One Health Resource Center

One Health Resource Center

December 13, 2017
6 min read

Q&A: Facing climate change, ID clinicians must ‘expect the unexpected’

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Image of Aedes albopictus mosquito
Climate change can increase the risk for mosquito-borne illnesses.
Source: James Gathany/CDC

According to the CDC, climate change can increase the risk for the spread of numerous infections like Lyme disease, West Nile virus, diarrheal diseases and more.

The buildup of carbon dioxide (CO2) in the atmosphere causes Earth’s temperature to rise, trapping heat that can disrupt many of the connected systems in our environment and present risks to human health, according to the CDC.

Warmer average temperatures lead to earlier spring seasons, shorter and milder winters and hotter summers — conditions that are agreeable to vector-borne and tick-borne diseases.

Kacey Ernst
Kacey C. Ernst

Infectious Disease News spoke with Kacey C. Ernst, PhD, MPH, associate professor and undergraduate program director at the University of Arizona’s Mel & Enid Zuckerman College of Public Health and a 2017 AAAS Leshner Fellow for Public Engagement, to discuss how the effects of climate change can impact the field of infectious diseases. – by Savannah Demko

How is climate change affecting public health today?

You first must understand what climate change is before you can really talk about the health effects. The amount of CO2 (and other greenhouse gases) in the atmosphere is at unprecedented levels. This CO2 in the atmosphere acts as an insulating blanket, increasing global temperatures and leading to greater atmospheric disturbances. The buildup of CO2 impacts public health by increasing extreme heat events, reducing air quality and increasing the intensity and frequency of storms that may directly kill or injure those in the affected areas. Climate change also indirectly impacts the environment in other ways. For example, increasing temperatures may lengthen the season for disease vectors such as mosquitoes, ticks and flies; and increasing water temperatures may facilitate water-borne pathogen replication. Our basic needs of food and water are also being impacted. Most of the excess CO2 in the atmosphere is being absorbed by the oceans and they are suffering from acidification. Declining fish populations mean malnutrition for populations reliant on fish as a primary protein source. Rising sea levels are leading to salinization of adjacent soils, rendering a poor harvest. Worsening droughts also lead to declines in agricultural productivity and access to clean water. These problems all lead to declining food and water security, which is the foundation for a healthy population.

Are current public health responses enough to stop or reverse climate change?

No. A recent study published in Nature Climate Change estimated that there is a 95% chance our global temperature will increase by 2°C by the end of the 21st century and less than a 1% chance that the warming will be less than 1.5°C. This means we are already on an irreversible path for some changes. However, it is up to us as a global population to determine just how severe this increase will become. Some projections indicate an over 4°C change if we continue with business as usual. This is devastating to the environment, animals and humans. If we can remain at a 2°C change, we can likely improve on already developing strategies to mitigate the impacts and even maintain some of the positive trajectories in global health that we have made over the past several decades, including reductions in the mortality rate of children aged younger than 5 years and the maternal mortality rate, and improved living conditions through access to clean water and sanitation. With the current political environment, changes at the national level, at least in the United States, are being stymied. Improvements and commitment to mitigation are, however, being carried out at the city level at unprecedented rates. Specifically in public health, our efforts to increase resilience against climate change impacts are limited. Public health is poorly funded and can typically respond only to immediate threats. Long-term planning and investment are crucial to reduce the public health impacts of climate change.


As the Earth continues to warm and the ice in the Arctic continues to melt, what can public health officials and professionals do to combat the effects of climate change?

As public health professionals, we are committed to improving health in communities every single day. All evidence indicates that the populations who will suffer the most from impacts of climate change are the same populations that are most vulnerable today. People who are very young, very old, sick, poor, and food insecure will all disproportionately feel the brunt of climate change impacts. Therefore, public health programming that reduces risk and creates resiliency in these populations now will also improve health outcomes under climate change conditions. Aside from carrying out our current activities, we as public health officials have an obligation to educate ourselves and each other on the health impacts of climate change. Just like our understanding of poverty and education as drivers of health, climate change needs to be taught as a fundamental issue that will impact a myriad of health outcomes. In addition, public health professionals need to collaborate more with specialists in other fields; city planners, utility companies and insurance companies need to work together to plan smarter cities that can be resilient against climate change effects. And finally, we must advocate to both local and national policymakers to ensure that the voice of public health is a nonpartisan advocate for change to improve everyone’s well-being.

