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The aim of vector control is to interrupt or eliminate local transmission of diseases, reduce vulnerability to disease, and prevent secondary infections from introduced diseases so they do not create further outbreaks.
This requires a strong organisational framework, well defined plans, skilled technical operators, appropriate equipment and sufficient financial resources.
The main targets of vector control measures worldwide are Anopheles species for malaria parasite control and Aedes species for a range of viruses eg Dengue, Zika, Chikungunya, West Nile fever.
For Aedes species, which can transmit several viruses as soon as they are infected, the ecology and behaviour are well known and vector control can be planned more easily.
For malaria, which is transmitted by 41 Anopheles species, choosing vector control methods is more complicated as each species has distinct ecology and behaviour — called bionomics. The behaviour is not well known for some species and can change in response to selective pressure when control methods are applied. South East Asia, in particular is a complex area because of the high species diversity.
Integrated vector management (IVM) is a new approach to the control of vector-borne diseases that reorients existing national and international disease control programmes to make them more effective. It is defined as “a rational decision-making process to optimise the use of resources for vector control” (WHO, 2016).
IVM establishes partnerships across multiple organisations to implement programmes in an efficient, cost-effective, ecologically sound and sustainable manner. It uses a range of interventions based on local knowledge about the vectors, diseases and disease determinants and encourages collaboration with the health services, other public services and local communities. IVM has adapted concepts from integrated pest management techniques in which insecticide application is only used as a last resort.
The key elements of an IVM strategy are (WHO, 2012):
Surveillance is an essential component of vector control to determine the local presence and abundance of the target mosquito species, before, during and after a control programme, for further planning and alerting of resurgence and reintroduction.
It is used to assess species distribution, densities, aquatic habitats, feeding and resting behaviours, especially where little is known about local mosquito species. In addition, it is a vital tool in monitoring and determining insecticide resistance and how it affects mosquito behaviour.
There are different techniques of surveillance depending on the mosquito species and the target disease(s). For malaria and West Nile virus surveillance mainly monitors mosquito populations, while for dengue, Chikungunya, yellow fever and Zika, it is more efficient to monitor infections in people. (CDC, 2016a) through public health services.
Mosquito population threshold levels for disease transmission at each location are calculated from the survey data collected from:
A range of mosquito collection devices are used depending on the species and local situation:
Reconnaissance of the areas where mosquito interventions are planned is essential for optimising the vector control measures. Geographical reconnaissance will identify the spatial distribution and number of structures for spraying, the mosquito breeding sites for interventions, and in developing countries, the houses that will receive insecticide-impregnated mosquito nets.
It is also used to record places where action has been taken, such as larvicidal treatments and places where public outreach events have been conducted.
The tools used are GPS devices, including mobile phones with appropriate apps, geographic information systems and computerised mapping. Despite proven results in academic programmes, the high capital investment cost of this type of exercise is often a prohibitive factor in the economies of countries most at risk of mosquito borne diseases.
Figure 1 shows an example of the use of geographic data for vector control by New York City Department of Health & Mental Hygiene.
Figure 1. Mosquito surveillance map for West Nile Virus in New York City. Example of geographic data for vector control by New York City Department of Health & Mental Hygiene (in Kass, 2016)
Environmental management involves removing breeding opportunities for mosquitoes. For Aedes species, which stay within tens of metres from their human hosts, breeding places are close to human habitation, around homes and businesses. For Anopheles species, breeding places are areas of water in surrounding natural areas as well, such as forests, ditches, tyre tracks in roads, paddy fields, swamps.
Remove or cover any kind of container that holds water. For A. aegypti and A. albopictus breeding site control, CDC classifies water containers into five general types (CDC, 2016 a):
This involves long-term modifications to reduce larval habitats, including:
Biological control introduces agents to affect reproduction, growth and activity of vector insects or change the transmission dynamics of a disease in an environmentally safe way, including:
Application of insecticides is done as complementary action to physical and biological control methods and only when there is no other option. Biocides are strictly controlled by legislation in most countries eg EU, US, Australia, and should as a minimum follow WHO guidelines.
WHO has promoted and coordinated the testing and evaluation of pesticides for public health since 1960 in the WHO Pesticide Evaluation Scheme (WHOPES) at: who.int/whopes.
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Kass, D. 2016. Considerations for Enhanced Mosquito Control in NYC in Anticipation of Local Zika Transmission. In CDC, Zika Action Plan Summit Presentations. Controlling and responding to mosquito borne illness. April 2016. (link)
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PAHO, WHO. 2003 c. Zoonoses and communicable diseases common to man and animals. Vol III. Parasitoses. Scientific and Technical Publication No 580. PAHO/ WHO, Washington.
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Some lessons from history describing the devastating human and economic toll of vector-borne diseases
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