If there’s one thing I’ve learned over the last decade-plus of blogging about medicine and alternative medicine, it’s that any time there is an outbreak or pandemic of infectious disease, there will inevitably follow major conspiracy theories about it. It happened during the H1N1 pandemic in the 2009-2010 influenza season, the Ebola outbreak in late 2014, and the Disneyland measles outbreak last year, when cranks of many stripes claimed that either the outbreaks themselves were due to conspiracies (usually, but not limited to, conspiracies to promote the “depopulation” vaccination agenda of—who else?—Bill Gates) or that nefarious forces were seizing on the outbreak to take away our freedom. The second thing I’ve learned is that inevitably people will try to impose their ideology on to the disease and try to use outbreaks to push their own ideological agenda. Indeed, the Ebola outbreak, for example, was rapidly seized on by politicians to promote quarantines and to halt immigration from the affected countries. This year, the biggest infectious disease-related story thus far is the Zika virus outbreak in Brazil that has been linked to microcephaly and other birth defects, and it’s a case of the same stuff, different year, lots of conspiracy theories.
The Zika virus is a mosquito-borne flavivirus related to dengue virus and transmitted primarily by Aedes aegypti mosquitoes. On the surface, this virus would appear to be relatively benign, with 80% of those infected by it remaining asymptomatic, while the other 20% suffer from what is usually a self-limited, relatively mild illness characterized by fever, rash, arthralgias (joint aches), and conjunctivitis. Of course, as we learn more, we’re learning that in some people it can be more serious. Still, in the grand scheme of things, after decades of being endemic in many tropical areas Zika virus infection probably didn’t seem so bad and didn’t appear to be much of a public health priority in the regions where Aedes aegypti mosquitoes live, mainly tropical regions in South and Central America, Africa, southeast Asia, and the Pacific islands. Then came the evidence that prenatal infection might cause microcephaly, and everything changed. Not surprisingly, conspiracy theories galore arose with social media speed, as did the ideologically motivated overselling of proposed solutions, such as bringing back DDT to combat the mosquito carrying the disease.
I don’t feel the need to go into a lot of detail about the various conspiracy theories and quack ideas that have sprung up over the last couple of months, given that Steve Novella, Tara Smith, and, of course, yours truly have deconstructed these in detail already. Suffice to say that the main conspiracy theories come from the anti-GMO fringe and the antivaccine movement. Even George Takei fell for one of them.
Et tu, George?
Of course, it is amusing to me that the cause and solution to the Zika virus health crisis would be pesticides. But is there even a crisis? Let’s examine the evidence before deciding whether it’s appropriate to unleash the white powder known as DDT.
Zika virus and microcephaly
Before embarking on a discussion of the reaction thus far to the Zika virus health crisis, it’s important to ask: What’s the evidence for a link between Zika virus and microcephaly? Microcephaly literally means “small head” and is characterized by an abnormally small head and brain with delayed brain development. But does Zika virus cause microcephaly? As Facebook relationship statuses like to say, it’s complicated. The existing epidemiological evidence is strongly suggestive but certainly not (yet) a slam dunk for inferring causation, and some have even questioned whether the number of cases of microcephaly has really increased as much as initially thought, further muddling the question and contributing to the “Zika virus didn’t do it” conspiracy theories.
A couple of weeks ago, Steve did a fine job of summarizing the evidence linking Zika virus infection and microcephaly. It’s a good, digestible summation, but unfortunately for him events are moving so fast that, shortly after his post went live, the CDC published a report in the MMWR, Notes from the Field: Evidence of Zika Virus Infection in Brain and Placental Tissues from Two Congenitally Infected Newborns and Two Fetal Losses — Brazil, 2015, and the New England Journal of Medicine published a case report that adds to the very suggestive evidence of a link between maternal Zika virus infection and microcephaly.
