Mosquito vector control methods

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

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):

1. Advocacy, social mobilization and legislation: promotion and embedding of IVM principles in designing policies in all relevant agencies, organizations and civil society; establishment or strengthening of regulatory and legislative controls for public health; empowerment of communities.
2. Collaboration with health and other sectors: consider options for collaboration with public and private sectors; strengthen channels of communication among policy makers, vector-borne disease programme managers and other partners.
3. Integrated approach: ensure rational use of available resources by addressing several diseases, integrating non-chemical and chemical control methods and other disease control measures.
4. Evidence-based decision making: adapt strategies and interventions suitable for the local ecology, epidemiology and resources, guided by operational research and routine monitoring and evaluation.
5. Capacity building: provide the required material, financial and human resources at national and local level for an IVM strategy.

Surveillance

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:

• egg survey: number per ovitrap per week;
• larval survey: larvae are counted in all or a sample of water containers to calculate:
  • House Index (HI): percentage of houses with at least one positive container;
  • Container Index (CI): percentage of containers with larvae;
  • Breteau Index (BI): number of containers with larvae per 100 houses; an index over 1% generally indicates control activities are required.
• pupal survey: pupae are counted in water containers to give: pupae per house, person, per hectare. They are raised in a lab to mature to adults so the species can be identified;
• female adult survey: number per trap per week.

A range of mosquito collection devices are used depending on the species and local situation:

• Ovitraps (Aedes spp): small containers containing water and straw, and a substrate for female mosquitoes to lay eggs on (wood, paper, cloth). These are placed at representative habitat sites in the surveillance area. They require regular checking and weekly removal to prevent them becoming breeding sites. Eggs are hatched and reared in a laboratory to identify the species. Ovitraps can be made lethal by the inclusion of insecticides that prevent larval development. This can be useful for areas with restricted or intermittent access.
• Gravid female trap: these are similar to ovitraps to attract adult female mosquitoes looking for sites to lay eggs and use funnels or sticky boards to capture them.
• Mechanical aspirators: an aspirator has a battery-powered fan to suck mosquitoes into a collecting chamber. They are particularly useful for collecting resting mosquitoes, which are more representative of the vector population. However, collecting a large enough sample can be labour intensive and require sampling a large number of houses due to the low density of mosquitoes resting in one location.

Geographical reconnaissance

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

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.

Source reduction

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):

• phytotelmata (natural water containers in plants eg tree holes, leaf axils, etc) — fill with sand/ concrete where possible;
• non-essential or disposable containers (food and drink containers, tyres, broken appliances) — collect and dispose/ recycle or cover; including a reliable public waste disposal service;
• useful containers (water-storage vessels, potted plants and trays, animal drinking bowls, paint trays, toys, buckets, septic tanks) — cover, clean and empty frequently;
• cavities in structures (fence poles, bricks, uneven floors and roofs, roof gutters, air-conditioner trays) — repair, fill cavities, clean, design buildings to remove water collection places;
• outdoor underground structures (storm drains, water meters, public wells, septic tanks) — cover, repair, clean, modify design, as appropriate.

Habitat modification

This involves long-term modifications to reduce larval habitats, including:

• regional water management projects;
• installing reliable water supplies for households. This eliminates the need for jars and tanks to store water in individual houses (if supply is reliable) and which can be larval habitats;
• farming practices that reduce standing water.

Mechanical control

• window and door screening;
• for malaria prevention, sleep in bed nets impregnated with insecticide (LLINs);
• drilling holes in containers, tyres, etc to drain water;
• removal or safe storage of scrap.

Biological control

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:

• biological larvicides: formulations of bacteria eg Bacillus thuringiensis, Bacillus sphaericus, as wettable powders for spraying, and granules or briquettes for manual dispersion;
• larvivorous fish: small surface-feeding fish that feed on mosquito larvae, eg Gambusia affinis, Lebistes reticulatus; There are many potential species worldwide. WHO has produced an overview of the use of fish for mosquito control, listing known larvivorous fish in different regions of the world, experience in their use and breeding techniques (WHO, 2003);
• mass trapping of mosquitoes: technology to attract and kill large numbers of mosquito is available and developing rapidly, but currently has low effectiveness (Bellini et al, 2014).

Chemical control

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.

• larvicidal application:
  • chemical larvicides: the active ingredients that can be used are controlled by biocide regulations and can only be applied by ground application of mapped breeding sites;
  • monomolecular films and oils. These are sprayed on a water surface and form a thin film that prevents immature mosquitoes from breathing air through the water surface.
• adulticide application:
  • Space spraying/ fogging: by aerial or ground application; recommended for use only in emergencies to rapidly reduce populations, as there is little evidence of long term effectiveness. It can contaminate water supplies and may not be acceptable to the local population; aerial spraying may also be illegal, as in the EU.
  • indoor residual insecticide spraying (IRS);
  • insecticide treated bed nets (LLINs): the most effective malaria control method;
  • outdoor barrier spraying around houses, in yards and on nearby vegetation;
  • mosquito repellents, such as deet for personal use. The use of wide area repellents is being investigated.

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.

Bibliography

Centers for Disease Control and Prevention. A–Z Index. (link)

CDC, Division of Vector Borne Diseases. (link accessed 7 Oct 2016)

Hampson K, et al. Estimating the Global Burden of Endemic Canine Rabies. PLOS Neglected Tropical Diseases, April 16, 2015. (link)

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)

PAHO, WHO. 2003 a. Zoonoses and communicable diseases common to man and animals. VOl I. Bacterioses and Mycoses. Scientific and Technical Publication No 580. PAHO/ WHO, Washington.

PAHO, WHO. 2003 b. Zoonoses and communicable diseases common to man and animals. VOl II. Chlamydioses, Rickettsioses and Viroses. Scientific and Technical Publication No 580. PAHO/ WHO, Washington.

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.

WHO. 2008. Public health significance of urban pests. WHO, Copenhagen.

WHO. 2014. A global brief on vector-borne diseases. WHO, Geneva.

WHO. 2016. Vector surveillance and control at ports, airports and ground crossings. WHO, Geneva.

WHO. Rabies fact sheet. (link accessed 14 Oct 2016)