BACKGROUND
Vector control aims to reduce the transmission of malaria and thus decrease malarial morbidity and mortality [WHO-MPG]. The approach to vector control depends on three factors:
1. malaria burden
2. feasibility and applicability with respect to time
3. sustainability
In the following, three primary methods of vector control will be presented and analyzed: larvicides, insecticide treated bed nets, and indoor residual spraying.
LARVICIDES
Control efforts to reduce larval prevalence include altering the environment to reduce the number of breeding sites, as well as introducing agents to kill larvae. Modes of control can consist of: filling drains and ditches, covering storage containers with mesh, biodegradable oils, chemical larvicides, and bed net use. Biodegradable oils may be applied to the surfaces of water in order to suffocate larvae while the chemical Methoprene affects larval growth. [CDC] While this may achieve efficient larval control, vector resistance to larvicides has been documented. Moreover, the application of larvicides can be expensive due to the necessity of having regular trained personnel. [WHO, 1993]
Bacterial Larvicides
VectoBac (Bacillus thuringiensis israelensis) and Vectolex (Bacillus sphaericus) are the bacteria larvicides that target mosquitoes. The bacteria are ingested by mosquito larvae which, ultimately kills them. These bacteria are only toxic to insects, not other mammals, because they recognize binding sites found solely in the guts of insects. This type of larvicide is considered safe for the environment and depending on the applied concentration, can be effective for up to 4 weeks. [EPA] A 2006 study in rural Kenya showed these bacteria larvicides reduced Anopheles larval density by 95% at a cost of under $0.90 for an individual per year. [Fillinger et. al, 2006]
Larvivorous Fish
Another effective way to control Anopheles larvae is through the introduction of larvivorous fish. A study, taking place in a village in Northern Somalia, found that the introduction of the larvivorous fish Oreochromis spilurus spilurus into village reservoirs, on average, reduced larval prevalence by 52.8%. This fish has the ability to withstand chlorination levels of up to 1 mg/ L and could be a relatively cheap method to control malaria. [Mohammed,A, 2002] Some larvivorous fish are able to While use of larvivorous fish is effective, this method raises questions of sustainability, especially in areas with dry seasons, which may cause reservoirs to dry out. While the drying of reservoirs and other water sources would kill these fish, other types larvivorous fish have eggs that can survive the dry season.
INSECTICIDE TREATED BED NETS
Another method of managing the Anopheles vector is by using bed nets. Bed nets are used to reduce contact between humans and mosquito and thus reduce transmission rates of the Malaria Plasmodium parasite.
While both untreated and insecticide treated bed nets (ITN) are used in the same manner, the effectiveness of untreated bed nets are severely impaired by openings and holes created by everyday wear and tear or improper use. However, if insecticide treated bed nets have holes or small openings; they still confer protection unto an individual due to the repellants in the insecticides. It has been found that ITNs reduce mortality by 
20% in some African countries while untreated bed nets were only half as effective. If enough individuals in the community utilize ITN’s, the mosquito population is reduced in such a manner as to confer protection unto those in the community without bed nets, a concept best known as community coverage. [CDC]
ITN’s last for about 3- 5 years and need to be re-treated every 6-12 months and more frequently if they are washed or exposed to sunlight. Pyrethyroid insecticides, like deltamethrin, are used to treat bed nets because they are not very toxic to humans, yet are toxic to insects at very low levels. Standard ITN’s cost $5 and re-treatments cost anywhere from $0.50 - $1. Long lasting ITN’s do need to be retreated and cost about $10. [CDC] It was estimated that in areas of widespread ITN use, only 5% of ITNs are retreated with insectide in order to retain potency. [RBM]
Children and pregnant women are the most susceptible to malaria and 75% of the annual deaths due to malaria occur in African children under the age of 5. [RBM] ITNs can reduce the mortality rates of women and children, although there is speculation about delays in immunity for these children. A study beginning in 1997 and ending in 2003, in Kenya demonstrated that ITN’s significantly reduced mortality in infants without contributing to increased mortality in older children.