I’ve already discussed the MMWR report in detail before; so I won’t discuss it again. Further adding evidence of both biological plausibility and a potential link is the case report published online last week in the NEJM. This report describes the case of a 25-year-old previously healthy European woman in Ljubljana, Slovenia who had worked as a volunteer in Natal, the capital of Rio Grande Do Norte state in Brazil since December 2013. She became pregnant near the end of February 2015 and became ill during her estimated 13th week of gestation with a high fever, a rash, musculoskeletal pain, and eye pain. Infection with Zika virus was suspected, but no diagnostic testing was performed. Ultrasound at 14 and 20 weeks showed normal fetal growth and anatomy. The patient ultimately returned to Europe around her 28th week, and a 29 week ultrasound examination showed fetal anomalies. Another ultrasound at 32 weeks showed intrauterine growth retardation, a head circumference less than the second percentile for gestation, along with abnormalities of the brain, including calcifications. The pregnancy was terminated, and the fetus showed prominent microcephaly, with almost complete agyria (lack of the normal folds on the surface of the brain). Other abnormalities included inflammation and evidence of a viral infection in the neurons.
A detailed family history failed to find any genetic syndromes, and other causes of microcephaly were ruled out. The brain tissue was subjected to next-generation sequencing, and:
A complete ZIKV genome sequence (10,808 nucelotides) was recovered from brain tissue. Phylogenetic analysis showed the highest identity (99.7%) with the ZIKV strain isolated from a patient from French Polynesia in 2013 (KJ776791) and ZIKV detected in Sao Paolo, Brazil, in 2015 (KU321639), followed by a strain isolated in Cambodia in 2010 (JN860885, with 98.3% identity) and with a strain from the outbreak in Micronesia in 2007 (EU545988, with 98% identity).
The complete genome sequence of ZIKV that was recovered in this study is consistent with the observation that the present strain in Brazil has emerged from the Asian lineage. The presence of two major amino acid substitutions positioned in nonstructural proteins NS1 and NS4B probably represents an accidental event or indicates a process of eventual adaptation of the virus to a new environment. Further research is needed to better understand the potential implications of these observations.
In other words, the virus isolated from this microcephalic fetus appears to have come from Asia. It also appears to have two mutations in nonstructural proteins, the significance of which are unknown. Again, this is not slam-dunk evidence that Zika virus causes microcephaly, given that it’s only one case. (For instance, something else could have caused the microcephaly and at the same time made the fetal brain more susceptible to infection.) Even so, it adds to the accumulation of evidence linking Zika virus to microcephaly.
Still, there remain many questions, such as why there haven’t been outbreaks like this before, given that Zika virus has been endemic in many areas since at least the 1950s and there have been outbreaks of Zika virus before in Africa, southeast Asia, and the Pacific Islands, as well as several countries in Central and South America where Zika virus outbreaks are currently being reported. Yet, despite how widespread the virus is, there haven’t been reports of increased incidence of microcephaly before this. Clearly, there are many unanswered questions that will require large scale epidemiological studies to answer. In the meantime, taking precautions and developing a vaccine are reasonable interventions.
Since Zika virus is mosquito-borne, prevention is impossible without mosquito control in the affected areas.
It was at this point that I couldn’t help but note an argument that was popping up in certain spheres about the Zika virus and how to control its vector. For instance, Gil Ross of the American Council on Science and Health (ACSH), an organization I’ve discussed before that apparently never met a pesticide it didn’t like, posted an article entitled “Stopping Zika Virus in Its Tracks, by Unleashing DDT,” while Josh Bloom of the same organization wrote a very pro-DDT article that might as well have been entitled “DDT is good for you.” Meanwhile, Dr. Jane Orient, whom we’ve also met before, executive director of the American Association of Physicians and Surgeons (AAPS), an organization that we’ve also encountered before, published an article in WorldNetDaily entitled “Zika Virus: What Should We Do About It?” It was this passage from Dr. Orient’s article that got me thinking and made me decide to write about DDT:
Travel restrictions would greatly harm the economy of Latin American countries, especially as Brazil is preparing to host the Olympics. Of course, there is no screening at all of illegal entrants to the U.S. The key public health measure is mosquito control. Mosquito-borne diseases, after a time when it was thought that even malaria might be wiped out, began increasing worldwide when the U.S. banned the most effective public health weapon of all time: DDT. If Zika causes rethinking of this disastrous decision, even though other deadly threats like malaria have not, it will save millions of lives – and even help us win the war on bed bugs.
