Section 3: MALARIA & MODELLING
Control
Contents:
F:
From the biology section you are aware that the transmission of malaria is a complicated process. The first step in trying to control malaria was to find out the means of transmission, and the image on the left shows a gateway in Calcutta which pays homage to the man who discovered this.
In order to try and control the spread of the disease, you can intervene with preventative measures at many stages of the transmission cycle.
This page outlines three stages of control:
- prevention of malaria transmission to humans by reducing the number of mosquito bites,
- prevention and treatment using drugs which kill the parasite in the human,
- environmental controls.
Gateway to a malaria laboratory in Calcutta.
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This section introduces the main public health practices which can limit malaria transmission by trying to reduce the number of mosquitoes and number of mosquito bites
Reducing number of bites: Bednets
Bednets are finely woven nets which are hung over a bed to stop mosquitos from being able to get to the human (see the picture on the left). Bednets are an effect method to reduce malaria transmission as they stop the mosquito biting the human.
Bednets stop more humans being infected or infectious humans from transmitting the parasite. If you look back at the lifecycle diagram on the biology pages the use of bednets breaks the cycle at stages 1(&7) stopping an infected mosquito biting a human and so passing on malaria, and stage 5, stopping an uninfected mosquito from becomming infected by biting an infected human.
The use of bednets is especially important if the room does not have insect screens (which cover doors and windows to stop mosquitoes from getting in) or air conditioning (as the windows are more likely to be open).
Often the bednets are impregnated with an insecticide which also helps to reduce the number of mosquitoes. They need retreating every six to 12 months or more frequently if they are washed.
One bednet study in Tanzania showed a reduction of 27% in child deaths when bed net use was supported and encouraged.
The number of bites can also be reduced by wearing long sleeved shirts and trousers to reduce the amount of exposed skin, the use of insect repellents on clothes and exposed skin (especially those containing DEET), and spraying bedrooms to kill any mosquitoes before sleeping. As Anopheles mosquitoes bite between dusk and dawn night time is a particularly important time to protect against mosquito bites.
Spraying and other controls
Spraying to limit mosquito numbers is normally split into two categories:
- larvicide, which is the reduction of the larvae at the aquatic stage of the life cycle (this is discussed in more detail in the section on environmental controls);
- insecticide, which is the spraying, now normally indoors, of the adult mosquitoes.
1. Larvicide
The number of mosquito larvae can be controlled through the use of chemical insecticides but other methods such as:
- the spreading of biodegradable oil onto the water surface which suffocates the larvae;
- the use of mosquito eating fish;
- or the use of biological agents that will kill the larvae.
Click on the images on the left which show various larvicide methods to see bigger versions.
These controls work well when there are known breeding sites for the mosquitoes, such as lakes or ponds. However the Anopheles mosquito which is the major vector in Africa can breed in very shallow pools or puddles which may only exist for short periods of time. This makes it very difficult to implement this type of control in Africa.
2. Insecticide
Indoor spraying coats the surfaces walls etc. with an insecticide that will kill mosquitoes when they come into contact with this surface. The insecticide usually lasts for a number of months.
This technique is called Indoor Residual Spraying. Between 1955 and 1960 the Global Malaria Eradication Campaign used DDT and dieldrin as insecticides.
However, because there are adverse environmental impacts of DDT and also because some mosquitoes became resistant to these insecticides, the switch was made to other insecticides.
The Campaign was affective in ridding some areas of malaria and reducing the burden of the disease in many others, but unfortunately these new insecticides were too expensive to be used by many countries that still had endemic malaria.
Another method used to control the number of adult mosquitoes is to literally suck them up into a giant vacuum.
Click on the images on the left to see bigger versions.
Bednets.
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DEET
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Spraying Oil.
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Mosquito fish.
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Indoor residual spraying
Mosquito vacuum
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There are two ways in which drugs are used to control malaria:
- drugs are given to prevent malaria (prophylaxis)
- and drugs are given to treat infected persons.
Sometime the same drugs are used for both purposes.
The different drugs given work in an number of ways but they often act on the parasite in the schizont stage of the lifecycle (stage 3 - the erythrocytic cycle, see the biology pages). As this stage is when the symptoms occurr, the drugs alleviate the symptoms caused by the parasite in the human. Other drugs act on the gametocytes (stage 4) therefore reducing or preventing the spread of the disease.
Medical advice should always be sought before visiting an area with known malaria transmission.
Some of the commonly used drugs include:
Chloroquine: this can be used both as a treatment and for prophylaxis but its effectifness is increasly limited due to widespread resistance to the drug by P. falciparum
Mefloquine: this can be used both as a treatment and for prophylaxis and is suitable for parts of the world where malaria is resistant to chloroquine. However, there is reisatnce to this drug found for P. falciparum in Cambodia and parts of Thailand.
Other drugs such as Primaquine sulphate can be used to treat what is termed benign malaria. This is where the person is not noticeable ill bit is still harboring parasite with their liver which can lead to recurrent attacks of the disease as is the case with P. vivax.
