Does Climate Change Impact Infectious Diseases?

Written by Rebecca Gordon

Looking out the window this winter, we might notice a good view of dead grass, warmer temperatures, and a greater chance of disease. Cold temperatures and snow arrive progressively later each year, leading to mild winters. Over the past century, winter temperatures have increased by 3 degrees Fahrenheit [1]. This warming could lead to ecological impacts on diseases transmitted by insects because of the extended season for insect breeding and biting. In more temperate regions, we might think of ticks and Lyme disease.

Figure 1. A tick is going on a quest for a blood meal. Photo credit: Erik_Karits via Pixabay

Lyme disease became evident in the 1980s in New England (USA). It is now estimated to sicken over 450,000 people and has spread throughout North America [3]. Ticks can emerge from their winter slumber earlier in the spring, explode in population during the summer, persist well into the fall, and potentially survive during mild winter months. Cold temperatures below -5C (23 F) can kill exposed ticks, but they can live down to -20C (-4 F) if buried deep in leaf litter [2]. Thus, as the average temperature over winter increases, ticks and tick bites can be a year-round concern, contributing to the rise of tick-borne disease.

But what of those in the tropical and subtropical regions? These regions tend to be warm year-round and experience greater humidity, greater biodiversity, and higher rates of poverty. Poverty is one of the greatest risk factors for Neglected Tropical Diseases (NTDs), along with high population density, lack of proper housing, political instability, poor water sanitation, and lack of access to medical care and disease treatments. Currently 17 diseases are categorized as NTDs, as identified by the World Health Organization (WHO), and they affect over a billion people around the world. Some NTDs include viral (Dengue fever and rabies), bacterial (Leprosy, Buruli Ulcer, Trachoma, Treponematoses), and parasitic infections (Chagas Disease, Leishmaniases, and Schistosomiasis just to name a few) [4]. In addition to the increased risk of disease, individuals in the tropics are more susceptible to the effects of climate change, such as rising sea levels, droughts, floods, and extreme weather. All combined, factors associated with climate change can negatively impact the lives of people living in tropical regions, influencing human migration which can have an effect on the regions in which these diseases are localized. Climate change will also affect the environment in which the animal vectors of these NTDs can live, adding to the danger of the diseases spreading to new areas.

Figure 2. Picture of a mosquito, a transmitter of countless diseases. Photo credit: ekamelev via Pixabay.

For instance, dengue has increased in range and intensity, with outbreaks in Florida, Texas, and Hawaii, due to the increasing range of Aedes mosquitoes from warmer climates. Dengue is a mosquito-borne NTD that causes high fever, headaches, muscle and joint pain, shock, bleeding, and death. It is predicted that the abundance of Aedes will increase as some species of this mosquito are able to over winter in mild, temperate climates and the eggs are desiccation-resistant, or dehydration resistant. Beyond mosquito-borne NTDs increasing in latitude, it has increased range in altitude: as the temperatures have increased, places like Nepal have experienced outbreaks of dengue, where no outbreaks have previously occurred. Dengue does not have a vaccine, nor is there a direct treatment– merely symptom management. This leaves environmental management and a heavy reliance on insecticides to reduce the incidence of disease [5].

As people move from place to place, disease cannot be expected to stay in one place. A lack of hard winters present an opportunity for insect vectors to survive the colder months without dying off, leading to increased numbers the next year. Furthermore, increasing temperatures at higher latitudes can spread disease vectors into areas unbeknownst to that disease. This adds to the urgency to tackle climate change, reduce poverty, and track disease spread.

References

[1] “Climate Change Indicators: Seasonal Temperature” EPA. Environmental Protection Agency. Accessed December 20, 2021.

[2] Ogden, N. H. “Possible Effects of Climate Change on Ixodid Ticks and the Pathogens They Transmit: Predictions and Observations.” Journal of Medical Entomology 58, no. 4 (2021): 1536-1545.

[3] Beard, C. B. “Rise of Ticks and Tickborne Diseases in the United States—Introduction.” Journal of Medical Entomology 58, no. 4 (2021): 1487-1489.

[4] Mackey, Tim K. “Emerging and Reemerging Neglected Tropical Diseases: a Review of Key Characteristics, Risk Factors, and the Policy and Innovation Environment.” Clinical Microbiology Reviews 27, no. 4 (2014): 949-979.

[5] Tidman, Rachel. “The Impact of Climate Change on Neglected Tropical Diseases: a Systematic Review.” Transactions of The Royal Society of Tropical Medicine and Hygiene 115, no. 2 (2021): 147–68.

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