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.

Epidemiology of Malaria

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QUESTION:

Epidemology of malaria

ANSWER:

I’m afraid that without a bit more detail in the question, it is difficult to answer – there are many facets to the epidemiology of malaria, from the distribution of the disease globally to its prevalence in different age and risk groups among human populations. Human migration and movement can also play a large role in the epidemiology of malaria, as can mosquito vector behaviour and population dynamics.

Malaria is also known to vary widely based on climatic conditions, and there are concerns that climate change will affect what we know about the epidemiology of malaria, making it harder to control. There are also cases of cross-over between human forms of malaria and those from other primates, such as with Plasmodium knowlesi in macaques in south-east Asia. If you re-submit a more specific question, I’ll be happy to answer it more fully!

Cases of Malaria

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QUESTION:

What are reasons for the increasing number of cases of malaria?

ANSWER:

As I recently wrote in answer to another Q&A post, it is difficult to determine whether cases of malaria are indeed increasing or not. For one, a large number of cases are not reported every year, making accurate estimates difficult. Secondly, the world’s population is growing, and it is growing at the greatest rate in Africa, where the majority of malaria cases occur. As such, even if the proportion of people with malaria decreases over time, due to health initiatives such as distributing long-lasting insecticide treated bednets or free treatment, the total number of cases may still rise. Another problem we face in the fight against malaria is climate change: as the world’s patterns of rainfall and temperatures change, new areas become susceptible to malaria transmission, putting more people at risk. However, what is very encouraging is that deaths from malaria seem to be decreasing on a global scale. Malaria No More is an organisation dedicated to eliminating deaths from malaria by the year 2015; more information about their methods and some of their success stories can be found on their website.

Malaria in Southern United States

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QUESTION:

Why is there not a prevalence of malaria in the southern United States when we are bitten almost daily by the “little beasts?”

ANSWER:

Malaria once was relatively common in the southern United States. Transmission used to be possible due to the favorable climatic conditions for the development both of the mosquito as well as the malaria parasite. Huge advances in the control and treatment of malaria were made directly as a result of increased interest in the disease after the US occupation of Cuba and the building of the Panama Canal in the early years of the 20th century. This vastly reduced the number of cases of the disease, but the final, concerted effort to eradicate malaria came in the 1940s.

This was due to a federal public health program called the National Malaria Eradication Program (NMEP), and as a result of its actions, malaria transmission was halted throughout the United States by 1951. The program was launched in 1947, coordinated by the newly formed Communicable Disease Center (now the Center for Disease Control and Prevention, or CDC) and mostly involved reducing the number of mosquitoes in and around people’s homes. This was done through the wide-spread spraying of DDT—during the years of NMEP, it has been estimated that more than 6.5 million homes were sprayed with the insecticide. Alongside spraying, mosquito breeding habitats were also removed, through wetland drainage, and human monitoring and treatment efforts were stepped up. By 1949, malaria was no longer considered a disease of public health importance, and it was declared eradicated from the United States in 1951.

Having said that, the species of mosquito that transmit malaria still exist in the USA, and particularly in the southern states, which means that there is always a risk of small, localized outbreaks of the disease, particularly during hot and wet seasons.

Climate change may also increase the zones where malaria is at risk of being able to develop within the United States. For this reason, the CDC continually monitors the small number of cases reported each year in the USA (there were about 1500 cases in 2007—all but four of these cases, however, were the result of travelers to malarial areas outside of the USA bringing the disease back with them) to ensure that they are prepared and well-informed should an outbreak arise.

Introduction of malaria

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QUESTION:

What is the introduction of malaria?

ANSWER:

Malaria is introduced into the human host via mosquitoes, who pass the parasite into the human blood stream through their mouthparts as they take a blood meal. Since the development of both the mosquito and the parasite are temperature-dependent, and tend to favour warm, humid conditions (mosquito larvae require pools of still water to develop into adults), the distribution of malaria is limited to parts of the world where these temperature and humidity conditions are met.

However, cases of malaria can also be introduced into other parts of the world by travellers coming from malarial regions or by mosquitoes, already infected with malaria, being accidentally transported into non-malarial countries in aeroplanes or in ships. These kinds of introductions resulted in about 1500 cases of malaria reported in the United States in 2007, a country which is usually free of malarial transmission. These introductions are rare, and also short lived, as the conditions do not favour the continued survival of the transmission cycle. However, this is one of the worries regarding climate change; it may make some areas of the world, which currently do not have malaria, more climatically suited to transmission of the disease, thus allowing for introductions of the mosquitoes and the parasite in the future.