The damaged babies are a terrible tragedy. How can we prevent more? Instead of waiting for some future vaccine against a virus that may prove innocent, we could stop transmission now with effective mosquito control in affected areas. We could also immediately stop deliberately exposing women who might be pregnant to medicines or vaccines without thorough safety testing.
I can’t help but note that in her article Dr. Orient embraced antivaccine claims that Tdap and/or the MMR vaccine was causing the microcephaly, while insinuating that the whole thing might be trumped up as a means to promote an agenda that includes combatting anthropogenic climate change, loosening restrictions on abortion in Central America, keeping immigrants out of our country, and, of course, letting loose the DDT if it turns out this Zika virus-microcephaly thing is real.
Of course, it’s easy to pick on Dr. Orient; she’s a bona fide crank who runs an organization that resembles the AMA if it were crossed with the more extreme elements of the John Birch Society. Besides, just because she’s a crank doesn’t mean she might not be right on this one issue. Also, she’s not the only one pushing to bring back DDT. A quick Google search about DDT and Zika virus brings up a slew of articles with titles like “Zika Virus Shows It’s Time to Bring Back DDT: Our most effective tool has been banned,” “WHO Says Zika Virus a Global Threat; Health Experts Push to Lift DDT Ban,” “The Experts Want To Unleash DDT To Fight The Zika Virus,” “Physician: Lifting DDT Ban Could Stop Mosquito-Borne Zika Virus,” and “To Combat Zika, Bring Back DDT.”
One can’t help but also notice that there is one thing nearly all of the publications and persons calling for a return of DDT share, and that’s a certain political orientation, something made a bit clearer in titles of other pro-DDT articles written about the Zika virus outbreak, such as “DDT might have stopped Zika, but environmentalists chose mosquitos over people” and “Mass Murders and Radical Environmentalists,” the latter of which seriously argues that environmentalists, through banning DDT, have killed more people than Hitler and Stalin, concluding with the utterly unhinged and reprehensible statement that the “title of ‘Greatest Mass Murderer of all Time’ goes to the late Rachel Carson and all of her radical environmentalist followers.” Yes, Rachel Carson is the target of most pro-DDT ire, a typical title being a little less histrionic; e.g., “Zika Virus: Rachel Carson’s Deadly “Good Intentions” Legacy Continues.”
Basically all these articles argue that, if only we would let loose with the “excellent powder” of DDT, Zika virus would no longer be a problem, frequently with the claim that the only thing preventing us from doing this are those pesky environmentalists, starting with Rachel Carson over 50 years ago! But do they have a point? Not really, at least not a particularly good one. Their narrative, in which Rachel Carson inadvertently led to the deaths of tens of millions because her book persuaded the US, having eradicated malaria within its own borders, to ban the tool we used to do it so that poor countries couldn’t do the same and in which all we have to do is to bring back DDT to defeat malaria and other mosquito-borne illnesses like Zika virus, is simple and attractive. Unfortunately, because it ignores a lot of nuance, it is also mostly wrong.
Rachel Carson, Silent Spring, and DDT
Dichlorodiphenyltrichloroethane (DDT) is a colorless crystalline organochloride that is also tasteless and almost odorless and has been used for many decades as an insecticide, particularly for eliminating disease-carrying mosquitos. It was first synthesized in 1874 by a German graduate student in chemistry, Othmar Zeidler, under the supervision of Adolf von Baeyer. Not much was done with it until 1939, when Swiss chemist Paul Hermann Müller discovered its insecticidal properties. After that, DDT soon was in widespread use during World War II to control malaria and typhus. After the war it was used as an agricultural pesticide, and its use skyrocketed. Because DDT was such an effective pesticide with such apparently low toxicity to humans and animals, in 1948 Müller won the Nobel Prize in Physiology or Medicine for his discovery.
