Introduction
Malaria is major life threatening disease, transmitted through mosquitoes and caused by parasites. Plasmodium virus causes malaria in humans. A recent study and WHO Statistics revealed that around 625,000 individuals died because of Malaria in the year 2012. Malaria has affected more than 200 million people in over 75 countries in the past 50 years. The sub-Saharan Africa covering to about 50% of malarial cases alone. DDT (dichlorodiphenyltrichloroethane) was introduced in the year 1874. The insecticidal properties of DDT was discovered in the year 1935 and has been used worldwide ever since. Many early studies had revealed the potential threats of DDT in animals and humans. DDT is a known carcinogen. However metabolites of DDT (mainly DDE) have found to be most prevalent in humans. Exposure of DDT to women cause serious health problems including Breast cancer while in men exposed to DDT can cause infertility and also birth defects. In 2006 WHO also considered DDT to be continued as it can help in reduction of malaria cases and it also outweighs benefits to risk. Not only consideration for indoor use, but DDT is also considered as one of the 12 pesticides for indoor use recommended by WHO.DDT is considered to be a potent human carcinogen but however its use in a systematic manner, has many benefits and outweighs the risk of toxicity.
Significance of Malaria
Malaria is major life threatening disease, transmitted through mosquitoes and caused by parasites. Plasmodium virus causes malaria in humans. There are 4 types of plasmodium species causing malaria in humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium malaria and Plasmodium ovale. A recent study and WHO Statistics revealed that around 625,000 individuals died because of Malaria in the year 2012. Majority of the deaths have to been found in African children, the estimate number ranges from 450,000 to 780 000. The disease can be prevented and cured. Over the years increased amount of prevention tactics have decreased the prevalence of malaria. Latest study according to WHO, in December 2013 that more than 200 million cases of malaria was observed last year. Among the major types, most common infecting malarial parasites are Plasmodium falciparum and Plasmodium vivax. Plasmodium falciparum is the deadliest of the 4 types. In certain forested and rural areas of south-east Asia, Plasmodium knowlesi is said to have caused malaria in humans. Althoug this species is widely found infecting monkeys and other primates. (Cohen JM et al; 2010)
Transmission Factors
Anopheles mosquitoes are the vectors in the transmission of malaria. There are around 20 species of Anopheles species which carry the plasmodium parasite and infect humans. There are a couple of factors in malarial transmission, vector type, human host and environment being the major considerations. Anopheles species prefer shallow water for breeding. Although rice fields and streams are most common. A study by premier scientist have revealed that the burden of malaria increases in certain places where Anopheles species life span is longer. This might be one of the reasons for high rate of malaria in African countries. (WHO Statistics, December 2013)
Malaria has affected more than 200 million people in over 75 countries in the past 50 years. The sub-Saharan Africa covering to about 50% of malarial cases alone. Asia, Latin America and European Union have also found be affected with malaria. However of all cases reported so far, the mortality rate of malaria is high in certain groups of individuals. Young children and non-immune pregnant women are likely to be infected with malaria. HIV infected pregnant women and no-immune international travelers have a higher chance to acquire the disease. In non-immune pregnant women, complications can arise and may also lead to fetal mortality. Individuals with HIV/AIDS are highest in mortality cases since their immune system is diminished and cannot sustain the attack of the plasmodium parasite to the body. (Sinka ME et al; 2010)
Treatment and Prevention
WHO recommended drugs chloroquine and sulfadoxine-pyrimethamine (SP) were initially used for the treatment of malaria. However the resistance of Plasmodium species to these drugs have major concerns over malarial medication. Resistance by these species might be blamed on to patients who withdraw from treatment prior to completion. Most malarial medications are a combination of drugs and if a particular patient withdraws, there are some parasites within the body who then acquire resistance after a period of time due to lack of drug treatment. This not only causes reoccurrence of malaria but also brings down the health economy. Global Plan for Artemisinin Resistance Containment (GPARC) was released in 2011 for treatment of malaria by WHO. (Sinka ME et al; 2010)
Control of vector in malarial burdened sectors can prevent or decrease the cause of malaria. Use of insecticides was found to be the most beneficial way of eradicating vector Anopheles and thus malaria. (WHO Statistics, December 2013)
Impact of DDT
DDT (dichlorodiphenyltrichloroethane) was introduced in the year 1874. The insecticidal properties of DDT was discovered in the year 1935 and has been used worldwide ever since. According to EPA (Environmental protection agency) in the United States of America, they have used approximately 1,350,000,000 pounds (lbs) of DDT ever since its discovery. Many early studies had revealed the potential threats of DDT in animals and humans. However metabolites of DDT (mainly DDE) have found to be most prevalent in humans. It was confirmed after laboratory testing and studies that DDT has a half life cycle of around 65 years, both in humans and the environment. However it is said that DDT had benefited a large number of populations and also eradicated the occurrence of malaria in the early 1960’s. (Steve Corin and Sean A Weaver, 2005)
Mechanism of action of DDT
Epidemiological studies have shown that DDT may lead to endocrine disruption. This in turn may cause lower testosterone levels. Certain chemicals (metabolites of DDT) might block or bind on receptors of hormones. This will cause normal functioning of any particular signal transduction. Some of the examples would be like, ο, ρ’-DDT (metabolite of DDT) mimics the primary endogenous estrogen - 17β-estradiol (E2). Another example is the binding of ρ, ρ’-DDT to endoplasmic reticulum (ER).
