We recently solicited your questions for Sonia Shah, author of The Fever: How Malaria Has Ruled Humankind for 500,000 Years. Her responses cover the effect of Rachel Carson and Silent Spring on malaria; bed nets and their alternatives; and the history of malaria in the U.S. Thanks to Sonia and everyone who participated.
“[M]osquitoes infect between 250 million and 500 million people with malaria every year”?
Is there an extra zero in there somewhere? That number is way too high, since even on the low end everyone in the entire world would have malaria in 30 years. From the CDC:?”WHO estimates that in 2008 malaria caused 190 – 311 million clinical episodes.” This is within the lower bound of the range, but they use the word clinical episode whereas you use the word infect. There isn’t a good link for defining what a clinical episode is, but it appears to be a period of time in a clinic – which means that if you have a disease that isn’t cured, every time you visit a doctor is a clinical episode. It looks like there are perhaps 500 million people with the disease, which is a very high number but does not indicate such a high infection rate, unless everyone is cured of the disease every year. -Jon L.
No extra zeros. Hundreds of millions of people indeed sicken with malaria every year. The illness doesn’t last long, and many people suffer multiple episodes every year. In a highly endemic locale like rural Malawi, a child might suffer twelve episodes of malaria before the age of 2!
The greatest predator of mankind is the mosquito. It is the the top vector for infectious disease. Mosquitoes are mankind’s greatest pest.
Can an environmentalist morally defend the eradication of the mosquito? What would a world without mosquitoes look like? Should we “Save the Mosquito”? – Drill-Baby-Drill Drill Team
Not so fast. Fighting malaria doesn’t mean war on all mosquito-kind. Mosquitoes consider malaria parasites unwanted intruders, just like we do, and their immune systems instinctively attack the parasites. Most successfully ward it off, and so, while we may find them quite annoying, we cannot blame them for malaria. It’s only about a dozen species of mosquitoes-all from the genus called Anopheles-that allow malaria parasites to roost in their bodies, infecting hundreds of millions with malaria every year.
Conservationists need not worry about these dozen species, either. To wipe out malaria, we don’t need to kill them all-just the malaria-infected ones. These tend to be the elderly mosquitoes anyway, for it takes the malaria parasite over a week to develop sufficiently to make the mosquito infective.
I contracted malaria three times over the course of a year despite scrupulous use of bed nets. The first bout resulted in 10-day hospital stay, while the second and third just required medication.
I realize that’s just my personal anecdote, but I think the Western fixation on “just use bed nets” is quixotic. Using a bed net every night in hot, humid tropical areas is uncomfortable at best – and then there is the fact that most people are not willing to sit under a bed net for 12 hours a day while mosquitoes are active. It’s also impossible for someone to tell if the bed net’s insecticide treatment, which is what makes it effective, is still potent or if it must be treated again.
So what is the alternative to bed nets? – Quill
In clinical trials, sleeping under a treated bed net reduces the number of kids dying from malaria by 20 percent. That’s nothing to sniff at, but no net, no matter how religiously used, is going to provide 100 percent protection from malaria.
There are many alternatives to nets-personal repellents, window screening, environmental management to reduce mosquito larval sites, preventive and therapeutic drugs, indoor insecticide spraying, the list goes on and on. In the 1930s, when malariologists figured out that England’s malaria had vanished because English malarial mosquitoes preferred pig and calf blood to human blood, and the local livestock population had boomed, they called for a pig under every bed as a solution to malaria.
But until we get a highly effective malaria vaccine (the most promising one at the moment is only around 60 percent effective) or effect massive economic development in poor, rural, malarious regions, we’ll need to protect ourselves using multiple methods simultaneously. There’s no stand-alone solution.
How much did DDT’s ban set back anti-malarial work? – Brett
The Rachel Carson-inspired ban on DDT in the United States arrived in 1972-nine years after the international campaign to eradicate malaria fell apart. So the ban had little to do with the demise of that unprecedented effort.
But while DDT has never been banned from use in public health work, the bad smell emanating from the chemical still affects anti-malaria work today. There are certainly places in the world where DDT could be useful in the fight against malaria. But hardly anyone manufactures DDT anymore. As DDT fell out of public favor, chemical companies stopped making it. (They’d never made much money on it anyway, since it was off-patent.) Today, there are only two factories that make DDT that could be used in public health campaigns. One is in India. The other is in North Korea.
I’d like to second Brett’s question (#3) – to be more specific, are there stats on malaria prior to Rachel Carson’s Silent Spring, and after the subsequent efforts to ban DDT? – Metamorf
There are, and they are not pretty.