There’s no doubt that DDT was incredibly effective—at first—and cheap too. In 1955, the World Health Organization initiated its Global Malaria Eradication Program that relied largely on DDT for mosquito control and rapid diagnosis and treatment to reduce transmission. The program was very successful—again, at first. Unfortunately, failure to maintain the program and the development of resistance to DDT (evolution in action!) conspired to roll back or even completely reverse early victories.
It was in the middle of this history that Rachel Carson published her book Silent Spring in 1962. The book was a powerful indictment of what Carson saw as the overuse of DDT and other pesticides, which she termed “biocides,” mainly because such chemicals almost never limit their toxic effects to just the intended organisms. In the book, she documented a number of adverse effects of DDT and other pesticides on the environment and human health. Specifically, Carson blamed DDT and pesticide use for thinning of birds’ egg shells and declining bird populations, as well as described it as potentially carcinogenic based on animal studies. She described how DDT entered the food chain and accumulated in the fatty tissues of animals, and how one of DDT’s greatest advantages as a pesticide, its ability to persist where it is sprayed and remain toxic to insects for long periods of time, was also what contributed to its potential harmful effect to the environment and humans. Much of what she found has stood up, although in fairness it must also be noted that Carson probably did vastly overstate how carcinogenic DDT is.
Despite this indictment, contrary to popular belief, it was not a book that was just about DDT, nor was it a book that called for the outright banning of DDT or other pesticides. Indeed, Carson was at pains to point out that she did not advocate banning DDT or pesticides. At one point she wrote:
It is not my contention that chemical insecticides must never be used. I do contend that we have put poisonous and biologically potent chemicals indiscriminately into the hands of persons largely or wholly ignorant of their potentials for harm.
Which was really hardly arguable at the time, because it was largely true. In fact, I doubt there are too many environmentalists today who would argue with this. She was also correct about the evolution of resistance:
No responsible person contends that insect-borne disease should be ignored. The question that has now urgently presented itself is whether it is either wise or responsible to attack the problem by methods that are rapidly making it worse. The world has heard much of the triumphant war against disease through the control of insect vectors of infection, but it has heard little of the other side of the story—the defeats, the short-lived triumphs that now strongly support the alarming view that the insect enemy has been made actually stronger by our efforts. Even worse, we may have destroyed our very means of fighting.
Yes, those last two sentences refer to exactly what happened: Mosquitos developing pesticide resistance in the face of too-zealous use of DDT and other pesticides, which she later described explicitly:
The whole process of spraying seems caught up in an endless spiral. Since DDT was released for civilian use, a process of escalation has been going on in which ever more toxic materials must be found. This has happened because insects, in a triumphant vindication of Darwin’s principle of the survival of the fittest, have evolved super races immune to the particular insecticide used, hence a deadlier one has always to be developed and then a deadlier one than that. It has happened also because, for reasons to be described later, destructive insects often undergo a “flareback,” or resurgence, after spraying, in numbers greater than before. Thus the chemical war is never won, and all life is caught in its violent crossfire.
It’s easy for us in 2016 to forget just how heavily pesticides were used in the 1940s-1960s and therefore conclude that Rachel Carson was unduly alarmist. However, back in the day, governments engaged in aerial insecticide spraying campaigns over farms, forests, cities and suburbs. Farmers routinely applied high concentrations of insecticides, and the residential use of these chemicals was common in everything from shelf paper to aerosol “bombs” designed to clear out a house of insects. Aerial residential and forest sprayings were often carried out whether the residents affected wanted them or not. You can get an idea from this late 1950s film called Goodbye, Mr. Roach:
It’s not for nothing that the 1950s were called the golden age of pesticides. In other words, pesticide use in Rachel Carson’s time was incredibly indiscriminate by today’s standards.
