Arguments
Malaria: biting into the climate change debate
Posted on 23 April 2013 by Sarah Finlay-Jones
This article was written by a student in the Science Communication program at the University of Western Australia. In the unit, Introduction to Scientific Practices, students learn how to effectively present scientific information for different audiences. As part of the process of learning to distil complex science into simple, engaging formats, structured tutorials are given on how to use The Debunking Handbook. This article was submitted as part of an assignment on writing an engaging article for a non-specialist audience.
With modern drugs, insecticides and the prospects for vaccination, malaria should be a thing of the past. However, our ability to keep malaria at bay is now facing a new challenge - climate change.
Malaria is an infectious disease which results in the infection of red blood cells, and reduces the blood’s oxygen-carrying capacity. The parasite Plasmodium that causes malaria can be divided into four different species. The most common species of malaria is P. vivax, and is found in temperate climate zones, as well as in the tropics. Within the tropic and sub-tropic regions, where malaria is a large problem, P. falciparum is also a very common species of malaria. Female mosquitoes are the transmitters of the malaria parasite, and therefore the infection rate of malaria relies heavily on the survival of female mosquito populations.
Mosquitoes transfer the parasite when consuming blood from another organism. In humans, the malaria parasite only takes 30 minutes to enter the liver once it has entered the bloodstream. From here an infection develops within the red blood cells, which eventually burst and infect other red blood cells. The malaria parasite can also be transferred from humans to mosquitoes.
Predicting the effect of climate change on malaria prevalence is a complex exercise. Several models of global climate change have been used to make predictions based on a variety of possible climate change scenarios. These studies concluded that climate change will have a definite impact on the prevalence of malaria, but will affect different regions to different extents.
A major component of the threat climate has on humanity is that average global temperatures have been rising over the past century, and are predicted to continue rising.
The time in which it takes the parasite to develop whilst in the female mosquito is exponentially related to temperature, meaning that only a slight increase in temperature can result in a several-fold greater increase in the development rate of the parasite. However this rapid increase is only present for a restricted range of temperatures, after which the proportion of surviving parasites rapidly decreases, as the accelerated development of the parasite can no longer outweigh the decreased life expectancy of the mosquito.
For P. vivax, the minimum temperature at which it can develop is between 14.5 and 15°C, and between 16 and 19°C for P. falciparum. At temperatures beyond 32-34°C, the development rates of both P. vivax and P. falciparum begin to decrease.
With increasing environmental temperature, the rate at which mosquitoes can digest human blood also increases. This means that with increasing temperature, mosquitoes will be able to consume more blood, and therefore bite and consequently infect, more humans.
Another scenario which these climate change models predict is an increase in global rainfall levels. Sustained rainfall is a key factor for the survival of mosquito populations. This is because mosquitoes usually breed in freshwater pools or marshes, rather than in flowing water. If rainfall does increase as predicted, we will see an increase in the number of suitable breeding habitats for mosquitoes, which will in turn lead to mosquito populations increasing, and therefore the total population of infected mosquitoes is also set to increase.
The effect climate change has on malaria prevalence is expected to affect different regions of the world in different ways. In the temperate climates of Asia, Northern Europe and North America, the mosquito vector is already present, but it is too cold for the parasite to develop. An increase in temperature in these geographical regions will result in the climate becoming more suitable for parasite development, leading to the possibility of an epidemic.
Highland regions, such as those in Eastern Africa and the Andes region in South America, will also experience an increase in the prevalence of malaria due to this same increase in temperature. However the areas that are most at risk of an outbreak of malaria are those regions bordering areas where malaria is currently prevalent.
The predicted effects of climate change on malaria prevalence could have devastating effects on global health, as well as the economy, as the cost of curing and preventing this often deadly disease is large. In order to prevent the disease from becoming more widespread, action must be taken now, before mosquito populations and malaria infested regions become too large for us to combat.
Bibliography
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It's ironic that I was drinking a glass of tonic water (quinine) when I ran into this article. :-)
Is there a map showing the potential expansion of malaria throughout the world?
Great article! Thanks for all the references. I remember talking at a conference with some folks modelling malaria. According to this quick talk climate change is indeed a compounding factor, but second to the interplay of urbanization vs hygiene. I could not find such disucssion on the references. Do we know the "importance" of each factor? I´m pretty sure they ballparked 10% for climate on their models.
- On one side urbanization, populazion growth and populations shifts in general, which increasing vectors of malaria.
