding Wolbachia spread after release, optimal release methods and a better understanding of the likely effects in terms of preventing human disease,” Professor O’Neill explains.