Influence and backlash
Silent Spring was an immediate sensation beginning before the book itself was published. That’s because parts of Silent Spring were serialized in The New Yorker in June 1962 and then later published as a book that September. Advance sales totaled 40,000, and October Book-of-the-Month sales hit 150,000. By the time Carson died of metastatic breast cancer in April 1964, a million copies had been sold. The book was also hugely influential, particularly after Carson appeared on a CBS Reports television special, The Silent Spring of Rachel Carson, which was broadcast on April 3, 1963. An hour long special, it featured investigations by Eric Severeid interspersed with an interview with Carson, now visibly ill from her breast cancer, and interviews with experts, particularly scientist Robert White-Stephens, a spokesman for American Cyanamid, wearing black-rimmed glasses and sitting among beakers in a lab, trying to refute Carson. The response was overwhelmingly positive, as White-Stephens came off as loud and wild-eyed, while Carson appeared calm—anything but the hysterical woman her opponents had portrayed her as.
Not surprisingly, the attacks began very early, and, though predictably a lot of it did come from the chemical industry and scientists sympathetic or linked to it, there was also a very Cold War flavor in the criticism. Carson’s criticisms were often portrayed as, in essence, an attack on American values. She was not infrequently referred to as a Communist and attacked in misogynistic terms, for example in a letter to The New Yorker after serialization of Silent Spring:
Miss Rachel Carson’s reference to the selfishness of insecticide manufacturers probably reflects her Communist sympathies, like a lot of our writers these days. We can live without birds and animals, but, as the current market slump shows, we cannot live without business. As for insects, isn’t it just like a woman to be scared to death of a few little bugs! As long as we have the H-bomb everything will be O.K.
“We can live without birds and animals”? WTF? Believe it or not, this is how a lot of Americans thought over 50 years ago.
This accusation being a Communist was echoed by former Secretary of Agriculture Ezra Taft Benson in a letter to former President Dwight Eisenhower in which he said that Carson was “probably a Communist.” Elsewhere she was accused of being radical, unscientific, hysterical, and disloyal. Much was made of calling her a “spinster” because she was unmarried. David Hecht noted that initially criticism of Carson involved mainly scientists concerned with defending their vision of science from what they perceived as the threat embodied in Carson’s ecological perspective and that the “legion of critics” attacking her defended DDT by making it emblematic of technological progress, suggesting that her condemnation of it derived from a philosophy antithetical to the modern world.
Despite all this, it’s not for nothing that Carson’s book was credited with sparking the modern environmental movement. Silent Spring became part of popular culture. By the early 1970s, public opinion had shifted. This wasn’t entirely due to Carson’s work, of course, but there’s no denying that her book was very influential. Ultimately, the EPA held seven months of hearings in 1971-1972 where the evidence for and against DDT was presented, and in the summer of 1972, William Ruckelshaus, the EPA’s first Administrator, announced a ban on DDT use in the US for most uses, although he did not ban its manufacture or its sale overseas and there was an exception for public health; e.g., malaria control. Indeed, the US continued to manufacture and export DDT until the mid-1980s. In the following years, agricultural use was banned in most developed countries. Most recently, the Stockholm Convention on Persistent Organic Pollutants took effect in 2004 and restricted DDT use to vector control. Specifically, the convention exempts public health use within WHO guidelines from the ban. Today, approximately 3,000 to 4,000 tons of DDT are used each year for vector control, usually application to the inside walls of homes to kill or repel mosquitos, a technique known as indoor residual spraying.
Rachel Carson revisionism to rescue DDT for fighting the Zika virus
Now that the World Health Organization has declared the Zika virus to be a “global public health emergency,” calls to bring back DDT by lifting the ban are, predictably, coming fast and furious. A few are somewhat reasonably nuanced. Most are nowhere near nuanced. For instance:
The Zika virus outbreak is the latest batch of blood on the dirty hands of Rachel Carson and the entire environmental movement which puts mosquitoes ahead of people. There was a time when we beat insect-born [sic] diseases through simple and easy methods that worked.
