Thursday, March 31, 2016

A Is For Aedes aegypti: Whining Killers vs Silent Spring #AtoZChallenge

Spraying DDT, 1958
General Background

Brazil, currently a hotspot for Zika, reported an almost 50-percent increase in cases of dengue fever reported over a three-week period in January 2016. Dengue is transmitted by the Aedes aegypti mosquito, which also transmits Zika, chikungunya and yellow fever.

Aedes aegypti is now found in most tropical and sub-tropical regions of the world. It has distinctive white markings on legs and thorax. A. aegypti are particularly well-suited to living alongside humans as they prefer to stay (and bite) in shady/indoor areas, and to breed and lay their eggs in stagnant water. They are particularly drawn to water containing the chemical signature of bacteria involved in the decay of leaves and other organics in water. So outdoor showers and lavatories, trash cans, flower pots, wash tubs - all can provide a breeding ground for the mosquitoes, even if they hold as little as a tablespoon of stagnant water. The eggs themselves can remain viable even after spending up to a year in a dry state (during drought, for example) and thus hatch a new swarm after a dry spell or
cold season.

Chikungunya, dengue (sometimes referred to as "break bone disease" because of its associated pain) and now Zika have been rising in incidence in the Americas. Zika has proven particularly worrisome because of evidential links to births of babies with microcephaly.

A. aegypti used to be referred to as the yellow fever mosquito. The United States suffered its last major outbreak of yellow fever in 1905, in New Orleans. Yellow fever was never endemic to the US - it arrived via port traffic and trade, which is why most epidemics occurred in port cities like New Orleans and Philadelphia. Once here, the disease was spread via the native mosquito populations.

Malaria was once a problem in the US as well. Ever wonder how those two scourges were eliminated in the US?

Observation, sanitation, fumigation

The link between yellow fever and mosquitoes was known and pretty much proven by 1900. During the last yellow fever epidemic in the US, Walter Wyman, the surgeon general of the U.S. Public Health and Marine Hospital Service, was appointed by Roosevelt to institute a public health campaign. Ships were quarantined and fumigated. Workers fumigated the city, screened the cisterns and water tanks, and destroyed breeding grounds for mosquitoes. Fines were given to residents who failed to comply with public health measures.

The same methods, with the addition of improved drainage, plumbing, public reservoirs and the use of window screens in housing helped to reduce malaria as well. The introduction of chloroquine and especially DDT after WW2 completed the process of eliminating malaria in the US and much of Europe.


DDT was synthesized in 1874 and its properties as an effective insecticide established in 1939. It was used extensively during World War Two to combat typhus, malaria and dengue. In 1945 it began its use as an agricultural pesticide.  DDT played a large role in the elimination of malaria from Europe and North America. However, there were mounting concerns over its effects on health and the environment. Rachel Carson explored these concerns in her famous book Silent Spring, published in 1962. The book argued that pesticides, particularly DDT, were poisoning wildlife and the environment and were endangering human health. Silent Spring became a best seller, and launched the environmental movement in the United States.  DDT was banned in the US in 1972, with some exemptions for public health uses. It was also banned in many other countries.

"The Stockholm Convention on Persistent Organic Pollutants, which took effect in 2004, outlawed several persistent organic pollutants, and restricted DDT use to vector control. The Convention has been ratified by more than 170 countries and is endorsed by most environmental groups. Recognizing that total elimination in many malaria-prone countries is currently unfeasible because there are few affordable or effective alternatives, the convention exempts public health use within World Health Organization (WHO) guidelines from the ban." *

 "It's hard to overestimate the impact that DDT's early success had on the world of public health," writes Malcolm Gladwell for THE NEW YORKER. 
DDT saved millions of lives during its decades of use in countries all over the world.

Other Vector Control Methods

Aedes aegypti can be genetically modified with a self-limiting gene that prevents the offspring from surviving. Lab produced male mosquitoes, which do not bite or spread disease, are released into the environment to mate with the blood-feeding females. Their offspring then inherit the self-limiting gene and die before they can reproduce or spread disease as adults.  This method, however, opens a can of worms as we can not always predict the future effects of gene modification on a species.

Radiation can also used to sterilize adult males, which reduces the number of viable offspring in the mosquito population. An expensive and somewhat laborious way to combat a prolific breeder.

Proper sanitation, elimination of  mosquito breeding areas and the use bed nets and window screens are effective tools but prove expensive and problematic in large, poverty-stricken urban environments, especially since the mosquitoes can breed in as little as a tablespoon of water.

