Tackling insect-borne disease whatever the weather
We must reduce the poor's vulnerability to insect-borne disease regardless of climate change, says public health expert Ulisses Confalonieri.
Insect-borne diseases, including dengue fever, malaria, Lyme disease, tick borne encephalitis, West Nile fever and yellow fever, already affect millions of people in both tropical and temperate regions.
Changes in climate brought about by global warming will certainly affect, either directly or indirectly, these diseases' cycles. But exactly how will depend much on local or regional characteristics: the baseline climate; the insect, or 'vector', species involved in transmission; the natural and built environment and the human host populations.
Several recent research papers predict wider geographical distributions for insect-borne diseases, or more intense transmission rates, as global temperatures rise. The Intergovernmental Panel on Climate Change, for example, has acknowledged that tick borne encephalitis is dispersing northwards in Scandinavia and has shifted higher in the mountains of the Czech Republic, as a result of changing temperatures and seasonal patterns.
A small number of papers consider such links between climate and disease premature, pointing to data problems and methodological difficulties in the analyses. The evidence for changes in malaria transmission and distribution in Africa, for example, is not clear, with different authors presenting inconsistent results.
It is certainly true that climate variability and change can affect the complex life-cycles and interactions of vectors, pathogens and hosts (both human and animal) involved in insect-borne diseases. Physical climate parameters, such as temperature or humidity, can affect developmental phases of the vectors and pathogens as well as the hosts' population dynamics. For example, climate has been known to play a role in malaria transmission since the early 20th century, and is included in classical mathematical malaria models.
But the effects of climate change on any given disease will vary depending on the local context. For example, increased rainfall in a dry region may create breeding sites for malaria bearing mosquitoes to thrive, but the same increase in a humid environment, such as tropical rainforests, could wash immature mosquitoes away.
Malaria predictions based on global climate scenarios fail to consider these differences. Nor do they account for regional characteristics of the disease cycle: malaria behaves differently in Africa compared to South America, for example. Regional differences in vector species and human immunity, among other factors, will also influence transmission patterns. Responses to climate shall vary accordingly.
Insect-borne diseases will remain a major public health challenge in the coming decades. But most of the health risks climate change brings will be changing 'old scourges' rather than new problems. The reason there is no malaria in Europe or North America, as well as in most of South America, is not climate unsuitability but because of efficient disease control and monitoring, combined with the destruction of vectors' natural habitats through, for example, urbanisation.
Still, it is possible that environmental changes triggered by a changing global climate system may extend territories where insect-borne diseases are endemic.
The best hope of detecting such extensions is for control programmes to develop strategic surveillance at the borders of current disease distributions, either latitudinal or altitudinal — especially where climatic projections suggest a higher probability of change in the next few decades. Satellite-based environmental surveillance, using hydro-meteorological data, may be important in this process.
Even more useful than disease monitoring will be surveillance of the insect vectors themselves to detect changes in their ranges before disease transmission areas expand.
But the main challenge for societies struggling with insect-borne diseases is to make health systems more efficient through disease control and to make people less vulnerable to infection, for example by improving sanitation, housing and education. This should be a priority regardless of global climate change.
Ulisses E. C. Confalonieri is professor of public health at the Oswaldo Cruz Foundation (FIOCRUZ) in Brazil.