That method? DDT of course. Of course, if anyone thinks that controlling mosquitos was ever “simple and easy,” even in the era of maximal DDT spraying, well, I have a bridge I think that person just might be interested in buying. Here’s another:
Rachel Carson died in 1964, but the legacy of Silent Spring and its recommended ban on DDT live with us today. Millions are suffering from malaria and countless others are contracting the Zika virus as a result of the DDT ban. They were never given the choice of living with DDT or dying without it. The World Health Organization should recognize that DDT has benefits, and encourage its use in combating today’s diseases.
If you’ve read Merchants of Doubt by Naomi Oreskes and Erik Conway, you’d be aware that there has been a renewed concerted effort to discredit Rachel Carson, which is what I mean when I refer to “Carson revisionism.” Over the last 20 years or so, the story of Rachel Carson has been subjected to historical revisionism by the very same people who tried to deny the science demonstrating the harmful health effects of tobacco and who more recently have spearheaded anthropogenic climate change denialism.
This revisionism involves a narrative in which, because of the anti-pesticide and anti-DDT fervor ignited by her book that led to the banning of DDT in the US, and, if you believe the narrative, in the rest of the world, millions died of malaria whom DDT could have saved. Key components of the narrative are predictable: DDT is the only hope of controlling Zika virus. There wouldn’t be so many people dying of malaria if only those damned Rachel Carson environmentalists would let governments unleash DDT. After the DDT ban, malaria came roaring back, and now Zika virus is uncontrollable. DDT is harmless (or its adverse effects were grossly exaggerated.) Environmentalists care more about birds than people, and not lifting the ban will lead to a Zika virus pandemic. Oh, and liberals and environmentalists want to keep DDT banned as a population control measure because it was too effective.
Indeed, in a speech railing against climate science, dismissing secondhand tobacco smoke as harmless, and characterizing environmentalism as a “religion,” late author Michael Crichton, whom I’ve discussed before, characterized the DDT ban as having “caused the deaths of tens of millions of poor people, mostly children, whose deaths are directly attributable to a callous, technologically advanced western society that promoted the new cause of environmentalism by pushing a fantasy about a pesticide, and thus irrevocably harmed the third world.” In his novel State of Fear, Crichton even had one of his characters (a sympathetic one) say, “Banning DDT killed more people than Hitler.” He even put the death toll at 50 million. Sure, it was just dialog in a novel, but it’s clear from interviews (and the speech quoted above) that Crichton believed it. Tellingly, the plot of State of Fear involved eco-terrorists plotting mass murder to publicize the dangers of global warming. No wonder Chris Mooney described the novel as “pure porn for global warming deniers.”
There’s only one problem with the revisionist narrative promoted by Crichton before his death and still being promoted by conservative free market-friendly think tanks like the Heartland Institute and the Competitive Enterprise Institute. Although Rachel Carson was certainly not correct about everything (for instance, the evidence linking DDT and cancer turned out to be fairly weak, even over 50 years later, although not, as her critics claim, nonexistent), nonetheless this revisionist narrative is demonstrably a false one promoted by the likes of not just Crichton but Steve Milloy of Junk Science fame, who never met an anti-environmentalism myth he didn’t like, DDT included. The “Rachel Carson killed more people than Hitler” myth is basically a zombie myth that won’t die.
For example, one claim frequently made in defense of DDT is that in Sri Lanka, DDT dramatically decreased the incidence of malaria, such that when the spraying stopped only a handful of people suffered from the disease, but that from 1968 to 1970, malaria came roaring back to infect 1.5 million because Sri Lanka could no longer use DDT. In Merchants of Doubt, Oreskes and Conway point out that this doesn’t tell the whole story, namely that Sri Lankans did use DDT when malaria came roaring back in 1968 but it couldn’t control the mosquitos because resistance had developed; in fact they used ever greater amounts of it, to no avail. They had to switch to malathion, a pesticide to which their mosquitos hadn’t yet developed resistance.