Dangers of DDT vs human toll of new epidemics

“That concern about DDT has to be reconsidered in the public health context,” said Dr. Lyle R. Petersen, director of the division of vector-borne diseases at the Centers for Disease Control and Prevention. He said the damage to fish and wildlife stemmed from widespread outdoor use of DDT in agriculture, not the use of small amounts on walls inside homes to kill mosquitoes. **

While the World Health Organization has declared Zika a global emergency, most of the recommendations I've read in the media amount to things like asking that women in the affected regions forgo pregnancy in the next year, that pregnant women from other countries avoid travel to affected areas, and that women of childbearing age use repellents (like DEET), wear protective clothing and use contraception. Some of these recommendations are problematic and disruptive to daily life. Certain faiths discourage or outright prohibit the use of birth control. Tropical heat is not conducive to wearing clothing which covers the entire body. Repellents may be unavailable for purchase or prohibitively expensive for those already struggling to afford food.

Children born with microcephaly often face a shortened lifespan along with a multitude of physical and mental impairments. Families of affected children will bear the burden of struggling to secure proper medical care along with the cost burden of providing that care. The socio-economic fallout from Zika may be felt for many years, if not decades. Add to that the additional health problems resulting from an explosion of dengue and (possibly) chikungunya, and we have the makings of a medical catastrophe.

Half a century ago, DDT was applied copiously and sometimes indiscriminately. We now know that it persists in the environment for years and makes its way into the food chain and the bodies of organisms. Since then we have also come a long way in our understanding of toxins, as well as safer means of application.  We know that continued exposure to a particular pesticide often causes insects to develop immunity; but since DDT has not been used in many countries for 20+ years, the current mosquito population probably has no resistance. And DDT has always been recognized as an extremely effective insecticide.

In 2006, The World Health Organization (WHO) began recommending the use of indoor residual spraying (IRS) of DDT in malaria epidemic areas and in regions with constant and high transmission rates.

From the WHO news release: “The scientific and programmatic evidence clearly supports this reassessment,” said Dr Anarfi Asamoa-Baah, WHO Assistant Director-General for HIV/AIDS, TB and Malaria. "Indoor residual spraying is useful to quickly reduce the number of infections caused by malaria-carrying mosquitoes. IRS has proven to be just as cost effective as other malaria prevention measures, and DDT presents no health risk when used properly.”

“Indoor spraying is like providing a huge mosquito net over an entire household for around-the-clock protection,” said U.S. Senator Tom Coburn, a leading advocate for global malaria control efforts. “Finally, with WHO’s unambiguous leadership on the issue, we can put to rest the junk science and myths that have provided aid and comfort to the real enemy – mosquitoes – which threaten the lives of more than 300 million children each year.”  Read the entire 2006 WHO news release here.

     Update April 4, 2016:  The World Health Organization (WHO) announced that there is “a strong scientific consensus” that the Zika virus is a cause of both the birth defect microcephaly and the neurological disorder Guillain-Barré syndrome

**Pollack, A., NYT (January 30, 2016). "New Weapon to Fight Zika: The Mosquito". Retrieved February 13, 2016

Additional sources:
Szabo, L, USA Today (March 4, 2016) "Study Provides 'Strongest Evidence Yet' Linking Zika, Birth Defects". Retrieved  March 4, 2016

Roland Mortimer. "Aedes aegypti and dengue fever". Ltd, Microscopy-UK

Moyers B (September 21, 2007). "Rachel Carson and DDT". Retrieved February 13, 2016

Phillips D , Washington Post (February 12, 2016). "Brazil Reports Explosion Of Dengue, A Bad Omen For Spread Of Zika Virus" Retrieved February 13, 2016


  1. I remember DDT. Scary something like that can be so good and yet so bad.

    1. I agree. The best solution would be an all-out effort to clean up, provide mosquito netting and screens to home and other "mechanical" solutions rather than chemicals. But IF Zika is causing all of these birth defects and IF it continues to spread like wildfire, there's going to be a huge medical crisis. So, maybe judicious use of DDT would be warranted. I just wanted to put it out there and see what others think.

  2. We had what we referred to as "tuxedo" mosquitoes in Maryland, and they were aggressive suckers. They'd bite you multiple times, chase you inside, and generally make life hell. No amount of repellent kept them off of you, and they bite through clothing.
    I'd rather not go DDT, but something will have to be done, and I think that with rising global temps, this is just the beginning of our insect problems. Great piece for your A post.

    1. Oh yes, Cape Henlopen Delaware has those too. Went camping one time and I must have had 50 welts on my legs. Had to take Benadryl and coat my legs in Calomine lotion.
      And yes, tropical insects and diseases seem to be shifting north at an accelerated rate. We're going to have start addressing it.

  3. I found this very interesting as I live in Malaysia. It's the tropics and I've known about malaria and dengue since I was a child. My grandmothers used to burn incense and camphor at dusk - while the were doing this for religious reasons, I do remember the elders saying that it was to ward off mosquitoes as well.