More recently introduced prophylaxis include one that uses a combination of proguanil and atovaquone.
Also antibiotic drugs such as the tetracycline antibiotic doxycyline can be used in the prophylaxis of malaria when combined with quinine, which is the oldest known anti-malarial. This can also be used in the treatment of P.falciparum.
Most of these drugs are often referred to by their brand names which are not used in this list.
However, no drug treatment can be seen as 100% effective and travelers should always try and prevent being bitten by mosquitoes (see the prevention section above).
Finally, vaccines are being sought and developed which work in a number of ways. So far trials in some areas have been successful but this success has not been universal.
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Lifecycle. Click to link to biology pages
Map of areas where malaria is chloroquine resistant. Click for bigger image.
Plant from which quinine is derived. Click for bigger image.
Environmental controls focus on removing potential mosquito breeding sites and so reducing the number of mosquitoes.
At tropical temperatures mosquitoes can develop rapidly from egg to adult and therefore water bodies can be small, shallow and short lived (see biology pages for more details on mosquito lifecycles).
Suitable sites include tyre tracks, puddles, or within water trapped in old car tyres.
Although it is not possible to eliminate puddles, containers that are used to collect and store water can be covered. Drainage of shallow ponds and marshy ground near villages and the maintenance of irrigation system i.e. removal of vegetation to ensure water is kept moving and therefore not providing a suitable mosquito breeding habitat can also be successful.
Drainage site being cleared.
Click for larger image.
People in malaria endemic areas are bitten frequently and so exposed to malaria.
This image shows blood cells from someone with sickle cell anaemia. Note the elongated cells.
Immunity
People in malaria endemic regions are infected by malaria again and again. As they are repeatedly infected, immunity to the disease gradually develops.
There are several ways in which this immunity develops:
- Through the development of mechanisms that kill the parasites
- Through the development of mechanisms that limit the ability of the parasite to reproduce
- By limiting the body's immune system.
This third way in which immunity can develop sounds confusing as you would think that immunity would be developed by increasing the power of the immune system, not decreasing it. However, with malaria, some of the most severe symptoms can be a result of the immune system going into overdrive, and the response to this is that it damages the body, so by limiting the immune response, the affect of malaria is limited.
The development of immunity in malaria is complicated because unlike some other diseases where if you are infected once then you develop immunity to that disease, with malaria, immunity gradually builds up as you are infected again and again.
It is thought that part of the reason for the amount of time it takes to build up immunity is that malaria affects the immune system, so it doesn't respond immediately. But this slowing of the immune system may actually be beneficial as it will prevent the severe symptoms associated with over stimulation of the immune system.
Resistance
Other than being immune to malaria, a form of resistance to the disease can develop which is not linked to the immune system. It is thought that this resistance developed over time through natural selection.
How does it work?
This response to malaria was noticed through an apparent correlation between malaria endemic areas and areas where people suffer from a certain blood disorder known as sickle-cell-anaemia. It is thought that this disorder is a result of evolution which protects against malaria.
It is not known exactly how sickle cell anaemia provides resistance to malaria, but the answer is thought to lie in the red blood cells. Sickle cell anaemia means that red blood cells deform and become a sickle shape (hence the name). Think about the malaria parasite life cycle. Part of this takes place in the red blood cells. (See biology pages).
Researchers think that because in someone who has sickle cell anaemia, their red blood cells are deformed, the development of the parasite in the red blood cell stage of the lifecycle is inhibited, although no one knows exactly how this happens. There are several suggestions, including:
- The deformed cells, because of their shape, directly inhibit the growth of the parasite;
- Because the cells are deformed, the body's natural system which replaces red blood cells kills these deformed cells earlier than it would have done. This means that the parasite is also killed, and the new cells which develop will be uninfected.
Unfortunately, as well as providing this resistance to malaria, sickle cell anaemia is a serious condition as the deformed red blood cells can't fit easily through blood vessels and so these vessels may get blocked meaning that oxygen doesn't circulate round the body as it normally would and this can damage major organs. Also, because the red blood cells are killed more quickly, this can lead to anaemia.
Vaccinations
The development of a vaccination for malaria is being researched but no adequate vaccination has yet been found.
The development of a malaria vaccination is difficult because of the stages of the development of the parasite within the human. Most vaccines develop antigens which attack parasites in the fluid outside of the cells, but not within cells.
However, the main stages of development of the malaria parasite in the human take place within cells (the liver cells and the RBCs) (see biology pages). This means that if a vaccine is used which develops antigens to attack the parasite in the fluid outside of the cells, this will not stop the development of the parasite at the other stages.
To overcome this, researchers are working on the development of a vaccine which is a combination vaccine. This would attack the parasite in all stages at the same time.
2. Which 2 of these methods of controlling malaria are known as larvicide? (Select your choices then click submit. Unless you get both right you will be told you are incorrect. To try again, first click reset.)
4. Which of these drugs is now rarely used because malaria has become resistant to it?