ρ, ρ’-DDE, another metabolite of DDT can bind to ER. It is also studied that it has affinity to bid to androgen receptors, this on the other hand interferes with cytokines. The major metabolites of DDT that affect humans in some way or the other on the basis of scientific evidence are as follows: ρ, ρ’ –DDT, ο, ρ’-DDT, ρ, ρ’-DDE, ο, ρ’-DDE, ρ, ρ’-DDD, ο, ρ’-DDD and ο,o’-DDD. These have shown various affinity linked to androgen, endoplasmic reticulum or even act as androgen agonist. (Steve Corin and Sean A Weaver, 2005)
Effects of DDT on Human Health
The use of DDT became quite prevalent in the early 1960’s. It was used in large quantities as an insecticide and especially used my farmers. Eradication of malaria in regions with high use of DDT was seen but at the cost of human health and environment. A study reveals the high risk of breast cancer in women exposed to DDT directly. The exposure may be a result of occupational hazard or environmental. A study revealed that high levels of serum p,p′-DDT increased the rate of breast cancer in women 5 times. The study involved women from the batch of 1959 to 1967, since DDT was introduced and widely spread in these years. Test was conducted by taking serum samples of women who were at the mean age of 26 years. Serum analysis of p,p′-DDT, o,p′-DDT and p,p′-DDE. Of these p,p′-DDE ids the most widely found metabolite of DDT in serum samples. (Barbara A. Cohn et al. 2007)
Scientific evaluation of serum samples was from women exposed to DDT and women who were not exposed to DDT (control) were evaluated for any association with breast cancer. The study reveals women exposed to DDT showed 5 times more risk of having p,p′-DDE in their serum samples and eventually breast cancer. Women not exposed to DDT had no relation to breast cancer in any manner. (Barbara A. Cohn et al. 2007)
The samples collected from women were in the age group of 14 to 20 years during the peak use of DDT (1959 TO 1967). Among these women, mainly pregnant women were tested for metabolites of DDT in their serum. This link provided a strong backbone in the evaluation of breast cancer and other fetal related disorders. Many women also faced miscarriages or fetal deformity. Children born of women exposed to DDT suffered from various cardiorespiratory or neural health problems. This then related to DDT as not only a carcinogen but also a potent toxic chemical which can be passed from mother to fetus. This draws attention to the high health issues related to DDT exposure. (Barbara A. Cohn et al. 2007)
Another study evaluated the effect of DDT on reproductive health of men. The study was similar to the prior study carried out in women. However the number of men evaluated for DDT exposure and consequent reproductive health analysis was quite higher compared to women. The study involved around 2033 men from areas of Mexico, where DDT was used largely in the mid 60’s. The average age of men at that time was around 22. 18 years being the seniority level for evaluation. The DDT exposure of workers evaluated was on an average of 15 years. Retired workers had an average of 27 years of exposure to DDT. Many other criteria for evaluation of exposure, time period and age at time of exposure were considered. On an average 61µg/g of DDT was found in fat with men exposed to DDT. This mean was taken from the total of the 2033 cases evaluated. (Félix Salazar-García, et al. 2004)
Most workers had a similar lifestyle and evaluation of smoking and drinking habits were also noted down in order to avoid bias for the study. 805 of men consumed alcohol and around 45% were regular smokers. An evaluation of previous work experience and entry into malaria program (DDT use) was considered. More than 55% of men had a farming job which dealt with the use of DDT as an exposure. Bu the exposure time to DDT was reasonably low, with an average of 2-3 days per year. (Félix Salazar-García, et al. 2004)