In reality, DDT use peaked before Silent Spring and DDT use was decreasing because mosquitos were developing resistance, just as Rachel Carson had been concerned about. Sadly, it’s a myth that skeptics who should know better not infrequently fall for, sometimes people I never would have guessed as being susceptible to such messages.
Is there a role for DDT in controlling Zika virus?
Rachel Carson revisionism aside, it must be conceded that it is not entirely unreasonable to examine all the tools in our armamentarium and ask if there might be a role for DDT in controlling Aedes aegypti. However, the answer to such a question must be grounded in a scientifically-supportable and balanced understanding of the risks versus the benefits. My main reason for discussing Rachel Carson revisionism is because it represents a certain kind of antiscience that, because of ideology, vastly exaggerates the benefits of DDT and other pesticides while all but denying any potential harm. In the 1960s, the ideology behind attacks on Carson linked DDT with “progress” and the American way, while painting her as an enemy of modernity and science itself, a hysterical woman (who was probably a Communist) who would set back American agriculture to the inferior level of the Soviet Union at the time. Since the late 1990s, that ideology has been a certain free market belief system that chafes at any regulation of business, particularly regulation rooted in environmental concerns. Some of those of this ideological bent have latched on to DDT as an example of overweening government regulation spurred by a environmentalists who in their view don’t care how many people die of malaria, as long as the birds are saved (unless, of course, they are killed by wind turbines, which, in this view, is acceptable to these parodies of environmentalists).
Obviously, this view rests on a Panglossian view of DDT in which it is virtually perfectly effective and safe, even though nothing really is. Perhaps the most balanced discussion of the risks and benefits of DDT for mosquito control was published ten years ago in The Lancet by Walter Rogin and Aimin Chen, who concluded that there was sufficient evidence of health problems to be concerned. In particular, they noted that exposure to DDT at amounts that would be needed for effective malaria control might cause preterm birth and early weaning, abrogating the benefit of reducing infant mortality from malaria, while at the same time noting that the evidence for a link between DDT and human cancer was tenuous at best. Rogin and Chen then note that DDT can be very effective in eliminating mosquitos, but that there are a number of problems that are not so easily overcome that demonstrate the problems of using DDT just for malaria control:
However, the effectiveness of DDT can be compromised by insecticide resistance and social resistance to DDT indoor spray. Because of the irritating, excito-repellent nature of the DDT residue, some mosquitoes tend to leave before they have absorbed a lethal dose, or tend to avoid entering the house or resting on the wall at all. By the end of Global Malaria Eradication Campaign, some mosquito species had developed resistance to DDT, especially in India and Sri Lanka. In 1968, high amounts of resistance to DDT in Anopheles gambiae was reported in Upper Volta (now Burkina Faso); shortly thereafter, DDT had no effect on mosquito mortality, biting frequency, or resting in houses in trials undertaken in Togo and Senegal. In the 1980s when DDT was judged to control the resurgence of malaria in Zanzibar after the DDT spraying programme finished in 1968, resistance was found in A gambiae ss and A arabiensis. In 2002, 2 years after DDT residual spraying was reintroduced in KwaZulu-Natal to control the increase of malaria cases, resistance was recorded in A arabiensis, although A funestus was still susceptible to DDT. Social resistance to DDT indoor sprays occurs because bedbugs are resistant to DDT, and DDT leaves stains on walls, which residents then replaster. In practice, the efficacy of DDT spraying for vector control depends on the coverage of spraying, mosquito species, and resistance to DDT. Climate—especially rainfall, temperature, and latitude—could affect the stability of transmission, and thus also affect DDT efficacy. WHO points out that DDT spraying is “most effective in reducing the overall malaria burden in unstable transmission areas, areas with marked seasonal transmission peaks and disease outbreaks, and highland areas”.
