Climate Change and Infectious Disease
Various studies have predicted that one of the main effects of global climate change will be more extreme wet and dry seasons around the world. This is because as global temperatures rise, more water evaporation occurs, leading to more vapor in the atmosphere and the potential for stronger storms. This also works the other way around – cooler temperatures will result in colder seasons overall, less water evaporation, and therefore a drier season with less rainfall.
Global Temperature Record. Source: NASA
Picture the aftermath of a typical hurricane, typhoon, or powerful tropical storm. Widespread flooding commonly occurs after the storm has passed, creating an ideal breeding ground for mosquitoes. People are also likely to be out in the aftermath doing cleanup and humanitarian work, enabling mosquitoes to bite more people and potentially spread more disease. As extreme weather events become more common with climate change, it is important to understand how the spread of infectious diseases, including vector-borne diseases, foodborne diseases, and waterborne diseases may change as well as consider the accompanying implications for human health.
Vector-borne diseases are illnesses transmitted by the bite of an infected arthropod species or vector such as ticks, mosquitoes, and sandflies. Vector-borne diseases represent over 17% of the global infectious disease burden. Mosquitoes are common vectors of infectious disease and can infect humans and other animals with the virus or parasite. Infected mosquitoes can spread a variety of deadly viruses including Dengue, West Nile Virus, Chikungunya, and Zika, and parasites such as malaria. In fact, each year, almost 700 million people across the globe acquire a mosquito-borne illness and about one million people die from their infection.
A variety of demographic, social, and environmental factors affect the distribution of vector-borne diseases across the globe. Vector-borne diseases are typically most prevalent in tropical and sub-tropical regions and affect the poorest populations that have limited access to safe water, proper sanitation, and acceptable housing. Climatic conditions can also affect transmission patterns, spread, and re-emergence of vector-borne diseases. For example, mosquitoes thrive in warm climates, which will enable them to reproduce and develop more quickly and expand their habitats as temperatures rise in areas previously inhospitable to mosquito survival. By 2050, two mosquito species Aedes aegypti and Aedes albopictus will dramatically expand their range and threaten almost 50% of the world’s population. Changing precipitation patterns can also affect the distribution of vector-borne diseases. More precipitation might enable more vector-breeding sites, however, drought may also increase the number of vector-breeding sites due to the need for rainfall collection in storage bins, which facilitate vector-breeding activity. Thus, while the relationship between infectious disease patterns and climate change is highly complex, we do know that climate change will have tremendous implications for the spread and control of vector-borne diseases.
The Centers for Disease Control and Prevention (CDC) also highlights another category of disease that has the potential to increase as a result of climate change: foodborne and waterborne diseases.
Foodborne disease is caused primarily by microbes that attach themselves to food at any stage of its production or cultivation ranging from planting to packaging. As a result, it can often be difficult to determine where the disease outbreak occurred along the production chain. The World Health Organization estimates that over 200 diseases are caused by eating food that is contaminated with bacteria, viruses, parasites or chemical substances such as heavy metals. The resulting outbreaks are estimated to have led to an additional 600 million illnesses and 420,000 premature deaths in 2010. These effects are felt primarily in low- to middle-income countries and will likely increase in these areas as climate change progresses. Climate change is also responsible for an increased survival rate and distribution of pathogens on food material. Certain conditions under climate change are ripe for these pathogens to reproduce more readily and be more potent.
Waterborne diseases can spread via bacteria that live in water or food and are particularly prevalent in the aftermath of a natural disaster. These diseases can be transmitted via fluids, flies, fingers, floods, and fields. One of the most well-known waterborne diseases is diarrhea which is caused by a variety of infectious agents including Vibrio cholerae (cholera), E. coli, Salmonella typhi (typhoid), Campylobacter, and Cryptosporidium. Diarrheal disease accounts for 1 in 9 child deaths across the globe and is the second leading cause of death in children under five years old.
Another well-known waterborne disease is Cryptosporidiosis. In 1993, Milwaukee, Wisconsin experienced the heaviest rainfall event in over 50 years which resulted in the largest documented waterborne disease outbreak in United States history. Almost 400,000 people became infected with Cryptosporidium and 104 people died, many of whom were suffering from Acquired Immunodeficiency Syndrome (AIDS).
Just like vector-borne diseases and foodborne diseases, climate change can influence the spread of waterborne diseases. Higher temperatures can promote pathogen proliferation in water and higher rainfall amounts can increase the likelihood of contaminated rainwater. For example, a 1oC temperature increase was linked with a two-fold increase in cholera case counts in Zanzibar. Similarly, a rainfall increase of 10 mm above the threshold caused cholera cases to increase by 14% in Bangladesh. Flooding caused by climate change can also cause latrines to overflow, the loss of safe drinking water, and the displacement of people which can collectively foster the spread of infectious diseases.
It is clear that climate change has the potential to greatly increase both the frequency and efficacy of infectious diseases of all types. Preparations and safeguards can be implemented to curtail the negative effects of these diseases; however the most effective means of combating these changes will be to address the changing climate as well. Infectious disease productivity increases can be pointed to as one of the many reasons why we should take active steps to address climate change.