25 men had no children, of which 17 were married and 8 were not married. Of the 17 married men, most of them showed infertility problems. This led to further evaluation of DDT and its association with reproductive health. The number of pregnancies, around 9,187, around 87% were live births while 118 births were still or declared dead. No data was available for 127 pregnancies. These were only observed when around 72% of fathers were already exposed to DDT prior to pregnancy. And also after thorough evaluation 55 children suffered from major birth defects. An interesting observation was made that 425 birth defects were observed when fathers were exposed to DDT. To conclude a significant amount of workers exposed to DDT showed reproductive health issues and child birth defect. (Félix Salazar-García, et al. 2004)
Malaria and the use of DDT can have a significant impact on both humans and the environment. Malaria may cause much higher death rates compared to mortality caused by DDT exposure. The use of DDT is much more beneficial compared to its ban since this might increase malarial cases in areas like Africa and Mexico where malaria is highly prevalent. A better opinion is the use of DDT, since its benefits outweighs the risk of toxicity.
On a comparative note, Malaria causes around 200 million deaths a year, while exposure to DDT may not even have a scientific evidence proving mortality. Malaria has more mortality rate in children, though DDT exposed parents have influence and cause defects, the two can still not be compared. The use of DDT in areas where malaria is prevalent is much more beneficial since it can prevent the disease and also indirectly reduce mortality. (WHO fact sheets, 2006)
Considering the environment, DDT might have a higher half life cycle, but its use will reduce the breeding of Anopheles species, the main vector of Malaria. In 2006 WHO also considered DDT to be continued as it can help in reduction of malaria cases and it also outweighs benefits to risk. Not only consideration for indoor use, but DDT is also considered as one of the 12 pesticides for indoor use recommended by WHO. Environment protection agency (EPA) in the US conduct DDT programs to educate people from the benefits and also the long term effects of DDT exposure. This enables people to use DDT at a considerate amount and in a safer manner, thus not only eradicating malaria but also reducing health effects like infertility and cancer. (EPA government website. DDT and History)
DDT is considered to be a potent human carcinogen but however its use in a systematic manner, has many benefits and outweighs the risk of toxicity.
Alternatives to DDT
Many methods have evolved as alternatives to DDT as vector control for malaria. On broader terms the alternative methods used can be classified into two i.e Chemical and Non-chemical methods respectively. Chemical methods involve Insecticide-treated bed nets, Indoor residual spraying, Chemical larviciding and Repellents and attractants. Most of these face a universal problem of mosquitoes acquiring resistance to these chemicals. Except for larviciding, all the other methods show toxic properties. Although larviciding may not have toxic activity but it showed impact on the surrounding environment. It affects the mosquito at larval stage while other methods kill and show efficacy against mosquitoes in adult stage.
The next set of methods are non-chemical methods and mainly involve Elimination of breeding sites, Habitat manipulation, Irrigation management, Design of irrigation structures. House improvement, Predation, Microbial larvicides, Botanicals and Polystyrene beads. All of these methods show efficacy against adult stage mosquitoes and Botanicals showing against both larval and adult stage. The only advantage of non-chemical methods is that they show no toxicity botanicals being an exception. However in the use of larvicides, chances of resistance is mosquitoes is observed.
In terms of overall considerations of alternatives to DDT, many new technologies are being developed. In terms of cost, non-chemical methods serve the purpose and also are environmental friendly and have no chance of co-resistance in any way. The development of these new technologies have replaced the potent carcinogenic DDT and have also been used worldwide for eradication of malaria in an efficient and safer manner.
(Van den Berg H, 2009)
References
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Retrieved from: http://www.who.int/mediacentre/factsheets/fs094/en/
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WHO gives indoor use of DDT a clean bill of health for controlling malaria. WHO fact sheets. 2006.
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