Biting back

DDT, Rachel Carson and the fight against malaria (9 minute read)

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Today, when I say the letters DDT, most people think of something bad, a toxic menace. These negative associations date back to Rachel Carson’s book Silent Spring, which first brought the negative side of this chemical to public attention in 1962. In subsequent decades, DDT was banned in many countries. It’s hard to imagine that in 1948, the Nobel Prize for medicine was awarded to Paul Hermann Müller for developing its use as an insecticide.

At first glance, Müller may appear to be an unlikely recipient, as he had no medical qualification and had never done any medical research. Nonetheless, Müller was a deserving recipient, for his research saved millions of lives. When DDT first became available, it was used in the fight against the Anopheles mosquito, which carries malaria. The use of DDT was so successful that, in 1955, the World Health Organisation set out to eradicate malaria, a decade before it began its successful campaign to eradicate smallpox.

Today, though, malaria remains a threat to nearly half of the world’s population. In 2020, it killed more than 600,000 people, mostly children. Many, many millions of people were infected.

Why did malaria eradication go so badly wrong? Does it have something to do the with global reaction against DDT, following the publication of Silent Spring?

Some commentators would say yes, blaming Rachel Carson for the fact that DDT has been banned in many countries and for the resurgence of mosquitoes and malaria. Notable examples of this opinion include an editorial in the Wall Street Journal in 2000, which said that the suffering and death of millions in poor countries was Carson’s shameful legacy, and the libertarian think tank, the Cato Institute.

At first glance, they appear to have a point. Widespread use of DDT dramatically reduced mosquito populations and malaria cases. As the use of DDT declined and the chemical was widely banned, mosquitoes and malaria came back. But a closer look at the data shows some interesting discrepancies. The decline in malaria began before the widespread use of DDT began in the late 1940s – for a range of reasons including swamp drainage and better housing. But it is true that it began increasing again, in some areas, after 1970, around the time that countries began banning the use of DDT.

However, don’t blame Rachel Carson just yet. The timing of malaria’s comeback and the banning of DDT illustrates an important principle in science – just because one event follows another, it doesn’t mean that the first event caused the second. Something else happened in the late 1960s, something which had a far greater impact on mosquitoes and malaria – the World Health Organisation abandoned its goal of eradicating malaria and funding stopped.

Why was malaria eradication abandoned? It wasn’t because of concern about the toxic effects of DDT. There was another reason, far more practical. By the late 1960s, the most crucial tools for malaria eradication, including DDT, were losing their effectiveness. They didn’t work as well as they had in the 1940s and 1950s. Something strange was happening.

Image credit: Image by Егор Камелев from Pixabay

In his great work, On the Origin of Species, Charles Darwin described the process of natural selection. Those members of a species best adapted to a certain environment would have the best chance of surviving and procreating their kind. Those that were less well adapted would not survive, or at least would be less likely to produce offspring. As well as describing natural selection, Darwin described the way people produced domesticated animals and plants, by a process which worked just like natural selection. He had in mind, in describing this kind of selection, the intentional efforts of farmers and breeders, but the same kind of selection can happen when we don’t intend it.

Had Darwin been around at the time when DDT came into widespread use, he would have immediately noticed a potential problem. Even with an insecticide lethal to most insects, there will be variation in how much it takes to kill different individuals of the same species. If the amount of insecticide used is very high, there might be no survivors. But at lower doses, while some insects might be killed, there will also be some which aren’t. If those survivors go on to reproduce, then the ability to resist the poison will increase in the insect population. Eventually, we end up breeding insects which are resistant to the poisons we use to control them, just as surely as we bred cows which produced more milk and dogs with flat faces.

A first glance, there appears an obvious solution – make sure none of the insects survive. But there’s a problem with this, quite apart from the environmental impacts of pouring large amounts of insecticides into the environment. Unless the insect population in question is confined in some way, for example on an island, there is always going to be an “edge”. Poisons like insecticides never stay precisely where they are put, and so at the edge of where they are used, a lower dose is present. Then there’s the fact that most poisons eventually break down. In the case of DDT, this doesn’t happen quickly, but it does happen. Once it has started breaking down, there is less DDT present, and insects are exposed to a lower dose.

