Insect vectors can transmit viral, bacterial, protozoal and helminth organisms to humans and result in diseases which cause significant morbidity and mortality. Vector-borne diseases including trypanomiasis, which is transmitted by the tsetse fly, leishmaniasis transmitted by the sandfly, Lyme disease transmitted by the deer tick and dengue fever, West Nile virus, yellow fever and malaria all transmitted by mosquitoes could thus pose increased health risks to humans.
In particular, malaria is the most prevalent vector-borne disease globally. It is a disease which is still endemic in 100 countries, meaning that an estimated 3 billion people, or half of the global population are at direct risk of malaria-related illness and death. Up to 1 million deaths are attributable to malaria annually. The bite of Plasmodium-infected female Anopheles mosquitoes and transmission of the protozoan parasites is the cause. Most deaths result from Plasmodium falciparum infections and occur in children under 5 years of age in Sub-Saharan Africa. In addition, each year up to 500 million malaria infections cause illness and disease for children and adults in malaria-endemic countries. There are a number of mathematical models that describe future associations between specific climate change variables and mosquito habitats, breeding conditions and life-span.
Many foresee more permissive environments for mosquitoes and warn of increased malaria transmission and disease risk with global warming. However, a model recently published in the Journal Nature by Gething et al. has provided evidence that climate change will have a small impact on malaria epidemics.
Historically, malaria has been transmitted in both non-tropical and tropical regions. Up until the mid 20th century, it was endemic in parts of the northern hemisphere in Europe, as far north as Finland, and in North America, as far as Canada. The rates of malaria-related illness and death were not dissimilar to those in some malaria-endemic Sub-Saharan Africa countries today. In fact, Anopheles vectors which are capable of malaria transmission still have hospitable habitats in many developed countries today. In England, for example, there are currently six Anopheles species able to transmit malaria. However, due to ecological changes, improvements in human living conditions, and greater access to medical care, malaria has been eradicated from England early in the 20th century. As described in an article describing the risk of malaria today, "one is much more likely to be struck by lightning than to get malaria from an English mosquito".
Similarly, in the USA, malaria eradication was associated with improved living conditions and access to health care. In the US, however, intensive and determined use of the synthetic insecticide DDT (dichlorodiphenyltrichloroethane) to eliminate mosquito vectors was crucial. Although the risk of malaria is now minimal, annual insecticide treatments continue as a public health measure in the US. The goal of these campaigns is to control vectors which transmit West Nile virus, Lyme disease, and dengue virus, amongst others, rather than malaria. In former Soviet block countries it was primarily the advent of DDT that caused rapid reductions in malaria transmission in the mid 20th century.
Within highly malaria-endemic areas today, disease transmission often coincides with the seasonal rains. So, while rainfall, humidity and temperature certainly have important impacts upon malaria, poverty, fragile health and inadequate malaria control systems contribute significantly to malaria risk. Using climate change factors alone will not tell the complex story of malaria transmission, disease and death.
Two types of disease model, 'statistical' and 'process-driven', are commonly used to estimate climate change effects upon vector distributions and provide predictions about the future risk of disease transmission. Steve Lindsay, Professor of Public Health Entomology at the London School of Hygiene and Tropical Medicine considers that "both models work quite nicely" but the process-driven model is the "Rolls Royce" version. The statistical model is largely able to capture the existing climate envelope for vectors by considering pertinent environmental factors: temperature, rainfall and relative humidity. Unlike the statistical model, the process driven model mainly considers temperature as the primary parameter but also takes into account the biology of the vector.
It remains an important goal for modelers to also consider human habitats and environmental conditions, amongst other parameters, in order to provide comprehensive risk models for malaria in association with climate change. But, it's also clear that with increased model complexity could also come more uncertainty in making predictions. Serious concerns remain, however, that models which use specific climate changes variables, primarily temperature, humidity and rainfall, in isolation of other factors result in misleading interpretations of future malaria distribution and impact risks.
Some models predict that increased malaria transmission over a widely dispersed area will accompany changes in specific climate related parameters, posing malaria risks to countries which are currently malaria free. Other models warn of changes in patterns of malaria transmission in highly localized regions for example predicting increased malaria transmission in heavily malaria burdened regions. Further models forecast malaria becoming problematic in high elevation, or semi-arid areas which are currently malaria-free but surrounded by malaria-endemic regions.
The models provide "insight" affirms Lindsay but it also remains "important not to over-interpret conclusions". For Lindsay, statistical models provide "scenarios" and a "foundation for understanding vector borne diseases [and disease] transmission". In order to inform what these first step analyses describe it is important, says Lindsay, to "go to the field" and collect real-life data. At the moment, considers Lindsay "it's almost a routine tool to model distribution of animals by determining their climate envelope".
Paul Reiter, Professor of Medical Entomology at The Pasteur Institute provides a harsher commentary on many predicted associations made between climate change and malaria risk. He considers that many models foresee situations which are analogous to "pour(ing) more water into a glass that is already full". Reiter argues that many publications use models to "sidestep" discussion about the ecology and behavior of humans and the ecology and behavior of vectors. Commenting that mention of individual host or host population immunity is also rare, Reiter adds "it is illogical to suggest that increased temperatures will result in an increased incidence of infections." However, he does agree that "changes in rainfall patterns could alter stability and distribution." Importantly, for Reiter, is an awareness that the "principal determinants [of malaria transmission] are linked to ecological and societal change, politics and economics."
Remarking upon climate change as a "minor" factor to determine future malaria risk, Lindsay observes that with global warming "malaria risk might increase but we're controlling it". Lindsay's comment chimes with the sustained reductions which have been described for malaria morbidity and mortality rates over recent years. Associated renewed efforts and enthusiasm for malaria elimination programs also emphasize the success of malaria control. As such, malaria eradication enthusiasts can identify strong evidence to support the goal of malaria eradication within the next 30 years or so. The 2010 Roll Back Malaria and UNICEF World Malaria Day report estimates that between 2000 and 2010, 1 million lives were saved as a direct impact of the increased financial support for malaria interventions and research. Key among the successful interventions is the use of insecticide treated nets and prompt, effective treatment for diagnoses of malaria with parasites.
Over-emphasis on the global impact climate change in relation to malaria misses the mark on the immediate need to address persistent socio-economic and political factors which drive malaria transmission. Malaria control measures are working and furthermore, there is compelling evidence that reducing the burden of malaria will also have a real impact on reducing other 'unrelated' causes of childhood mortality.
Discussing the importance of climate on disease is not new; malaria even takes its name from an association of mosquitoes with humid, swampy climates which caused the disease of 'bad air'. However, while speculation on the importance of climate for malaria and other vector-borne diseases continues, it is crucial to support anti-disease measures with robust evidence and campaigns which go beyond speculated scenarios. It's important not to take the climate change ride with the wrong ticket. Loosing focus from what works will mean loss of lives, and could result in efforts to combat climate change and vector borne diseases both being discredited.
Economic development together with sustained, and expanded, malaria control and treatment measures are important current and future indicators of malaria risk for endemic countries. Although established as important primary determinants of environmental factors, socio-economic factors are not built into the current mathematical models of climate change; indeed, the question remains about whether such complex models would have real value. In terms of climate change and vector-borne disease, comments Lindsay "more fundamental environmental changes are occurring; urbanization, irrigation, deforestation... By comparison, climate change is subtle as a long term process".