Most people consider mosquitoes as merely an annoyance, oblivious that these tiny assassins have the potential and lethal capacity to kill more than a million victims a year around the world, due to the transmission of dengue/dengue haemorrhagic fever, Zika, Malaria, JE relentlessly continue to be major health problems for exposed populations. Chemical pesticides such as dichlorodiphenyl trichloro ethane (DDT), malathion, chlordane and pyrethroids applied with the aim of eliminating mosquitoes have given rise to other serious problems. Not only because mosquitoes have developed resistance against these chemicals, but also because the pesticides themselves pose threats to both human health and the ecosystem.
Chemical pesticide exposure among humans has been linked to immune dysfunction, neurological disorders, various forms of cancer, birth defects, and infertility. Toxic effects of the chemicals were identified by researchers as a cause of liver damage. It was noted that blood disorders occurred shortly after exposure to pesticides; besides the immune system becomes weakened enough in any living mammal so much so viral cells are able to grow more rapidly. Actions must be taken to control the injudicious use of insecticides, and encourage research to develop new agents either synthetic or natural. The potential health and environmental effects strongly support the importance of finding alternatives to these pesticides. It is, therefore, necessary to identify a safe, eco-friendly alternate source of larvicides to reduce mosquito menace. Under these circumstances, the possibility of utilizing biopesticides as an alternative to synthetic chemical pesticides was examined. Biological control alone or as a part of integrated vector management is believed to be a better alternative to the chemical control aimed against pest mosquitoes.
Basically, mosquito control approaches can be divided into four categories: (1) Source reduction and environmental management, (2) Biological control, (3) Chemical control and (4) Physical barriers and personal protection.
In general, source reduction and environmental management are the common approaches for providing long-term solutions to many mosquito problems. For example, the control of dengue/dengue haemorrhagic fever vectors, using source reduction by getting rid of unwanted containers and prevention of mosquito breeding in useful containers which breed Aedes, can be highly effective to a certain extent. However, diligence and cooperation of the public under a non-epidemic condition is usually less than satisfactory. This was well witnessed by the surge of cases of dengue/dengue haemorrhagic fever in many urban centres throughout the world. In addition, public education concerning the breeding habitats of environmental management, basic infrastructural improvements including better drainage, sub-soil piping and siphon-flushing system in hilly areas in the western coastal region of Peninsular Malaysia have proven to be effective in reducing Anopheles maculatus population with subsequent reduction of malaria transmission. Nevertheless, environmental management involves major investment of funding. Hence the initial investment may prove to be too costly if large-scale implementation is required.
At present, the success story on the biological control of mosquitoes has been that the use of microbial agents/bacteria, i.e. Bacillus thuringiensis H-14 (Bti) and Bacillus sphaericus (Bsp) against various species of mosquitoes. Due to specificity, Bti was found to be more effective against Aedes and Anopheles while Bsp was more suitable for Culex and Mansonia control in polluted water.
As in any mosquito-borne diseases control programme, the cooperation and participation of the community in any locality is vital to the success of any program. Depending on the socio-economic and political situation in that community, different approaches can be used to educate and mobilise the community concerning their role in the success of mosquito-borne control in their locality. Public health officials must be innovative and bold in their search for the most effective form of community participation and not slavishly adopt patterns which have been shown to be successful elsewhere. When necessary, in view of the peculiarities of the local situation modifications must be made to any pre-existing approaches or other recommendations approaches.
A) Source reduction/environmental management
Source reduction should be the best approach for controlling container breeders such as Aedes aegypti/Aedes albopictus. Public education and diligence is the key to success. Basic infrastructural improvements (better drainage, fully covered septic tanks, proper sewage disposal) will lessen the breeding of Culex quinquefasciatus and related species. Basic environmental management (sub-soil drainage, siphon-flushing system in hilly streams, water surface disturbance) has been successful in controlling Anopheles maculatus in Peninsular Malaysia.
B) Biological control – Pathogens
Use of microbial agents especially cultivated bacteria has shown to be effective in the control of mosquito immatures. E.g.: Bacillus thuringiensis H-14 – for control clean water breeders such as Aedes and Anopheles.
C) Chemical control – Larvicides
Conventional insecticidal larvicides include Temephos (ABATE) for clean water breeders such as the dengue-carrying Aedes species.
With the majority of larvicides, care must be taken to avoid contamination of drinking water and waters inhabited by non-target organisms of value, such as fish and crustaceans. However, temephos may be used to control mosquitoes that breed in drinking-water containers. Such treatment should always be made with pesticide formulations that ensure accurate and reliable dosage.
Temephos is used mainly as a mosquito larvicide and, because of its wide margin of safety and its effectiveness at low dosage, is suitable for the treatment of potable water at a dosage rate of not more than 1 mg/litre (1 ppm). Biologically, the liquid formulations are superior to the granular, but the appearance of potable water treated with the former may not be acceptable.
Adulticides used with fogging machines are carried out when cases of dengue victims are reported by hospitals within the victims’ residences of usually 100 meters radius. Unfortunately, such reports do not actually pinpoint the hotspots of dengue infection, as the window period for symptoms to surface is from 3-5 days after the victim had been bitten by them. As such due diligence tracking of frequented places is recommended to determine such hotspots.
As most if not all, adulticides are repellent by design with the carrier formulation, a systematic approach is required to provide a lethal dose against these insects. The moment we start the fogging machine the blaring sound is enough to drive away them from their harbourages. Moreover, the odour of diesel is offensive to them which they will fly their level best to avoid. The proverbial story of successfully shooting one bird out of ten birds on a tree will not have 9 remaining on that branch.
Doctors and nursing staff were advised on maintaining a high index of suspicion for dengue when dealing with febrile patients. Nursing of all fever cases under insecticide treated mosquito nets were recommended to be done in the wards.
Personal protection with repellent by application on our skin is recommended when at outdoors of suspected infested areas is advised.
This method should be complemented with the above SAD methods, as it provides facilities for the gravid female mosquitoes to lay their eggs thereby giving us easier and ample opportunities to reduce its population.
Just as we humans attend to the call of nature, our obligations are to look for public toilets (if we are away from our homes) to ease ourselves. In the event, there is no such facilities, we will still need to ease albeit inhumanely and likewise can be said of dumping rubbish wastes etc. The below diagram illustrates the advantages of BAD with SAD.
As both these campaigns require the mandate from the authorities, it is our fervent hope that the Ministry Of Health will consider our proposal and imposed this awareness to the public. Our confidence lies in the ability of your ministry to deliver this message of co-operation to all residential, commercial, industrial and public property owners/stakeholders and ensure the success of this program.
Thank you
ANG TAN LOONG
Friday, 28 October 2016
*The writer is a: