Malaria and global warming

How does global warming affect malaria

Malaria is a very serious disease that can be deadly if not treated.  The disease is caused by a parasite that is transmitted via the bite of infected female Anopheles mosquitoes. It can be prevented and can be treated using antimalarial medication.

According to the World Health Organisation, there were an estimated 228 million cases of malaria worldwide; in that year the estimated number of deaths was 405,000 children under 5 years accounted for 67% (272,000) of the total number of malaria deaths in 2018.

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Having said this, we may be looking at a situation that is getting worse; there may be millions of more deaths from Malaria as global warming sees a spread of the disease to areas which are historically too cool for the mosquito to survive.

A report by the BBC in 2014 discussed some research published in the journal Science.  The research was carried out at the University of Michigan in the US; Professor Mercedes Pascual who carried out the research said, “the impact in terms of increasing the risk of exposure to the disease is very large”.

 

Higher altitude - what is the risk?

In the past, higher altitudes have been blissfully clear of the risk of contracting Malaria. The reason for this is that both the parasite which causes the disease and the mosquito that carries it do not thrive well in cooler temperatures.  Studies are now beginning to show that as the world temperature rises, so does the temperature at higher altitudes and so now, these areas are not necessarily mosquito and parasite free. As a result, the risk of contracting malaria at higher temperatures is growing.

 

Highlands of Colombia and Ethiopia

The highlands of Colombia and Ethiopia are densely populated areas; they have become so heavily populated because people choose to live in the higher and cooler areas where less risk of contracting malaria exists.

These areas have detailed records of malaria cases and temperatures dating from the 1990s to 2005. It was discovered that in warmer years, cases of malaria shifted higher into the mountains and in cooler years dropped again.  It would appear to show a direct correlation between climate temperature and the incidence of malaria cases.

There is a lack of protective immunity against malaria in higher altitude communities.

The scientists carrying out the study for the University of Michigan were concerned about the communities living at higher temperatures in the areas they studied.  Co-author Menno Bouma, senior lecturer at the London School of Hygiene and Tropical Medicine said, “Our latest research suggests that with progressive global warming, malaria will creep up the mountains and spread to new altitude areas. And because these populations lack protective immunity, they will be particularly vulnerable to severe morbidity and mortality”. 

In order to explain this, the definition of ‘acquired protective immunity’ is:

  • Individuals who are repeatedly exposed to malaria develop antibodies against the sporozoite, liver-stage, blood-stage, and/or sexual-stage malaria antigens.

The concerns held by the scientists involved is that the populations that may, in the future, be exposed to the malaria antigen but have no natural immunity to it. This in turn could have devastating results in the communities.

 

Is this assumption about the spread of malaria correct - or is there another possibility?

We have been covering a study which predicts that a warmer climate will increase the spread of malaria - but will it? Researchers at Pennsylvania State University have said that following a study carried out in rodent malaria suggested that warmer temperatures appear to slow the transmission of malaria by reducing the infectiousness of the parasite. In the studies above there is an assumption that whilst increased temperature will increase malaria infections because the parasites involved will develop faster and that the mosquito will be able to acquire, maintain and transmit the parasite will be at a constant rate.

The PSU study has shown that temperature has a more complex effect; they have shown that although the parasites develop faster when at a higher temperature, fewer of them become infectious. Entomologist and author of the study Krijn Paiijmans said,

 “It is a trade-off between parasite development and parasite survival and if you don’t factor this in I think you come to the wrong conclusions”.

 

Conclusion

On first observations, it is easy to assume that as the temperature rises on earth the incidence of malaria will increase because both the mosquito and the parasite do not thrive at lower temperatures.

Is it correct to assume therefore that because one situation, i.e. lower temperature, results in one outcome that the opposite, higher temperature results in the opposite outcome?. As Sarah Reece, a malaria researcher at the University of Edinburgh, UK commented in her review of the study, “although interest in the effects of climate change on the transmission of malaria is increasing, interactions between parasites and mosquitoes are often overlooked. This study demonstrates the importance of paying attention to parasite ecology”

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