Vector Control Using Insecticides

At the end of the 19th century, it was discovered that certain species of insects, other arthropods and fresh water snails were responsible for the transmission of some diseases of pubic health importance. Since effective vaccines or drugs were not always available for the prevention or treatment of these diseases, control of transmission then had to rely mainly on control of vectors. The control programmes included among others, use of mosquito nets, drainage of gutters, filling of potholes and other water bodies used by insects for breeding. The 1940s sawed the discovery of DDT insecticide (dichlorodiphenyl tricloroethene) which was a major breakthrough in the control of vector-borne diseases. DDT also appeared to be effective and economical in the control of other biting flies (tsetse fly, simulium, and sand fly) and midges and of infestations with fleas, lice, bedbugs and triatomine bugs. The initial large scale success achieved in the control programme was short-lived as the vectors developed resistance to the insecticides in use, thereby creating a need for new more expensive chemicals. Interest in alternatives to the use of insecticides such as environmental management (source reduction) and biological control, has been revived because of increasing resistance to the commonly used insecticides among important vector species e.g. (malaria) and also because of concerns about the effects of DDT and certain other insecticides on the environment. For many vector species, environmental sanitation1 through source reduction and health education is the fundamental means of control; other methods should serve as a supplement, not as a substitute. Thus, in recent years, the practices of vector control have evolved, and environmental management and modification have come to the fore, both for disease control and for agricultural and other economic purposes2, this is a complex and multi-disciplinary field3. Effective application of any control measure must be based on a fundamental understanding of the ecology, bionomics and behaviour of the target vector species and its relation to its host and the environment. Effective vector control also requires careful training and supervision of pest control operations and periodic evaluation of the impacts of the control measures. In more recent years, less reliance has been placed on the use of a single method of chemical control; there is a shift towards more integrated vector control involving several types of environmental management supplemented by more than one method of chemical control and the use of drugs. More attention has been paid to community participation (a key component of primary health care (PHC) in eliminating breeding sites of vectors (including clearing of weeds/bushes near residential houses) and reducing vector densities.


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Finally, there is also need to provide on continuous basis information on single, effective and acceptable methods for vector source reduction and personal protection to individuals and families in the community at a reasonable cost. In a chapter of this nature only a few, but more important species of vectors could be discussed and even such a discussion could only be brief, emphasizing only the important features of the arthropod which serve to illustrate how the arthropod affects public health.

Malaria and its vectors
Human malaria is a number one public enemy and an illness caused by the bite of an infective female anopheles mosquito which transfers parasites called plasmodium from person to person. Four plasmodium parasites exists (P. falciparum, P. ovale, P. malariae and P. vivax) but only one (P. falciparum) is of vital importance in disease transmission in Nigeria. The important vectors in Nigeria are anopheles gambiae and A. funestus. These mosquitoes breed readily in ditches and collections of water in empty receptacles around the houses. The disease is endemic in Nigeria and over 90% of the population is at risk. Fifty percent of the population will have at least one attack per year, 300, 000 children and 11% of pregnant women die of malaria each year respectively and millions of dollars is lost each year in treatment of malaria. The table below shows some of these vectors and diseases transmitted by them. and Japanese encephalitis. For practical purposes and entomologically, it is useful to be able to differentiate between these three types of mosquitoes; so the main distinguishing features in each stage of the life cycle are shown below in the diagram Basically, the mosquito has four stages in its life cycle: eggs → larva→ pupa → adult. It is useful to understand the life cycle and natural history of the anopheles mosquito not only for epidemiological reasons but most importantly for the fact that all the four stages are targets to control mosquito vector. There are several features of the behaviour of anopheles mosquitoes which are important in understanding malaria epidemiology and also for planning mosquito control. Malaria is holo-endemic in most regions of Nigeria; ideal climatic conditions for the propagation and transmission of the infection are a temperature between 26 o C -30 o C and relative humidity of over 60 percent.

Disease control
The general methods of malaria control can be grouped into three measures directed against the parasite in man, measures directed against the vector and measures designed to prevent mosquito-man contact, these are summarized in table 2

Principal goal Interventions
Treatment Outpatient treatment of uncomplicated malaria Inpatient treatment of severe and complicated malaria Home treatment Prevention  Table 2. Interventions to control malaria 5 .
The first involves the use of appropriate anti-malarial agents to treat clinical malaria and the use of chemoprophylaxis among the vulnerable groups. The problem facing most tropical countries is serious as majority of patients with clinical malaria are either untreated or treated inadequately by self-medication. Also, their governments cannot afford to buy sufficient anti-malarial drugs for their needs and most people cannot afford to purchase effective treatments 7 . The annual per capita expenditure on anti-malarial drugs in most of sub-Saharan Africa is still <US$10. An adult (60kg) course of Chloroquine costs US$0.08 but the new artemisinin based combinations (ACTs) cost more than five times as shown in table 3.
This is currently unaffordable to most patients surviving barely on less than a US$1 per day. The treatment of malaria has become quite challenging, and the emergence of resistant strains of the parasite. In Nigeria where at least 80% 9,10 of the people live in rural areas and are supperstitious illiterates, early recognition and the right treatment are not adhered to. This encourages drug resistance. The Abuja declaration on Roll Back Malaria on 25 April 2000 and agreed to by African Heads of State sets an ambitious goals to reduce the burden of malaria (insecticide-treated nets, prompt access to treatment and prevention of malaria in pregnancy) by the year 2010 8 . Achieving high coverage in both IPT and use of ITNs among the vulnerable groups and the general population has remained elusive for many countries in sub-Saharan Africa 11 . A major barrier to net ownership is poverty as the price of a net represents a large proportion of the income of a poor household, this has been reported in various studies 12,13,14 .
Environmental control (source reduction) offers the best practical and easy measure to control disease vector as it eliminates the breeding places. The filling of mosquito breeding sites with soil, ash or rubbish and is most suitable for reducing breeding in small depressions, water holes or pools, which does not require much filling material. On the other hand, drainage of water can be achieved by constructing open ditches; however, the drainage systems used in agriculture or for the transportation of sewage and rainwater in cities often promote breeding because of poor design and maintenance.

