Climate, the Environment, and Infectious Disease

Overview

Humans have known for decades that climate conditions and geography impact the spread and introduction of disease. According to the World Health Organization, Roman Aristocrats would travel to the hills in the summer months to avoid mosquitos carrying malaria, whose survival depends heavily on factors like temperature, humidity, and rainfall. Many of the changes and prevalence of infectious disease patterns are a consequence of climate change. Warming temperatures and more extreme weather events are driving the frequency and intensity of infectious disease outbreak through increased vector breeding and survival and heightened parasite growth rates.

How Climate Impacts Infectious Disease

With the current COVID-19 pandemic, conversations about the connection between health, disease, wildlife, the environment, and environmental justice are becoming more frequent. New opportunities for human-disease interaction due to climate change are numerous and ever-changing.

The following examples provide an idea of just how much climate can play a role in the spread of infectious disease.

Human migration

Human migration as a result of climate change can cause people to come in contact with new infectious diseases that they are not immune to or bring diseases to new areas, creating opportunities for diseases to cross borders. Similarly, migration to urban areas as a result of climate change causes humans to live in closer proximity to each other, allowing for quicker spread of disease. After Hurricane Katrina, there were over 1,000 cases of norovirus reported amongst displaced families living in close proximity. These 1,000 cases arose from person-to-person transmission starting from over 20 unique outbreaks throughout the displaced populations.

Changes in Precipitation, Humidity, and Flooding

Increased precipitation, humidity, and flooding can increase pathogen exposure as contaminants can spread through precipitation runoff and floodwaters. Additionally, standing water such as ponds, swamps, ditches, or even abandoned birth baths provide an ideal breeding ground for disease vectors like mosquitos. The transmission of West Nile virus in the United States is influenced by aquatic habitat availability and is heavily dependent on climatic factors like precipitation and humidity.

Warmer Temperatures

Increased global temperatures allow warm weather vectors to survive and reproduce at higher latitudes. Development of pathogens transmitted by vectors also occurs more quickly at higher temperatures. Further, increased temperatures expand the duration of seasons with the highest exposures for many infectious diseases. Warmer winter and spring months are projected to cause an early onset of Lyme disease cases in the United States. According to the CDC, Lyme disease cases have tripled since the 1990’s. There were a record number of reported tick-borne diseases in 2017, with climate change as one of the likely influences. Similarly, a study conducted in 2016 found that the land area with a suitable climate for the mosquito carrying dengue, Zika, and yellow fever could increase by 13% under the highest warming scenario (4.8˚C increase over land from 2061 through 2080). This interactive map shows the projections for higher dengue risk in more regions by 2080.

Drought

Increased drought primarily leads to depletion of water sources giving rise to water-borne diseases due to worsened water quality in lower flow or stagnant conditions. However, drought can also lead to an increased risk of vector-borne diseases as it drives virus hosts closer to vectors who tend to congregate near remaining water sources in these conditions.

For example, exposure to West Nile virus in the United States has been known to spike during drought conditions with bird populations (virus host) in closer proximity to mosquitoes (vector). Droughts followed by heavy periods of rainfall are associated with increases in rodent populations, which could subsequently increase human exposure to rodent-borne diseases. 

Shrinking Wildlife Habitats

Erosion of wildlife habitats through human-caused events like rising sea levels, deforestation, or wildfires can bring animals harboring infectious zoonotic diseases closer than ever to human populations, amplifying the influence of disease on society. According to the Center for Disease Control (CDC), scientists estimate that more than 60% of infectious diseases in people can be spread from animals and that 75% of new emerging infectious diseases in humans come from animals.

Stay tuned for the next blog in this series that will dive deeper into the relationship between humans and wildlife and the influence of this close relationship on infectious disease.

The Interlinkage Between Infectious Disease and Environmental Justice 

Public health specialists have learned quickly through the COVID-19 pandemic, among other public health crises, that human health is directly aligned with social and environmental equality and justice. Infectious diseases disproportionately impact low-income communities and communities of color. This is due to higher exposure to poor air quality, pollution, and limited access to clean water. In addition, these communities can often bear the brunt of extreme weather events, drought, or flooding due to aging infrastructure and limited resources to respond.

Because of the connection between climate and health, vulnerable communities are disproportionately affected by the COVID-19 crisis. At a congressional briefing on water access in the United States earlier this month, Zoe Roller with the U.S. Water Alliance highlighted how the COVID-19 pandemic underscores the urgency of providing access to basic services like safe running water and sanitation to protect public health. According to the Closing the Water Access Gap in the United States report, over two million Americans are without access to running water and indoor plumbing, thus excluding them from taking part in the number one recommendation to curb the spread of disease: washing hands. Similarly, according to the CDC, people with chronic lung disease or moderate to severe asthma are considered high-risk for severe illness from COVID-19. Numerous studies show that the prevalence of asthma is higher in low-income households and these communities are being disproportionately impacted by the pandemic as a result. 

How Legislation can Prepare Society for Changing Disease Patterns

Policy makers have been working hard trying to combat the negative effects of COVID-19 and further prepare for future pandemic response. Because climate will continue to have a substantial impact on infectious disease and human health, policy can help better prepare systems and populations for these events. Understanding that health is both directly and indirectly related to climate, community resilience, and disaster preparedness can help prepare society to mitigate the negative impacts of a changing climate.

In the past three years alone, 254 bills in 26 states have been introduced related to disaster preparedness and more than 900 bills in all 50 states have been introduced in relation to infectious disease. The Infectious Disease Society of America outlines their support of policies that invest in public health infrastructure, implement prevention strategies, include infectious diseases as a risk associated with natural disasters, increase research to further understand the relationship between climate change and infectious disease, and more.

During the 2020 session:

• Washington state passed a bill concerning drought preparedness and response, linking climate change to potential water supply shortages and acknowledging the association with public health.
• Maine passed an act to strengthen public health infrastructure including measures such as equitable funding and service distribution and enabling coordination between emergency preparedness and public health professionals for example.
• New York’s disaster preparedness and emergency planning act could better prepare cities for the wide-ranging impacts of a natural or man-made disaster. This bill directly addresses the need to mitigate the impacts of climate change as it negatively affects public health.