In other words, DDT is no panacea, and its effectiveness very much depends on local conditions and the strategy used, not to mention the specific insects being targeted.
Indeed, there is reason to believe that letting loose the white powder would likely be far less effective than the pro-DDT apologists would lead you to believe because Aedes aegypti is particularly difficult to control. DDT is effective without mass indiscriminate spraying against many strains of the Anopheles mosquitos that spread malaria in part because of this:
Goldman said the pesticide is effective against the Anopheles mosquito, a night-biter that spreads malaria indoors while people are sleeping.
In many developing countries, DDT has proven effective when sprayed on the indoor walls of buildings.
“Basically the anopheles likes to rest on a wall surface between feedings and thus is poisoned by the DDT that is on the walls,” Goldman said.
“Also there are insecticide impregnated bed nets — usually with pyrethroids, and, infrequently, with DDT — to guard against malaria transmission while sleeping. Bed nets also are very effective.”
But the mosquito that transmits Zika is not the anopheles but another genus known as the Aedes, which also transmits dengue and chikungunya viruses, according to Goldman.
Aedes mosquitos bite outdoors, during the day, she said. Spraying walls with DDT won’t help.
Aedes mosquitos, in contrast to the Anopheles mosquitos, thrive in urban environments and tend to breed in close proximity to human populations, in particular in trash or discarded tires. They require a very tiny amount of water to breed, and their eggs can stay viable for a long time without water, ready to develop again once rains return. These characteristics make it relatively impervious to outdoor spraying, whatever the insecticide. Even though the mosquitos only survive two weeks and don’t fly very far, Dr. Thomas Frieden, director of the CDC, has likened them to the “cockroach of mosquitoes,” who “lives indoors around people and hides in dark places.” All of this means that spraying will be much less effective than it would be against Aedes. It could even backfire. How?
But according to Joe Conlon, a technical adviser to the American Mosquito Control Association and a former entomologist with the U.S. Navy, using DDT to control Zika is a terrible idea. “DDT seems like a silver bullet, but it isn’t.” First of all, the mosquitoes might be resistant to DDT. Conlon says the Latin American countries where Zika is blooming now used DDT heavily in the 1960s to kill off the Aedes aegypti, which also carries diseases like dengue and yellow fever. It worked, but the mosquitoes in the region developed robust resistance to the pesticide, which may still be lingering in the population. DDT resistance lasts a long time, he says, because the chemical persists in the environment so long. If you spray a wall with DDT today—a method commonly used because mosquitoes are known to rest on walls a moment after having a blood meal—it could still be coated by DDT in 20 years. The mosquito population continues to be bombarded by the chemical, so the resistance shows up in every subsequent mosquito generation.
And even if the mosquitoes aren’t already resistant, they will be. It only takes a “few generations” of mosquito to develop resistance, and when an Aedes mosquito’s life span is about 10 days, that’s not long at all. Conlon speculates mosquitoes could develop resistance within a year. “What’s even worse, resistance to DDT can stir cross-resistance to the other pesticides we use, like the pesticide we use to treat bed nets, to fight malaria.”
And that’s what bothers me so much about how ideologues have dragged DDT into the Zika virus debate. It distorts the debate that should be occurring, which is not a question of choosing environment or birds over humans or vice versa. That is a false dichotomy and the wrong question to be asking. Rather, deciding how to combat the Zika virus involves balancing one set of risks and costs, human, environmental, and financial, associated with different methods of mosquito control versus the risks and costs in human terms of a virus that hasn’t (yet) been conclusively demonstrated to be causing microcephaly. Add to that the consequences of bringing back DDT, which could reinvigorate resistance among mosquitos carrying malaria, and there are some real potentially bad consequences.
This is a situation where there is a lot of uncertainty involved, and all the available options are imperfect. Bringing back DDT could well be the most imperfect option of all.