    Aneeta from
    How to Tell a Great Story

    1. We used (and still use) citronella candles. I'll have to look into camphor or other natural oils and repellents.

  4. Very informative, but now I don't want to travel anywhere and I've never been to the Caribbean :(

    [Susan] from
    Color Me Writing

    1. I've been to the Caribbean many times, and never had a problem. I always wore lightweight dresses at night and a little repellent. (Avon makes something called Skin So Soft skin oil which is lightweight and whatever they put in it really repels mosquitoes and gnats.) The places I've been bitten nearly to death have been areas of Florida, South Carolina and Delaware!

    2. Good to know, especially as I've been contemplating a trip to Florida or Myrtle Beach for later this year! I've heard that Skin So Soft is a great mosquito repellent, but had forgotten that. Thanks for the reminder!

  5. I'm amazed at just what I learn on the A-Z Blogging Challenge. And am very grateful that the bugs we have in the UK are pretty innocuous compared to some parts of the world.

    1. My husband was from the UK and then moved here to the US. He was horrified his first summer - bugs, flies, gnats, mosquitoes everywhere! And I was amazed when he said his parents' home had no window screens, and that they could sit and eat outside on nice evenings without being bothered much by insects.

  6. An interesting and informative read. Thanks for visiting me.

  7. Thanks for the info. Chikungunya raced though the island last year and caused untold suffering. I'm so glad I didn't get it and based on what the doctors are saying, the side-effects last up to 3 years. And now there's Zika. We have 6 confirmed cases in Jamaica. :/

    1. It's really worrisome, especially now that it seems to cause Guillian-Barre and can be sexually transmitted as well. The bad news just keeps coming. I've stocked up on repellent and I actually have a hat with mosquito netting!

    2. DDT didn't save you from Chikungunya?

      Same mosquito carries Zika. Should we listen to experience?

  8. Omg! That was terrifying. Mosquitos and viruses and ddt. My skin is crawling at the thought.

    1. Between all of that plus the spread of Lyme via ticks...I really don't want to even go outside anymore.

  9. Thanks for your very detailed and information rich post.

  10. #DDT effectiveness declined steadlily, and dramatically after 1963 (that Malcolm Gladwell piece you cite, for example, tells the story of how DDT abuse in Africa killed the WHO's malaria eradication program in 1963, when it turned out mosquitoes in places WHO had never gone were already resistant and immune to DDT before WHO got there). Today, every mosquito on Earth carries at least a few of the alleles that make it digest DDT like a water soluble vitamin. DDT is well-known to be ineffective against Anopheles mosquitoes, which carry malaria. In fact the world's last maker of DDT, India, announced it will shut the plant down because malaria runs rampant despite more and more DDT applied.

    You note the Zika carrier, Aedes aegypti, also carries yellow fever (and chikungunya, and a few others). Consequently, it has been the target of DDT campaigns around the world, now suspended because almost all populations of Aedes are highly resistant to DDT. So there is little doubt that DDT could not be very effective against Zika carriers.

    But worse, Zika carriers bite outdoors, in daylight. DDT worked well against Anopheles because they bite dusk to midnight, indoors where people sleep. Consequently, indoor applications (Indoor Residual Spraying, or IRS) worked well, and still worked up through 2005, with limited damage out of doors.

    DDT is exactly the wrong stuff to use out of doors. As the courts and EPA found with extensive testimony, DDT is an uncontrollable poison outdoors, killing more non-target and beneficial species than the pests, and remaining as a killer for a long time (DDT still kills robins and other songbirds in Michigan, and California condors in California, 44 years after DDT use stopped).

    So, no, we shouldn't use DDT. It would let Zika run wild, but harm humans and Zika-killing organisms.

    You quote a clause noting that the US ban on DDT excepted use to fight mosquitoes, disease vectors. The Stockholm Convention has a similar carve out. DDT has NEVER been banned from fighting malaria or Zika. Use declines because DDT doesn't work. I don't know why people off the streets with no experience in tropical diseases, entomology or public health think that DDT can be a savior, when the public health people who fight diseases every day have already passed it over as an appropriate or hopeful solution. Maybe the experts really do know what they're doing when they don't ask for more DDT.

  11. P.S.: WHO's 2005 press release did not announce any change in policy. WHO never stopped using DDT. But in Uganda, they faced opposition from passive-aggressive tobacco growers, who claimed DDT used in IRS would contaminate their crops and prevent sales to Europe -- all specious claims.

    WHO hoped to tamp down opposition with the press release. Tobacco businesses sued to stop DDT use in Uganda, and lost. A DDT campaign started in 2006.

    But results of DDT were disappointing. Malaria was stopped by other insecticides (WHO approves a dozen for such use), and by bednets, but not by DDT in IRS. So Uganda abandoned the DDT-based campaign in 2008.

    Without DDT, bednets have been the foundation to cut malaria infections and deaths. Deaths are down 45% since 2000. Mostly without DDT, after the US banned DDT from farm use in 1972, malaria deaths have been cut by 90% and total infections by 50%.

    We should be asking how those successful, non-DDT campaigns achieved such good results.

  12. Thank you for taking the time to share this information.