ITN’s were effective and efficient against malaria in Sichuan, China. Provinces in this city regularly used bed nets, although they were untreated. Due to an increase in P. falciparum, a study was done to test the effectiveness of ITNs. The provinces bed nets were treated with deltramethrin for up to 5 years without evidence of resistance from the vector. ITN use resulted in a 61%- 71% reduction in malaria infection in children after just 5 months, compared to the control group. The government paid for the cost of spraying the insecticide on the nets and communities paid, on average, 30% of this cost. To treat the bed nets it cost $0.045 per person a year compared to $0.13 if prophylaxis was used or $0.18 if indoor residual spraying with DDT was used. [Cheng, H. et al.,1992]
INDOOR RESIDUAL SPRAYING AND DDT
Another common form of malaria vector control in many parts of the world is indoor residual spraying (IRS). By spraying a residual insecticide on the walls and other surfaces of a house, any insect that comes in contact with such a surface is killed. In this way, while IRS does not prevent vectors from acquiring blood meals, it does prevent their parasites from being spread. As the insecticide is effective for several months, if enough houses in a community are sprayed, malarial transmission can be decreased significantly. [CDC]
While the insecticide chemical used for spraying varies, the criteria used for determining whether or not spraying is feasible are the same. For indoor residual spraying to be effective, it must be, first and foremost, sprayed onto the walls of a house—living arrangements that are more transient in nature, such as thatched huts, are not localized enough for the chemicals to be yield optimal results. Along the same vein, people in danger of being bitten by vectors should be within these sprayed walls when the risk of being bitten in the greatest. Furthermore, spraying should not be done after an epidemic is documented, as by that time, it cannot be effective in preventing infection by the parasite. If spraying is done before, during, and after the transmission season, it is most likely to prevent such an epidemic. [WHO]
By far the most common insecticide used in IRS is DDT (dichlorodiphenyltrichloroethane). DDT has demonstrated significant results in decreasing the incidence of malaria in previous campaigns (most notably through the Global Malaria Eradication Campaign), and its low cost makes it an even more attractive option for IRS [Attaran & Maharaj, 2000; CDC]. However, as vector resistance to DDT grew in some areas, other options for insecticides were sought. At the same time, the environmental campaign against DDT was gaining global footing, and eventually, DDT usage decreased worldwide and was replaced by synthetic pyrethroid compounds [Liroff, 2000; CDC].
In South Africa, this history took a sudden turn. When a mosquito vector in the country demonstrated resistance to the synthetic pyrethroid compounds, programs switched back to using DDT in IRS—and as a result, malaria incidence decreased 80% [Liroff, 2000; CDC]. Naturally, a renewed interest in malaria proliferated in academic and activist circles around the world, and is now the subject of an intense debate focused on assessing its relative benefits and risks.
It is useful to understand the opposing sides of the DDT debate, as each perspective has many globally prominent supporters. Supporters of DDT cite its low cost and high malarial control rate as its primary benefits. Even as a chemical, DDT is unique compared to other insecticides used for IRS. Unlike many other insecticides, DDT is fatal to a vector not only upon contact, but it is also a vector repellant and irritant, even without direct contact [Roberts, 2002]. In addition, it is also longer lasting than most other IRS insecticides; while most compounds last 3-6 months, DDT exerts its repellant-irritant-toxic effects for over 6 months [WHO].
On the other hand, while the benefits of DDT as a malarial control are recognized, its detractors are concerned primarily with its detrimental environmental effects. The harmful effects of DDT were first brought into mainstream discourse through Rachel Carson’s landmark work, Silent Spring. Because DDT is a bioaccumulative compound, even small doses can eventually result in significant health risks. The US Agency for Toxic Substances and Disease Registry reports that DDT’s hormone-disrupting consequences are evident in human reproductive, nervous, and immune systems. DDT can accumulate even in breast milk and amniotic fluid, causing developmental harm to newborns and children [Liroff, 2000].
But in the field of vector control, proponents of DDT are prevailing. Statistics such as the following indicate why: when South Africa replaced DDT in IRS with other compounds, its rates of malaria rose 150%; concurrently, the incidence of malaria in Swaziland, which did not stop its use of DDT, was 20-40 times less [Economist, 2000]. In September 2006, the WHO renewed its support for DDT as a successful malarial control agent, and its use in IRS is emerging as a primary tool in controlling populations for malaria, along with medication and bednets [BBC].