- Education, hygiene and public health, which can be very effective in reducing those risks.
That seems to make sense but is hard to find research on that. This is the closest I´ve found:
http://www.malariajournal.com/content/10/1/188
Kudos for the blog, and the article!!
Never mind, here it is,
http://www.sciencedirect.com/science/article/pii/S0959378011000859
http://ars.els-cdn.com/content/image/1-s2.0-S0959378011000859-gr2.sml
The article is paywalled but the maps are freely available.
Malaria researcher Paul Reiter has a different take on this issue: The Malaria Myths of Climate Change.
His tone in that article strikes me as more polemical than objective, but it would be useful to see a point-by-point response - if there is one.
I think he's "polemical" for the same reason Roger Pielke Jr. is "polemical" about climate change and weather disasters (in particular hurricanes) and Jesse Ausubel is "polemical" about trends in carbon dioxide emissions.
They are men who have actually studied the science of the particular issues and are annoyed to see it misrepresented for the purposes of quickly reducing emissions of CO2.
I can understand why Paul Reiter would be particularly annoyed, because the focus on global warming actually turns attention *away* from measures that actually reduce malaria mortality, and thus promotes more malaria deaths than would otherwise occur.
Mark Bahner @5, like Pielke Jnr (who has built a career on the assumption that improved building codes have no effect on the risk of destruction by hurricanes), it appears to me that Reiter is pulling a swifty.
Specifically, consider the following two maps:
(Source)
The bottom map shows the historical distribution of malaria, while the upper map shows the frequency distribution of sickle cell anemia alleles.
Sickle cell anemia is very interesting for the study of evolution. It is invariably lethal without intensive modern medical care (and often with it) if it is inherited from both parents. If inherited from one parent, however, it confers a large degree of immunity to malaria. It follows that high frequencies of sickle cell anemia will only occur where there are high frequencies of malarial infection (also normally fatal without modern medical care). We can infer from the map above that the distrubtion of malaria in the second map leaves out an important component - the frequency of infection:
(Source)
Buggirl provides the following translation for the medically illiterate:
"To translate: the different colors relate to the level of infection in the general population (PR, or Parasite rate). ”Endemic” means that the infection is maintained in a community at a more or less steady state.
Epidemic/Unstable means that infections break out periodically in these regions
Hypoendemic: less than 10% of the population is infected with malaria
Mesoendemic: between 10% and <50% is infected with malaria
Hyperendemic: Between 50% and75% is infected with malaria
Holoendemic: over 75% of the population is infected with malaria"
As can be seen from map a in the second figure, the historical distribution of malaria is just as Reiter says; but the frequency of infection in London and lapland was very low, only occuring in occasional outbreaks. In constrast, in warm hot regions in Africa where sickle cell anemia is frequent, over 75% of the population was infected; or more correctly, 75% of the population without a sickle cell anemia allele.
High altitude, high latitude or ariddity significantly reduce rates of infection.
That picture is not complete. There is a far more extensive distribution of Thalassemia, another frequenlty fatal blood disease that is endemic in populations because it confers a partial immunity to malaria:
(Source)
You will note the complemental distribution with sickle cell anemia in areas of frequent endemic malarial infection.
So, will global warming push malaria into areas it has not occurred before? No, certainly not with modern control methods. Reiter is correct about that. But will it expand the area in which malarial infection is endemic and so frequent that having a genetic disorder that gives a 25% chance of each child dying makes good genetic sense? Absolutely. No scientist merely trying to correct the record would fail to mention the important factor of frequency of infection. Ergo Reiter's interventions are not in the interests of accuracy, but of politics.
Hopefully modern control measures can keep up with that risk, although global warming has a kicker there as well. The warmer the climate the greater the number of generations per year, and hence the quicker they will evolve immunity to various control measures.
Very good point, Tom Curtis. Thank you.
There is an important point missing her. Malaria has two reservoirs, mosquitos and people. You can wipe it out in an area by decimating the parasite in EITHER of the two.
Artemisinin triple combination therapy can wipe it out in humans before it ping pongs back to the bugs, but there are more pressing issues than climate change at this time. First, Artemisinin resistant strains are starting to appear mostly in SE Asia, probably associated with "cut" drugs. Second, we need a cheap test for infection because right now the more expensive Artemisinin based therapy is being over used.
(snip)
[Sph] Agreed, but still a violation of the comments policy.