Disease and climate change: What could the future look like?

By Laura Watson

Coronavirus is on all our minds right now – but what does our changing climate mean for the future of outbreaks?

Disease incidence and climate change
We have known that climatic conditions affect disease spread throughout human history. Civilisations as early as the Romans knew to retreat to hillsides during the warmer summer months, as malaria was endemic to the lowland areas during the warmer season. Climate change is likely to change the pattern of transmission for all kinds of diseases. For example, some pathogens may no longer be able to survive in certain locations, while they may become more prevalent in others. The Wildlife Conservation Society has identified 12 diseases which are likely to spread and get worse with climate change, including cholera, Ebola, plague and tuberculosis.
Disease spreading vectors like mosquitoes have optimal climatic conditions at which they survive and reproduce. This means that climate change might expand their range to include a much larger geographic area. Amplified seasonal patterns could also put areas at risk for longer portions of the year.

Human exposure to waterborne diseases could also rise as climate change amplifies the contamination of water supplies, as extreme weather events such as hurricanes increase, and sea levels rise. There are also links between increasing prevalence of diseases and ocean warming (such as red tide disease, caused by toxic algal blooms) and increased precipitation in certain regions (such as rift valley fever, and Hantavirus pulmonary syndrome).
People could also become more vulnerable to disease as the climate changes, as the health impacts of increased temperatures (such as increased stress) take hold.

The varying impacts of climate change on human health and disease (Photo: CDC)

The following two case studies illustrate the effects of already occurring climatic changes.

The West Nile Virus
West Nile Virus is a vector-borne disease originating in the West Nile region, transmitted by mosquitoes. It was first reported in around 1937, with human outbreaks reported intermittently since then. Transmission of the disease is impacted by weather conditions, and climatic conditions (temperature, precipitation, humidity and wind) are driving the expansion of the range of the disease’s mosquito vector, and therefore the geographic area affected by the disease.
Recent trends in increased rainfall and ambient temperature, as well as changing migration patterns of bird species have meant that the disease has migrated further north to the USA, Southern Europe and further south to Australia. The first case in the USA occurred in 1999, in New York City. Since then, it has spread across the country, with a reported 39,557 cases of the disease in the USA as of 2013, and only 4 states not reporting a case in 2018.
Climate change has been a key factor here for two reasons. First, increased temperatures correlate with increased viral replication rates, population growth, and transmission. Secondly, increased precipitation and flooding correlates with increased mosquito abundance, due to the use by the insect of stagnant water as a breeding ground. These trends are likely to continue, and the range of the vector and disease will continue to increase, meaning that monitoring of this (and other vector borne diseases like malaria and Zika) is crucial.

Cholera
Cholera is a waterborne bacterial disease which is likely to worsen with climate change. Its spread will be greatly affected by climate change as temperatures warm and precipitation levels rise, because cholera outbreaks, while sporadic, tend to occur in regions associated with higher temperatures and rainfall. In these conditions, water borne diseases can spread inland and thus into more densely populated areas. While there is as yet no clear understanding of the nature of the link between cholera and climate change, it is clear that with more weather extremes, cholera spread and incidence will be affected.
As well as the contamination of water supplies, the abundance and distribution of cholera is affected by sea surface temperatures, ocean currents and weather changes. It has recently been demonstrated that warming seas, a key impact of climate change, are linked to an increase in the presence of cholera bacteria in Europe and the USA. Overall it is clear there is a link between cholera and climate change – and like West Nile Virus, this could affect all of us.


What does this mean for the future?
While some diseases show clear links to climate change, the recent Covid-19 pandemic so far has not. This shows that not all diseases will necessarily become more prevalent as the climate changes. However, it is clear that monitoring, especially of vector borne diseases, remains crucial to understanding what the future will look like. Covid-19 has shown us that there are many diseases in the natural world which are currently unknown to humans. We must act urgently on climate change to lower the pressure on natural systems and to prevent dangerous future outbreaks.

Sources
https://www.who.int/globalchange/summary/en/index5.html
https://www.who.int/globalchange/environment/en/chapter6.pdf
https://www.ipcc.ch/2018/10/08/summary-for-policymakers-of-ipcc-special-report-on-global-warming-of-1-5c-approved-by-governments/ 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4342965/
https://health2016.globalchange.gov/vectorborne-diseases/content/west-nile-virus
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