DDT and Alzheimer's disease risk
Dichlordiphenyltrichloroethane, also known as DDT, emerged during World War II as something of a miracle chemical. The war had left cities across Europe devastated and struggling to cope with (among other things) poor sanitation, which created a fertile environment for the spread of disease. When Allied forces entered Naples soon after the Germans retreated, they discovered a typhus epidemic that was killing 25% of those infected; the number of infected was into the thousands. The Germans, before leaving, had laid waste to Naples’ water and sewer system, with the hopes of bringing about some sort of humanitarian crisis as punishment for Naples’ uprising against occupying German forces. They had succeeded.
The typhus epidemic in Naples was spread by lice, and so Allied forces needed a way to rid the city of the disease-carrying parasites. If you’ve ever had the misfortune of having to deal with a lice infestation of your own, you can imagine the difficulty of trying to accomplish this on a city-wide scale. Fortunately, the Marines had recently contracted DuPont Chemical to produce tons of DDT for them to reduce malaria rates among soldiers (by spraying to kill mosquitos). They sprayed Neapolitan citizens with DDT in an assembly-line fashion, and the epidemic was over within a month.
DDT was used in the late 1940s and 1950s to reduce malaria rates in many areas of the world. In 1948 the discoverer of DDT’s insecticide properties, Paul H. Muller, won a Nobel Prize in Medicine. However, as DDT use increased over these decades, there was a growing murmur about negative health effects that might be associated with DDT. This murmur grew to a roar in 1962 with the publication of scientist and author Rachel Carson’s book Silent Spring, which implicated pesticides like DDT in endangering the environment, wildlife, and humans. Carson’s book sparked the environmentalist movement in the United States, which precipitated a ban on DDT use in the U.S. in 1972, except in cases where it was necessary for public health (e.g. curbing epidemics). Most countries in the world have now signed on to a similar ban of DDT use.
You would expect that, because the ban was put into effect in the early 1970s, any health effects associated with DDT in the U.S. would have disappeared by now. But DDT is highly resistant to degradation, as are the products it primarily breaks down into: DDE and DDD. The half-life of DDT in soil can be up to 15 years, and in water it may extend up to 150 years. In the human body, DDT’s half-life is around 6 years; DDE's is 10 years. Because DDT and its breakdown products remain in the environment for a long period of time, and because they persist in the human body for years after exposure, most of us have some level of these chemicals in our blood today. The unanswered question, however, is: are those levels affecting our health?
In an attempt to answer that question, Richardson et al. published a study this week in The Journal of the American Medical Association that compared levels of DDE in patients with Alzheimer’s disease (AD) to levels in control patients who didn’t have the disease. The authors found levels of DDE in AD patients that were 3.8 times higher than the levels found in their control group. They determined that the participants in the top ⅓ of their sample in terms of high DDE levels had more than 4 times the risk of developing AD than those with lower blood DDE levels. Participants in the ⅓ with the highest DDE blood levels also had the lowest scores in a questionnaire that assesses cognitive impairment. The researchers also looked at the participants who possessed a form of a gene called Apolipoprotein E4 (ApoE 4), which is known to increase AD risk. They found that those who had the ApoE 4 variant (as opposed to ApoE 3, which doesn’t suggest AD risk) had the lowest cognitive scores in the sample, suggesting that this population might be most susceptible to any effect DDT might have on increasing AD risk.
This study by Richardson et al. is what is known as a case-control study, meaning that DDE levels were compared between participants known to have the disease and those without it. A case-control study, however, does not have the ability to determine a causal link between an exposure and a disease due to a number of inherent limitations to the design (such as the fact that there could have been a variety of other influences on the participants’ development of AD). However, the Richardson et al. study provides a basis for further investigation into a link between DDT and AD.
The banning of DDT and research suggesting it has links to human disease is not completely uncontroversial. Some believe that DDT was banned too quickly, and that the risks (which still have not been fully elucidated) are unknown and thus don’t outweigh the potential benefit DDT could have in preventing the spread of diseases like malaria. The banning of DDT, however, made a previously naive populace more vigilant about the chemicals it is exposed to on a daily basis. Although we are frequently reassured that we are unlikely to experience any ill effects from the more than 80,000 chemicals that are used in the United States, the majority of them have not been rigorously tested for their impact on human health. Thus, the DDT controversy has helped to raise people’s awareness to the fact that their best interest is not always looked after unless they see to it themselves. Regardless of DDT’s potential benefits in treating malaria, it was foisted on a population before its potential negative health effects were understood. This recklessness significantly contributed to the force behind the movement that would eventually lead to DDT being banned.
However, due to lax regulation of the chemical industry in the United States, it is inevitable that we will have a number of other chemicals to add to the notorious history that DDT is a part of. The law (Toxic Substances Control Act, or TSCA) currently used to regulate the introduction of new chemicals into processes that are likely to affect the general population is designed to make it very difficult for those chemicals to be banned from use. Furthermore, the chemicals aren’t required to undergo strict safety testing before they can be used, whether that use be in the manufacture of garbage bags or children’s toys. There are, however, groups working hard to reform TSCA. If you’re interested in finding out more about those reform efforts, here are some links through which you can do that:
Richardson JR, Roy A, Shalat SL, von Stein RT, Hossain MM, Buckley B, Gearing M, Levey AI, & German DC (2014). Elevated Serum Pesticide Levels and Risk for Alzheimer Disease. JAMA neurology. PMID:24473795