Mycology against malaria

Mouthparts of a mosquito larva

15 August 2014 by Tom Marshall

Insect-borne infections take an appalling toll across much of the world, and they're turning up in new places. Tom Marshall finds out how fungi could help us fight back.

Insect-transmitted disease is a tragic fact of life for many in tropical and subtropical areas, killing people and livestock and causing immense suffering and economic loss. Now climate change and ever-more-intricate transport networks mean the threat is spreading. Warmer, wetter weather creates new habitats for the vector insects that transmit infections.

A recent midge-borne Bluetongue outbreak cost France alone an estimated €1 billion, and people have already died from emerging diseases in southern Europe. Mosquitoes previously confined to the tropics are appearing in new areas; members of the tough Aedes genus are now firmly established in Italy and have been found as far north as the Netherlands, hitchhiking in cargoes of old tyres and the popular houseplant lucky bamboo.

Some experts even fear that malaria - a deadly threat that's been almost unknown in Europe for generations - could be on the verge of returning, spread by the Anopheles mosquito genus. Less familiar diseases are appearing too; authorities have already found insects that carry viruses like chikungunya and yellow fever in the wild.

Threats on the horizon

The European Centre for Disease Prevention & Control monitors the fringes of Europe to get early warning of new dangers before they arrive in force, but it's likely some will get through. Professor Tariq Butt of Swansea University is involved with several projects aimed at developing new techniques that will help in the fight against insect-borne diseases both in new territories and in the regions they've troubled for millennia.

He's an expert on entomopathogenic (insect-killing) fungi - particularly Metarhizium anisopliae, or green muscardine fungus. This is deadly to many arthropods, and has already been turned into an innovative biological pesticide that kills crop pests without harming beneficial insect species.

Butt's attention has now turned to insects and other arthropods that carry disease, and he thinks Metarhizium can help fight continent-spanning scourges like malaria and dengue as well as rarer infections like Crimean-Congo hemorrhagic fever.

Until recently efforts to control insect-borne disease have relied on chemical insecticides, but these have major drawbacks. They're costly, they can pose serious health risks if used incorrectly (the Stockholm Convention on Persistent Organic Pollutants says ten of the 12 most dangerous and persistent chemicals are pesticides) and they kill bugs indiscriminately - not just disease carriers, but predators that eat them, pollinators that are vital for local crops and anything else in the area.

Target species eventually develop resistance to chemical treatments too, so ever-greater quantities must be applied. Because of all this, policymakers are imposing increasingly stringent controls that mean many formerly-popular pesticides are no longer on the market at all.

Known strains of Metarhizium are deadly to vectors including mosquitoes, midges and ticks. The fungus kills adults and juveniles alike, but ignores non-target species and doesn't pollute the environment. It's harmless to humans, and target insects can't develop immunity. Butt is working with projects investigating its potential all over Africa, Turkey and the Middle East, collaborating with organisations from NGOs and pest-control companies to the US military.

Metarhizium won't solve all our insect problems alone. But it's a promising tool, taking its place as part of an integrated control strategy alongside methods like pheromone lures, mass trapping and releasing sterile insects.

Going underground

One promising initiative targets sand flies, which spread Leishmaniasis, a widespread disease that causes painful sores, fever and serious organ damage. It's been of particular concern to the US military since it started appearing in personnel who'd been posted to Iraq and Afghanistan.

The initiative has brought Butt's group together with the US Naval Medical Research Unit No. 3 (NAMRU 3) in Cairo, a major part of whose mission is to research health threats to both US personnel posted abroad and local people. Butt has worked closely with Dr Alia Zayed, deputy head of NAMRU 3's Vector Biology Program and a professor at Cairo University with long experience of biological insect control.

First trial in Ghana - chief of village receiving instructions on applying the fungal material to sand fly breeding sites

Until now, spraying has been the usual measure against sand flies, but this has many disadvantages. Its cost is a particular problem for the remote rural communities that suffer most from Leishmaniasis, and it causes widespread ecological damage. It also reaches only adult flies on the wing; it can't touch juveniles, which usually live underground in rodent burrows and livestock pens. The researchers decided finding a way to kill the young flies before they become dangerous would let them control the sand fly problem sustainably.