By the time Silent Spring was published, insects resistant to DDT were already appearing. Carson, in fact, devoted an entire chapter to the problem. She noted that prior to 1945 there were about a dozen insect species which had developed resistance to traditional insecticides, such as lime sulphur. By 1960, there were 137 insect species which were resistant to DDT or similar chemicals, including 28 species of mosquito. The World Health Organisation was sounding the alarm. And it wasn’t just mosquitoes that the World Health Organisation was concerned about, Carson said. Houseflies, lice and fleas were also becoming resistant.

At the same time, another problem was developing in the fight against malaria. For centuries, quinine had been used for both prevention and treatment of the disease. However, it had a number of drawbacks – it gave incomplete control of malaria, it had nasty side effects and it was difficult to produce. Research during the 1930s led to the development of chloroquine, which was effective, easy to produce and cheap. From the mid-1940s, chloroquine was successfully used to control malaria around the world.

When the World Health Organisation set out to eradicate malaria, then, they had two highly effective tools – DDT to control the mosquitoes and chloroquine to control the disease. But, just as DDT was becoming less effective because resistance was developing in mosquitoes, chloroquine began to become less effective too. The malaria parasite was becoming resistant to the drug, undergoing the same process of evolution as the mosquitoes, only this time inside the human body.

The twin failures of DDT and chloroquine are part of a larger problem. Whenever we repeatedly use a chemical against a particular species, whether insect, weed, bacteria or a parasite like the agent which causes malaria, we create the conditions for evolution of resistance. Mosquitoes haven’t just become resistant to DDT, they’ve become resistant to more modern insecticides as well. So have other insects. Resistance is a huge problem with bacteria, which are becoming increasingly difficult to control (this issue is deserving of an article of its own, so I’ll return to the topic in future). Resistance can even develop in viruses, such as HIV.

Is there anything we can do to prevent resistance from developing? The answer here is both yes and no. We can’t prevent the process of natural selection from happening, but we can manage the way that we use chemicals like insecticides so that we are less likely to produce resistant pests. The most crucial action is not using the same type of insecticide over and over again, whether necessary or not. This was the usual practice in Rachel Carson’s time, and it does still happen, but those who use insecticides are mostly much more aware now. Farmers are also much more likely to work with nature and only use insecticides as a last resort, which was the approach that Carson herself encouraged. Integrated pest management, which combines careful observation of crops and pests with biological control and targeted use of insecticide, is widely used.

In the case of malaria and mosquitoes, resistance remains a problem. Malaria has developed resistance to every drug we’ve used against it, including the most recent, artemisinin. This doesn’t mean it’s untreatable, but treatment now involves a combination of drugs, and even they sometimes fail. Mosquitoes too are continuing to evolve resistance to insecticides. One of the newer approaches to mosquito control is to use bed nets where the netting has been treated with insecticide – but mosquitoes are developing resistance to that insecticide too.

There are some pieces of good news. One of the safest and most effective control methods is a form of biological control which can be sprayed on bodies of water to control mosquito larvae. While it does kill some insects related to mosquitoes, like blackflies and fungus gnats, it’s a lot less harmful than many chemical insecticides. It’s theoretically possible for resistance to this biological control to develop, but so far it hasn’t, even in laboratory experiments. It was this biological control which was used in New Zealand to eradicate the southern salt marsh mosquito, an Australian species first detected in New Zealand in 1998.

But there’s another tool which is crucial in our fight against deadly diseases that I haven’t yet mentioned – vaccines. For years, scientists have been working on developing a vaccine, but it proved difficult, due to malaria’s exceptionally complex life cycle. Funding was a problem too, since malaria affects the poorest people of the world far more than it affects the wealthy. But, finally, there is now a vaccine available. It was approved in 2021 and is being used to vaccinate children in parts of sub-Saharan Africa where malaria is a problem.

Malaria is still far from being defeated. The vaccine is, at best, only modestly effective, reducing severe disease by 30%. It can play only a small role in our continued struggle against malaria and malaria-carrying mosquitoes. Despite all our progress, we remain no more than a single step ahead of one of our deadliest foes.

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