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Will Bugs Creep North as Climate Heats?

Onome Akpogheneta

With predictions of temperatures rising by the end of this century, what will happen to the bugs that carry disease when the world warms? Will diseases of the Southern Hemisphere become more prevalent in these countries? Will the insects carry their disease burdens to the North? For many statistical models the effect of climate change on vector-borne diseases are real causes for concern. But perhaps predictions about climate change and disease migration support actions to limit climate change rather than highlight the most likely of disease-spread scenarios.

It has been widely hypothesized that global warming will bring new permissive environments for biting insects that can carry viral, bacterial, helminth and protozoal diseases. These insects include mosquitoes, ticks, sandflies, snails and blackflies; all are sensitive to climatic changes. Mosquito-borne diseases including malaria, dengue, viral encephalitides, West Nile fever, filariasis and yellow fever are some which have caused climate change associated concerns and investigations in recent years. Fears persist that newly resident infected mosquitoes could harbour and import diseases to regions and countries from which they have been absent or eradicated. The result could be greater disease prevalence and more disease outbreaks within countries of the Global South, as well as northern migration of disease epidemics. Fears have grown that climate change disease migration could become a global health threat in the 21st century.

While fears have grown, reliable evidence to reflect long-term climate change effects on mosquito-borne diseases has not kept pace. "Detailed scientific evidence remains scanty," says Peter Byass in an 2009 article in Global Health Action magazine; he considers that "entomological studies [are] expensive [and] not seen as high priority for disease surveillance." It is precisely because of "scanty" historical and longitudinal data on vector habitats and associated disease outbreaks that climate change predictions will likely be unreliable for vector-borne diseases. Predictions for simplistic long-term correlations should be made with disclaimers; there's simply not enough data to prove cause and effect.

"We still need evidence of the [long-term] effect of climate change," says Tarekegn Abeku, senior technical specialist in disease prevention at the Malaria Consortium and a contributor to the 2007 report of the International Panel on Climate Change. Abeku says "several studies have shown malaria in highlands might be caused by changes in weather" but these were short-term changes. Short-term studies can inform long-term predictions, but in order to reach conclusions, Abeku says, "you have to have data for decades" and even then the picture is still "very complex."

So what contributes to this complexity? Surely associations could be simply made between climate change factors and disease vectors. Wouldn't more rainfall, higher humidity and higher temperatures simply mean more hospitable areas for mosquitoes? It has long been known that seasonal rains, with hot, humid conditions are frequent correlates with mosquito-borne disease epidemics and outbreaks. Short-term climatic anomalies, such as the El Nino Southern Oscillation, are also known to lead to epidemic spikes and have been associated with malaria and dengue outbreaks in Africa, Asia and South America. In addition, higher temperatures can mean shorter maturation periods for mosquito larvae, and faster blood digestion with more frequent feeding for adult females. But there is a downside for the bugs, too; warming above 34oC could have a deleterious effect on their habitats and lifespan. Higher temperatures are predicted to make arid and semi-arid areas drier, while making mid-to-high latitude areas wetter. As ecosystems change, so too will the distribution of mosquitoes; both increased and reduced rainfall could mean shifting habitats, but these generalized effects will not move in a uniform direction for all mosquitoes.

"It's almost a routine tool to model distribution of [mosquitoes] by determining their climate envelope," says Steve Lindsay, professor of public health entomology at the London School of Hygiene and Tropical Medicine, but it's "important not to over-interpret conclusions." Statistical models and exploratory analyses can provide "insight," "scenarios" and a "foundation for understanding vector-borne diseases [and disease] transmission" but for Lindsay it's crucial to "go to the field" and collect real-life data.

Hypotheses of disease importation with vector migration are unsubstantiated and often overlook evidence that mosquitoes capable of carrying disease are resident and living comfortably in many locations where particular diseases are not endemic. Within 5 years after West Nile Virus was introduced to the U.S. in 1999, the disease spread rapidly from the northeastern corner across the country. "There is little evidence that the entry and establishment of West Nile Virus in the U.S.A. was influenced by climate change," says Walter Tabachnick in the Journal of Experimental Biology this year. Tabachnick describes that the "vectors were present in the U.S.A., and entry of West Nile Virus was not contingent on climate change in North America."

Until the mid 20th century, malaria was endemic in parts of the Northern Hemisphere; in Europe, as far north as Finland, and in North America, as far as Canada. Malaria caused illness and death in these areas at rates comparable to those in some malaria-endemic parts of sub-Saharan Africa today. In England today, there are currently six species able to transmit malaria. Although these malaria-capable vectors persist, Lindsay et al comment in the Malaria Journal that "one is much more likely to be struck by lightning than to get malaria from an English mosquito."

Climate was not a factor associated with malaria eradication from the developed world. It was due to ecological changes in insect habitats, in particular effective mosquito control measures, improvements in human living conditions, and greater access to medical care that malaria was eradicated prior to the advent of man-made global warming. While malaria began to wane as a worldwide public health problem by the mid 20th century, dengue and yellow fever, which belong to the same virus family and share the same mosquito vectors, began to emerge. According to WHO surveillance data, prior to 1960, dengue cases were reported in fewer than five countries; after 1960, the disease began to emerge in all continents, with cases reported in nearly 60 countries by 2007.

The migration of mosquito vectors (and the viruses themselves) had already occurred centuries earlier with transnational shipping and commerce in the 17th and 18th centuries. By the 20th century vectors were already permanent residents in today's disease endemic and non-endemic countries. According to WHO, the re-emergence of dengue after 1960 was associated with "rapid population growth, rural-urban migration." In addition, an increase in water containers and discarded items provided urban larval habitats. It was independent of climate change that mosquito vectors for dengue and yellow fever viruses adapted efficiently to new habitats, maintained close proximity with humans, and became especially well domesticated disease vectors.

Vector-borne diseases rely on the availability of human populations as much as their vectors for disease transmission. As such, it's important to consider more widely how climate change will likely affect a host of other socio-economic and human migration factors. The effect of climate change on human migration patterns could mean movement away from or movement toward disease carrying vectors, decreasing risk for one disease while increasing risk for another, depending on which diseases are endemic where. If global warming drives human migration toward urban areas, it could mean a movement away from largely rural endemic diseases, such as malaria, and toward urban epidemics with diseases like dengue. For filariasis, however, urban-rural human migration patterns may not suggest a clear epidemic outcome; some filariasis vectors have adapted to rural habitats, while others are urban or semi-urban residents.

To make robust associations between climate change and vector-borne disease means ignoring the absence of evidence. Climate change won't cause uniform changes for all vectors and all disease. It will have direct impacts not only on vectors, but also on pathogens and human hosts. Disentangling these multiple climate change effects and proving direct, causal relationships between specific diseases and climate change will likely continue to prove problematic and complex for many decades to come.

Sources

Climate Change and Population Health in Africa: Where are the Scientists?

Challenges in Predicting Climate and Environmental Effects on Vector-Borne Disease Episystems in a Changing World

Assessing the Future Threat from Vivax Malaria in the UK Using 2 Markedly Different Modeling Approaches

World Health Organization: Impact of Dengue

Onome Akpogheneta has a PhD from the London School of Hygiene and Tropical Medicine. Her work has been published in Infection & Immunity, Parasite Immunology, The Faster Times, MalariaWorld, The Periscope Post and Livestrong.