Malaria in Summer

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QUESTION

Does malaria only occur during summer season?

ANSWER

That depends on where you are. The transmission of malaria depends on the presence of the mosquitoes which are required to transmit the disease (they do this when they bite you).

Many kinds of mosquito transmit malaria, though all are of the genus Anopheles. These different species have different climatic requirements, but all lay their eggs in pools of stagnant water, and the larve likewise live in this stagnant water until they develop into adults. As such, malaria is only transmitted when there are suitable pools of standing, stagnant water available for mosquitoes to breed, and also when the temperature is suitable for mosquito development (optimum temperature for mosquitoes is 25-27 degrees C—the malaria parasite develops most rapidly around this temperature as well, though can survive in temperatures about ten degrees cooler as well).

In some regions of the world, this combination of conditions is only met in the summer time, which means that malaria transmission only occurs during this season. In other parts of the world, such as coastal West Africa, conditions are suitable for mosquito breeding and malaria development all year round, which means that malaria transmission occurs throughout the year.

Geographic Range of Malaria

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QUESTION

Has the geographic range of malaria increased over the past 20 – 30 years? I have read that preventative measures have helped lower rates of infection, but I’m interested in the extension of the range itself.

ANSWER

That is a very interesting question, and one that garners quite a lot of debate. Preventative measures have actually also helped to limit the range of malaria globally. For example, malaria used to be relatively common in the Mediterranean basin and south-eastern United States, but control measures (mainly based around killing mosquitoes and removing suitable mosquito habitat) has largely eradicated malaria from these areas.

However, there is concern that on-going and future climate change has and will change the  distribution of malaria globally. For example, some predictions have suggested that malaria might be able to re-establish itself in the Mediterranean and Middle East, due to higher rainfall and higher winter minimums of temperature. Additionally, malaria may be able to spread to higher altitudes in areas where it is already present at low elevations. This is of huge concern in places like Kenya: Nairobi, the capital city (with around 5million people), sits at 1660 m altitude, and as such currently has generally negligible levels of malaria transmission. However, if climate change enables malaria to move up to this altitude, a huge number of people will be at additional risk of infection. Worryingly, there is some evidence from the Kenyan highlands that these changes are already underway.

Having said this, there are also parts of the world which might see malaria transmission ease as a result of climate change. This is particularly the case where rainfall is expected to decrease, or change significantly in relation to temperature. Moreover, some scientists think that on-going control efforts, particularly with respect to the distribution of bednets, vector control and greater coverage of diagnosis and treatment will continue to reduce the geographical spread of malaria over and beyond the changes associated with climate change. These scientists have compiled a map of Plasmodium falciparum transmission now as compared to data from before control interventions were rolled out—the reduction of transmission risk in many parts of the world, are clear to see (see below).

changing plasmodium falciparum map

Maps showing changes in transmission risk and endemicity of Plasmodium falciparum malaria between approximately 1900 (a) and now (b). (c) shows the balance of change in malaria transmission between the two time periods: the higher the negative number, the greater the reduction in malaria transmission. A positive number indicates increased malaria transmission. The different classes of malaria transmission risk are as follows: hypoendemic, prevalence < 10%; mesoendemic, PR ≥ 10% and < 50%; hyperendemic, prevalence ≥ 50% and < 75%; holoendemic, prevalence ≥ 75%. Image reproduced here from Gething et al., (2010), 'Climate change and the global malaria recession', in Nature, volume 465, pages 342-345.

What countries have malaria?

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QUESTION

In which countries can it be found?

ANSWER

Malaria is found in many parts of Africa, Latin America, Asia, and Oceania. The exact distribution depends on climatic variables (namely heat and sufficient rainfall, for the development of the mosquitoes which transmit malaria) and also how successful a particular country or region has been at controlling malaria.

For example, malaria was once found in parts of the U.S. and Europe, but successful campaigns to control mosquitoes and treat cases led to elimination of transmission. The same is true in other parts of the world; for example, in Malaysia, transmission is successfully controlled in Kuala Lumpur, but malaria can still be found in many parts of the surrounding countryside.

Please visit the CDC website for an interactive map of malaria distribution around the world.

Malaria in Africa

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QUESTION

Why do Africans catch malaria more than others?

ANSWER

There are a number of reasons why malaria is more widespread in Africa than in many other parts of the world. However, it is worth mentioning that other parts of the world, such as India and south-east Asia, also have very high prevalence of malaria, especially in rural areas.

The high transmission of malaria is Africa is predominantly due to two factors: climate and control measures, or rather, the lack of them.

Malaria is spread by mosquitoes of the genus Anopheles, and so in order to persist, an area must have a suitable temperature for the development of both the mosquito as well as the malaria parasite. This limits malaria transmission to the sub-tropics and tropics, primarily. The area must also have sufficient rainfall and areas of standing water, since the malaria mosquitoes lay their eggs in stagnant water, which the larvae live in until they pupate into adults. This means that malaria transmission cannot occur in desert regions.

Unfortunately, a large portion of Africa, and particularly West, Central and East Africa, are climatically very well suited to the development of mosquitoes and thus the transmission of malaria.

In addition, many countries in Africa are not as developed as other tropical countries. This means that health resources have not been as focused on control efforts in Africa—for example, Malaysia very successfully reduced malaria transmission by a huge amount through a combination of vector control (namely spraying households with insecticides and filling up stagnant water pools so larvae couldn’t develop), distribution of bednets (which reduces mosquito biting rate) and better diagnoses and treatment facilities.

All of these efforts are beginning to be developed and rolled out in Africa as well, so hopefully in the near future we will also see a dramatic reduction in malaria transmission in Africa.