Butt's laboratory isolated a fungal strain that kills the flies efficiently, and Zayed and her colleagues applied it together with another Egyptian strain to burrows and animal pens around trial villages in Ghana. "So far the results have been phenomenal," Zayed says; the fungus reduces the adult sand fly population by more than 85 per cent under controlled field conditions. The trials will be repeated in Ghana this year to confirm the results. If all goes to plan, and if the drop in sand fly numbers has the expected effect on local people's risk of disease, it will be a huge step forwards.

Applying the fungus to sand flies' breeding sites in the soil will allow less of it to be used in a more focused way. Building resistance against the fungus isn't a concern - its killing methods are the result of millennia of evolution, so they're exquisitely deadly. And in most places it can be grown locally on cheap, widely-available materials, letting people set up their own pest-control cottage industries.

Extermination isn't the goal - that would be prohibitively expensive and probably impossible. Zayed's aim is more modest; simply to learn to control sand flies in built-up areas and around people's houses by attacking what were previously safe underground refuges. "We can't kill them all, but we can lower the population to reduce the risk that people nearby will be bitten by an infected fly," she explains.

Pushing back malaria

Perhaps the biggest target for efforts at insect-borne disease control is malaria, which killed some 660,000 people in 2010, 90 per cent of them in Africa. About half the world's population is at risk of the disease. Metarhizium has huge promise here too. Zayed and Butt are working with US biocontrol firm Suterra, to develop new mosquito attractants and repellents; the company is sponsoring one of the PhD students working on the subject in Butt's team.

The team hope the new compounds will be used to attract mosquitoes away from people's homes and into traps or monitoring facilities. The US Army is also involved, providing funding and letting the researchers use its wind tunnels to test how attractants affect mosquitoes' movements in different conditions. "The attractant could be pheromones or food, or anything else the target insect is attracted to," says Zayed. "It's just like attracting ants with sugar."

New repellents are another goal; chemically-treated mosquito nets are a vital weapon against the disease, and NGOs like the Gates Foundation have invested heavily. But it seems some of the benefits of this approach are fading. Mosquitoes adapt, for example appearing earlier in the evening before people retreat behind their nets, and seem to be developing resistance to widely-used repellents like Deet. Alternatives are urgently needed.

"The days when we thought we could just cover the landscape in insecticide are long gone; we need a many-pronged approach," Butt says. "So we keep the mosquitoes away from people with repellents; we mask the body odours that attract them; we develop new lures and use them to bring the insects to places where we can easily kill them."

He's been working on trials across Africa, and some of Metarhizium's capabilities surprised even him. Dealing efficiently with adult insects is par for the course, but for some reason the fungus can even kill mosquito larvae under water, and Butt was recently astonished to discover how. It doesn't even need to use its standard methods of infiltrating and killing adult insects with its armour-piercing threads; instead, genetic analysis shows that the larvae are effectively stressed to death.

Their immune systems evolved to deal with an underwater environment, and have never encountered anything remotely like a soil-dwelling, insect-killing fungus. So the young mosquitoes' own stress response kills them; just scatter the spores onto the water's surface and larvae beneath eat them and start dying.

These discoveries suggest exciting new possibilities. The African rainy season turns the landscape into a mosquito's paradise, full of ponds and puddles suitable for breeding. To deal with them by spraying you'd have to blanket the countryside in insecticide. But if you can attack the larvae under water, you can wait for the dry season, when the breeding grounds are far fewer and smaller, and dose them individually with fungal spores.

Butt's plan is to clean out the mosquitoes from towns and create a buffer zone around to prevent re-infestation - mosquitoes can only fly a few miles, so treating water sources near a city should largely neutralise the threat to its inhabitants. "We're not going to eradicate them - just supress them to levels where they're not dangerous to humans," Butt says. If we can reduce the mosquito population when it's already small, during the dry season, then it won't be able to expand again to such a dangerous extent when the rains arrive.

Trials of these fungal mosquito control methods have already taken place in countries including Turkey and Tanzania; more are scheduled for Ghana in 2014. Other collaborations are under way in places including Kenya and Brazil.

Butt emphasises that a problem as widespread as malaria in sub-Saharan Africa needs a whole range of methods - an approach known as Integrated Pest Management. He wants to develop new tools that health professionals can use as appropriate in different situations. "Rather than relying on a single approach, we need to aim for a more diverse and sustainable strategy that uses several different methods together," he says. "When it comes to disease control, there's no silver bullet."