The Development Of Vectrol Control Programmes

As Sunday is World Malaria Day, I thought that readers might be interested to understand a little more about this disease, where it comes from and how we at Rentokil are working to stop it as part of our public health agenda.

The first thing to know is that malaria is a mosquito-borne infectious disease. It is particularly widespread in tropical and subtropical regions, including parts of the Americas, Asia and Africa.

Geographic distribution of malaria

Source: Wikimedia

There are approximately 350–500 million cases each year, which kill between one and three million people – that is equivalent to almost half the population of Ireland.

anopheles mosquitoMalaria is caused by a parasite of the genus Plasmodium, which is transmitted by the bite of a female Anopheles mosquito (the primary vector). Only female mosquitoes feed on blood, so males do not transmit the disease. The female mosquitoes prefer to feed at night and usually start searching for a meal at dusk; this will continue throughout the night until they take a meal.

When a mosquito bites an infected person (the host), a small amount of blood which contains the malaria parasites is taken. These parasites develop within the mosquito for about one week, and when the mosquito takes its next blood meal, they are injected with the mosquito’s saliva into the person being bitten. After spending between two weeks and several months in the liver, the malaria parasites start to multiply within red blood cells, causing symptoms that generally include fever and headache, but in the most severe cases can lead to death.

So why are there so many cases of malaria? The continued existence of malaria in an area requires a combination of:

  • high human population density,
  • high mosquito population density,
  • high number of infected hosts and
  • high rates of transmission from mosquitoes to humans.

We seek to combine our understanding of these elements with a thorough understanding of both the biology of the mosquitoes and the epidemiology of the disease, to create an effective, integrated approach to vector control.

Over the last quarter of a century we have pioneered an approach we call  ERDM. This uses a combination of four principles:

Exclusion, Restriction, Destruction and Monitoring.

These principles enable us to make informed and targeted interventions which affect the balance of the human to mosquito ratio, the number of infected hosts and the rates of transmission. Significantly lowering one or more of these elements eventually leads to the disappearance of the parasite, and therefore the disease, from that area.

The darker side of this equation is that unless the parasite is eliminated from the whole world, it can quickly re-establish itself if conditions revert to a combination that favours the parasite’s reproduction.

Indeed, many countries are seeing an increase in the number of imported malaria cases due to extensive travel and migration.

Mrs Mosquito image

Image from Virginia Health Bulletin vol.14, #6 Extra, May 1922. Source: VCU Tompkins-McCraw Library Special Collections on Flickr -

So how do we implement these four principles?

Exclusion – is one of the easiest methods of intervening in the mosquito / parasite lifecycle.

The simplest method of exclusion is to use mosquito nets: they help keep mosquitoes away from people and therefore greatly reduce the infection and transmission of malaria. Nets do not provide a perfect barrier, but their efficacy is enhanced when treated with an insecticide (like permethrin or deltamethrin) designed to kill the mosquito  before it has time to search for a way past the net.

Since Anopheles mosquitoes feed at night, people can hang a large “bed net” above the center of a bed, draping it down so it covers the bed completely. The World Health Organisation and a number or charitable foundations distribute nets freely to Malaria epicentres around the world.

Bed netting

Source: Ethan on Picasa

Restriction and Destruction – focus efforts to eradicate the disease by eliminating the female mosquitoes that act as the vector for the disease; this is achieved by either removing their breeding sites or using selective insecticides to kill at various stages of their lifecycles.

Larvicides are probably the most effective way of interfering with the lifecycle, in as much as the adult mosquito is not allowed to develop. The use of these chemicals requires specialist knowledge to identify and monitor potential water sources where larvae can develop and then to make appropriate treatments that do not overburden the environment.

The more traditional approach to destruction is outdoor chemical spraying. This still has a role as a first stage to knock down the numbers of adult mosquitoes, but then other interventions are needed.

Indoor residual spraying (IRS) is the practice of spraying insecticides on the interior walls of homes in malaria-affected areas. After feeding, many mosquito species rest on a nearby surface while digesting the bloodmeal, so if the walls of dwellings have been coated with insecticides, the resting mosquitos will be killed before they can bite another victim and transfer the malaria parasite. In the past DDT was the chemical of choice, but many countries have now forbidden its use in preference to pyrethrum and its derivatives.

The danger with any large scale spraying program is that if it is applied inappropriately, or if the active chemicals utilised are not rotated, such practices can lead to the build up of resistance within the species we are trying to destroy. This is why we would always recommend using a professional pest control organisation to identify and deliver the most appropriate control strategies in the region that is affected.

Monitoring – is the final and perhaps most important stage. It ensures a society continues to be protected once control has been achieved.

Rentokil runs monitoring programs in several areas of the world, and the results are linked into our epidemiological disease models. By tracking trends using our surveillance tool Pest Net Online, we get an early warning when mosquito activity is increasing to a level where intervention is needed. This minimises the likelihood of a resurgence of the disease, should an active disease host reservoir be present.

So there you have it – a quick introduction to the art and science of vector control. However, one final thing…

Whilst tropical regions are affected most, malaria’s reaches can extend into some temperate zones with extreme seasonal changes. And thanks to climate change, it is possible that malaria – and indeed other diseases which have historically been associated with more ‘exotic’ climates – will extend even further across the world.

Furthermore, malaria is not just a disease commonly associated with poverty. As a major part of a vicious circle that hinders economic development of many countries, malaria is also a cause of poverty.

That is why Rentokil is investing now in the development of vector control resources across the 54 countries in which we operate.

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