Abstract
In a mammalian or avian host, malaria parasites may face many challenges, but they enjoy two special privileges. The first is that their host provides an almost unvarying temperature, set at a level that happens to be optimal for parasite growth and multiplication. The second privilege is that the host has a life-span that is much longer than that of the parasite. Provided that the parasite does not kill the host – a kind of kamikaze outcome that is in the interests of neither party – the host is likely to live long enough to give the parasite plenty of time to move on.
But malarial protozoa have evolved a second more precarious component of their life-cycle, exploiting the need of female mosquitoes to nourish their eggs with blood. In committing the sexual part of their development to an invertebrate vector with a natural life-span of around 2 months, plasmodia sacrifice both of the privileges enjoyed in their previous warm-blooded host. The mosquito, being poikilothermic, adopts the temperature of the surrounding air, and the parasite has no choice but to do the same. At approximately 18°C the parasite develops so slowly that it would require about 2 months to complete its development up to the stage of filling the mosquito’s salivary glands with sporozoites – by which time there is only a 1 in approximately 300 chance that the mosquito will still be alive. At about 22°C the parasite can complete its development more than twice as fast, with a 1 in approximately 6 chance of completion during the lifetime of the vector and therefore of transmission to a new vertebrate host. At higher temperatures parasite development is faster still.
These characteristics of the life-cycle of plasmodia account for two features of the disease with which we are all familiar. Once infected, a person can develop a fulminant malaria illness at any altitude or ambient temperature to which s/he subsequently travels; but transmission of malaria cannot occur if the ambient temperature is consistently below about 17–18°C.
At low temperatures of this order, mosquitoes themselves develop more slowly through their larval stages, so that there are fewer of them available to serve as malaria vectors anyway.
The dependence of malaria transmission on ambient temperature has been recognized for millennia. Malaria flourishes in hot climates. People move to hills not only for the pleasantness of cooler air, but because they get less malaria. Even in tropical areas, altitude is a major determinant of ambient temperature. It has been uncommon for malaria to flourish above altitudes of approximately 1500 metres above sea level (Figure 1).
Ethiopian highlands 2014. Photo by Anne and Gordon Stinson.
With current predictions about global warming, it is not surprising that the question has frequently been asked: will malaria transmission move to higher ground as temperatures rise? This is an important concern, because many people live in areas just higher than 1500 metres – for example 37 million Ethiopians live at altitudes between 1600 and 2400 metres. 1 An upward extension of malaria transmission could lead not only to many more infections, but to infections among people without immunity, who are likely to be more susceptible to severe and sometimes fatal disease than are their partially-immune compatriots living in lower and warmer territories.
It may seem logically obvious that malaria transmission will extend upwards with global warming, but the phenomenon has been surprisingly difficult to prove by direct observation, and remains controversial. Hay and colleagues approached the question the other way round, 2 and asked whether there had been any temperature rise in four high-altitude sites in East Africa where increases in malaria had been documented in the 1980s and 1990s. They found no evidence of increases in well-documented ambient temperatures at any of the four sites between 1911 and 1995. This study did not prove that a temperature rise would have no effect on malaria, but only that in these sites, and at that time, the upsurge of upland malaria probably had other causes. This study was challenged by a re-analysis of the same data at the same four sites and period by another group, 3 who found ‘evidence of a significant warming trend at all sites’.
Many authors have emphasized the complexity and multiplicity of factors – in addition to altitude and consequent ambient temperatures – that may affect the transmission of plasmodia. To name but a few, these factors include rainfall and foliage, density of the human population, human migration, access to health services, indoor temperatures (especially for endophilic mosquitoes, and for poorly ventilated huts with many occupants 4 ), diurnal temperature patterns, deforestation, malaria control measures, drug resistance 5 and insecticide resistance. Some of these factors correlate to some degree with ambient temperature and therefore are confounders in assessments of the role of climate or altitude in affecting malaria.
In a new approach
1
to the question of whether a rise in ambient temperature may lead to an extension of malaria transmission to higher ground, a collaborative group involving Ethiopian, Colombian, British and American universities have studied records of P. falciparum malaria cases in 124 municipalities in the Antioquia region in western Colombia (1990–2005) and in 159 kebeles in the Debre Zeit area of central Ethiopia (1993–2005), both areas around 1600 metres in altitude (Figure 2). They found that when the environmental temperature was highest, especially in two hot years 1997 and 2002, malaria cases were found at higher altitudes than at other times, in both Colombia and Ethiopia. The authors believe that the similarity of findings in two different continents strengthens the case for temperature being the principal cause of the upward spread of malaria illnesses in these populations.
Ethiopian highlands 2014. Photo by Anne and Gordon Stinson.
If global warming occurs as predicted, both logic and observation seem to point to a likelihood that many highland populations will become exposed to malaria, either in the form of epidemics or of a gradual extension of the parasites’ territory. Should this happen, the steady decline in WHO estimates of the annual number of malaria illnesses and deaths worldwide over the past few years (most recent estimates are roughly 207 million and 627,000 respectively 6 ), may be slowed or even reversed.
What then should we be doing about the risk of increasing highland malaria? Our current experience and thinking – about elimination of malaria from low-transmission sites, and the prediction and identification of epidemics – must be brought to bear on areas at risk: rapid and accurate diagnosis of human malaria infections in these areas will allow concentration of efforts on the most threatened populations.
To make such preparedness possible requires recognition of where these mid-altitude risk-areas are. We need to make sure that communities and health professionals in those areas are informed of the dangers and are equipped with the necessary materials and protocols to act quickly if malaria starts to get above itself.