Here’s what happened. Between 1958 and 1963, 93 countries joined a world-wide effort to eradicate malaria by spraying houses with DDT. The global malaria burden when they started was around 350 million cases a year. The plan was to kill sufficient numbers of mosquitoes fast enough-in less than six years-that large numbers of mosquitoes didn’t become inured to the toxin.
The trouble was that DDT was also being used in agriculture, so mosquitoes became resistant to it earlier than expected. And many houses that were supposed to be sprayed with DDT were not. In one village in India, for example, out of 63 houses, 10 doors were locked, 35 residents refused access and 1 house was forgotten by the spray team. Seventeen of 63 houses got sprayed.
And so malaria cases fell, briefly, to about 100 million worldwide, but then resurged back to their pre-DDT levels, which is where they remain today.
“[T]he drugs and DDT ‘spray-gun wars’ of the 1950′s – have failed so miserably”?
I’ll pile on with the DDT question. This is the first time I’ve heard anyone say DDT didn’t work, even from the environmentalist camp. Unless, like most environmentalists, you consider failure to control the population of brown people who talk funny and breed too fast “not working”. – J
Not to split hairs, but it depends on what you mean by “working.” Did DDT reduce the number of countries plagued by malaria? Yes. Did DDT alleviate our global malaria problem? No.
The global DDT blitz worked well in places where central authorities could overcome the skepticism of the public. And it also worked well in wealthy countries and island nations where malaria was relatively light to begin with. Of the 93 countries that signed on to the spray gun?war, 18 succeeded in eradicating malaria by 1970, all of them islands, industrialized, or socialist nations.
The malaria that was left behind is resistant to our cheapest drugs and to our cheapest insecticides, and entrenched in the poorest parts of the world. It’s by almost every measure harder to control.
Was malaria ever endemic in the US.? If so, how was it eradicated? – Jz
It was, and it took about a hundred years to get rid of it.
European explorers and African slaves introduced malaria to the United States in the seventeenth century, and malaria quickly took hold in the colonies from Massachusetts to Florida. They called it “the seasoning” and the “ague” (rhymes with “plague you”). After the American Revolution, pioneers brought malaria out west, where it sunk its tentacles into the Midwest and got as far west as California. By the mid-nineteenth century, songwriters were penning lyrics such as “Don’t go to Michigan, that land of ills…the word means ague, fever, and chills.”
We got rid of malaria slowly-by draining wetlands, screening the windows and doors of our houses, resettling away from low-lying coastal and riverside areas (thanks to the introduction of rail), and by economic development of the rural South, which long suffered year-round malaria, including the most vicious kind, falciparum malaria. A DDT campaign in 1951 hammered the final nail in malaria’s coffin, but it was already a vanishing disease by then.
What is it about the mosquitoes that carry malaria that prevents their spread to temperate regions? What prevents temperate-region mosquitoes from carrying or spreading malaria? Is there any danger of malaria eventually spreading to mosquito populations in climates like central Canada, where summers can be pretty miserable because of the mosquitoes? – Mantonat
Ironically, malaria-carrying mosquitoes are not the kind that drive you nuts in the summer. They’re the silent, stealthy types. When they bite you, you don’t even notice.
And they live all over temperate regions, including across North America. There are some in my backyard here in Baltimore. There’s no biological barrier preventing them from carrying vivax malaria, the type of malaria that is common in temperate regions. The barrier is human behavior.
If you or I got sick from malaria (say, by getting infected while traveling overseas), we’d feel very ill and so would likely rush to the doctor for parasite-killing drugs, and otherwise stay behind screened windows and doors where mosquitoes cannot reach us. The parasites die inside our bodies before the mosquitoes get to taste our blood.
I am curious about the role of animal reservoirs for the malaria parasite. Do infected birds or mammals supply fresh infections to mosquitoes? Is this a factor in the difficulty of eradicating the plasmodium from an area? – David Zaumeyer
It could be.
Up until a few years ago, the malaria parasite Plasmodium falciparum, which is responsible for the vast majority of human deaths from malaria, was considered an exclusively human pathogen. (It is one of five malaria parasite species that infect homo sapiens. There are a plethora of other malaria parasite species that infect everything from birds and lizards to mice and chimps.)
Scientists recently found evidence, however, that gorillas in Cameroon and Gabon are also infected with Plasmodium falciparum. That means that mosquitoes that feed on infected gorillas could pass their parasites on to humans. It’s never been a problem in the past, probably because we’ve never been particularly close to eradicating malaria anywhere in sub-Saharan Africa. But it could be in the future.
It’s hard enough to get people to sleep under bed nets, let alone gorillas.