A high-profile donation from the Bill and Melinda Gates Foundation threw a spotlight on neglected tropical diseases (NTD) earlier this year, but many fear progress toward eliminating some of these diseases could be scuppered by climate change.
NTDs are a group of chronic, disabling conditions - ranging from bacterial and parasitic infections to snakebites - affecting more than a billion people worldwide. These diseases contribute to an ongoing cycle of poverty and stigma, with many infected people unable to work or attend school. Controlling NTDs will be critical to the health and economic growth of developing countries - which are also disproportionately vulnerable to climate change.
Mark Booth, deputy director of the Wolfson Research Institute for Health and Wellbeing, said, “There’s a great deal of uncertainty. We’re not sure how much climate change will affect [disease] transmission rates. We’re not sure what will happen with the environment and how people and governments will adapt.”
Many researchers are concerned changes in rainfall patterns could result in a loss of “endemic stability” for diseases carried by mosquitoes, ticks and other disease hosts, and that environmental conditions could become favourable for disease transmission.
Rising average annual temperatures have already been blamed for increasing numbers of malarial mosquitoes at higher altitudes in Tanzania and Kenya. Increasing numbers of malaria cases have also been reported in the highlands of Madagascar, Rwanda and Ethiopia. While malaria is not considered an NTD, similar patterns of expansion are feared for other tropical diseases carried by mosquitoes and flies. Temperature, rainfall and runoff could also affect the growth and spread of microbial diseases.
But many researchers believe there is simply not enough information available to warrant alarm.
Speakers at the International Society for Neglected Tropical Diseases (ISNTD) Bites conference in London, in October, were divided over the extent to which climate change is already having an impact on NTD control and whether it should be given priority over other disease drivers.
World Health Organization (WHO) senior scientist Diarmid Campbell-Lendrum said, “Unfortunately I think the discussion has got in to a rather unconstructive, polarized debate. There are those on one side that blame all changes in NTDs on climate change - a disaster scenario - and those that think climate change has nothing to do with it at all.”
He continued, “I think climate change really has to go in to the mix of all of the things that we need to consider. Nobody disputes that there is evident climate sensitivity in some of these diseases, so it would be illogical to exclude it.”
Booth is calling for more research into the issue. “My feeling is not enough attention is being paid to climate change in relation to NTD control,” he said. He believes that climate change is “so complex that no one discipline - epidemiologists, ecologists, climate scientists - will be able to work it out.”
Booth describes a “paucity” of information, emphasizing the need to increase field work on environmental changes and to create a more robust surveillance system for NTDs.
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Courtney Murdock, a researcher at Pennsylvania State University, echoed these sentiments. “We don’t have enough of an understanding of the basic ecology of mosquitoes and their parasites - we still don’t know much about how they interact with the environment and how changes to microclimate will impact their physiology, development in larvae stage, their biting range, etc.,” she said. “Some of this work has already been done in the lab, but we need to move this into the field.”
Other human factors
Climate change, caused by the release of human-produced greenhouse gases into the atmosphere, is expected to have wide-reaching effects in relatively short order. But other human factors are sure to have an effect on the spread of NTDs, as well.
Alan Fenwick, a professor of tropical parasitology at Imperial College London, said that everyone working in his field “is aware of climate change.” But, he said, “in my opinion, more of what we’re seeing at the moment that impacts these diseases is man-made - so we’re talking about, for example, changes in water distribution, irrigation schemes… We also see increasing urbanization, with more people moving in to cities.”
Julian Entwistle, from pest management business consultancy Xenex Associates, believes there is evidence challenging the notion that climate change will lead to the rapid expansion of some NTDs, and says the movement of goods should be considered foremost among the drivers of vector-borne diseases.
The international trade in car tyres, for example, facilitates the spread of mosquitos that carry the NTD dengue fever and other illnesses; the water that accumulates in tyres offers a breeding ground for the insects. To control the spread of these diseases, said Entwistle, tyre importation could be regulated and tyre storage locations could be reported and inspected.
Additionally, many NTDs are linked to poor sanitation, meaning advances in living standards could mitigate their spread. “Movements into areas of relative affluence - that have running water and sanitation - we know can stop NTDs. But if [people] are moving into packed slums, this has the opposite effect,” Fenwick said.
But in most cases, the consequences of human activity are simply unknown. Here, too, better research networks and disease surveillance will be essential.
“Although the argument over determinants is scientifically interesting, what we really need to do is fund a stronger disease control program that’s more robust in response to increased risk," said WHO’s Campbell-Lendrum.