The use of insecticides
The first house-spraying campaigns after the Second World War, showed the capacity of this interventions to produce profound reductions in malaria transmission in a wide variety of circumstances. In Africa, the intervention was used in 1960s and 1970s but later abandoned except in some countries in southern and eastern Africa where residual insecticide spraying (IRS) remained the cornerstone of malaria control strategy. The evaluation of the local context of the 3 chemicals confirmed that residual effectiveness of the insecticides lasted for at least 4 months.
Residual spraying of houses involves the treating of interior walls and ceilings using a handheld compression spraying and is effective against mosquitoes that favour indoor resting before or after feeding. For a more detailed discourse of notable insecticide formulations, spray pumps, spraying techniques and maintenance of equipment, see Jan A. Rozendaal 15 . The figure 2, below summarizes and describes the IRS management principles; Mosquito nets are an old technology and most people in sub-Saharan Africa are aware of the existence of nets. In some countries such as Nigeria and the Gambia nets have been in regular use for over hundred years. Similarly, in some parts of Africa, net ownership is a well established social norm and nets are widely available. For instance, recently, it was found that 70% of households (HHs) in Dar es Salaam (Tanzania) owned a mosquito net 16 and around 35% of HHs were found to own a net in urban Burkina Faso 17 . Insecticide treated mosquito nets have had significant impact in reducing morbidity and mortality among children under-five years old and pregnant women where ITNs have been appropriately and extensively used in malaria endemic areas. The potential epidemiological advantages and public health benefits of treating nets with insecticide for protection against malaria were recognized in the mid-1980s. Specifically, the efficacy of insecticide treated nets (ITNs) for the control of malaria in children under-5 years of age has recently been demonstrated by several large scale studies 18,19,20,21 which find reductions in all causemortality, ranging from 16% to 63%. These insecticides which have been approved by WHOPES (World Health Organization Pesticides Evaluation Scheme) are safe. They have the following properties; provide personal protection from mosquito bites, effective against other insects: bedbugs, flies and cockroaches, community and household mass effective which may be more important in some contexts 22 and mosquito nuisance effect. There have been reports also of dead insects on the nets and on the floor, less mosquito noise and that ITNs providing a better night's sleep than a net alone 16 .
Most nets (and other materials) need to be treated and retreated with insecticides to increase their effectiveness. There are several insecticides that can be used which have proved to be safe.  Table 4.
For a detailed description of preparation of insecticide mixtures and treatment methods, the reader should consult guidelines on the use of insecticide-treated mosquito nets 23 .
A single impregnation of a cotton or nylon mosquito net will provide protection for 1 year 24,25 . Nylon tends to retain permethrin and deltamethrin better than cotton. The impregnated nets can be washed and can tolerate small tears/holes without markedly reducing the protective effect. Recently, long lasting nets (LLNs) have been developed and have the advantage of retaining insecticidal activity for years (so the nets will not lose its potency with repeated washings). Despite various government policies, cost of the nets remains a significant barrier and a long obstacle to the Roll Back Malaria goal of universal coverage -defined as one long lasting insecticide-treated nets for every two people in the household with 80% usage. ITN development is a public good. The development of insecticide resistance in the 1950s and recently by vectors has been a cause for concern. There have been reports of resistance to DDT in a wide range of sub-Saharan African countries 26 , but this has not reached an operationally significant level. With the exception of the Gezira region of Sudan 27 , widespread loss of vector susceptibility is not yet a big problem in Nigeria. A worrying new development is the emergence of knockdown resistance to pyrethroid insecticides in natural populations of anopheles mosquitoes in Cote d'Ivoire and Burkina Faso 28,29,30 where insecticides are widely used in cotton production.
Currently, pyrethroids are the only insecticides used for net treatment and are also increasingly used for spraying, so there is threat that if widespread resistance develops, the interventions will gradually become less cost-effective over time.
Finally, there is need to describe importantly the WHO integrated vector management (IVM) concept. Vector control is well suited for integrated approaches because vectors are responsible for multiple diseases and since interventions are effective against several vectors (use of insecticides) the concept of IVM was developed as a result of lessons learnt from integrated pest management which was used in Agriculture. Integrated vector management (IVM) is a major component of the global campaign against malaria. The revised strategic plan for RBM recommended that from 2006 -2010, 80% of the population at risk need to be protected using effective vector control measures. IVM creates synergies between various vector-borne disease control programmes. Utilization of single method could be optimized to control more than one vector-borne disease, eg ITNs can control malaria, lymphatic filariasis and to some extend leishmaniasis. IVM operates in the context of inter-sectoral collaboration. The application of IVM principles to vector control will contribute to the judicious use of insecticides and extend their useful life.