How does climate change affect the spread of zoonotic and mosquito-borne diseases?

The risk of transmission of vector-borne and zoonotic pathogens is dictated by a complex set of factors. It is best thought of as a pyramid, with the base of the pyramid being environmental suitability for pathogens, nonhuman reservoirs (animals) and vectors such as mosquitoes. This environmental suitability is influenced by temperature and precipitation patterns that alter vegetation cover, water availability and other factors that support the vector and nonhuman reservoirs. As the climate changes, environmental conditions will change along with it. While conditions may become more suitable for transmission in some areas, other areas may have reduced risk. For example, if an area experiences significant drought and high temperatures, mosquitoes may not survive as long to transmit pathogens. In addition, extreme conditions for the nonhuman reservoirs may lead to their decline and a reduction in transmission. However, there is evidence that limited biodiversity may allow more generalist species to thrive, and this can lead to higher and more rapid spread of infections when the remaining host is competent to transmit infections. In general, it is thought that warming conditions will increase the poleward and altitudinal expansion of many mosquito vectors into areas that were otherwise unsuitable for their establishment. Although establishment of vector populations does not necessarily mean disease transmission will follow, it cannot occur without them.


Who is at greatest risk for these diseases?

Transmission occurs only when there is presence of the host, pathogen and disease vector with environmental conditions that allow all three to interact. Individuals at greatest risk are those already residing in areas with transmission because they already have these connections in place. Although this may seem like an obvious point, the fact is that climate change conditions may act as an accelerant in areas where transmission is already occurring. For example, the length of the mosquito season may increase because of warmer temperature conditions, or increasing malnutrition may exacerbate the impact of malaria on children. Areas with newly established vector populations that are most vulnerable to transmission are those that are more likely to have introduction of the pathogen. Therefore, populations that currently live at the margins of current transmission are more vulnerable to emergent disease. For example, warmer temperature conditions in Ethiopia have led to malaria transmission at higher altitudes. In addition, areas that have longer seasons, higher population movement and poverty are also more vulnerable to the establishment of local transmission. This is due to the increasing odds that an infected traveler returns to transmit the pathogen to the local mosquito populations. Poverty enhances the likelihood of mosquito contact and further spread.

How will this impact the field of infectious diseases?

Climate change is not the only major factor that is influencing the field of infectious disease today, and its impacts will be coupled with several other factors that are occurring simultaneously: globalization and urbanization. The exchange of people, goods and livestock is being conducted in a changing environment, and some of the environments may become more receptive to the establishment of disease vectors and pathogens. We need to account for these factors when predicting and detecting infectious disease transmission. We need to enhance our capacity to model not just environmental suitability for pathogen spread but the social factors, mobility patterns and trade networks to better assess the risk of pathogen emergence for both the long and short term. In addition, there are great implications for surveillance. Our global infectious disease surveillance systems are still weak in many regions of the world. The recent pandemics of Ebola and Zika viruses underscore that point. We need to enhance our capacity to detect and control pathogens more rapidly to prevent spread. We also need to keep investing in technology to screen and detect new pathogens quickly and efficiently. These changes will improve our ability to respond both now and in the future.


What is the take-home message for infectious disease clinicians?

For clinicians, you should expect the unexpected. Or at least expect it to increase in frequency. The adage that when you hear hoofprints you should think horses instead of zebras is still true, but the scale is tipping slightly toward zebras. Always ask about a patient’s travel history. Know the signs and symptoms of infections that are occurring in adjacent countries and jurisdictions. You cannot detect what you do not test for. Educate your patients. Talk to patients about climate change, its potential health effects and how they can reduce their risk. For public health professionals, it is important to work closely with the infectious disease clinicians to identify and report any unusual presentation or pathogen. The sooner control measures can be put into place, the less people will be infected.


CDC. Climate change increases the number and geographic range of disease-carrying insects and ticks. Accessed on Dec. 12, 2017.

Raftery AE, et al. Nat Clim Change. 2017;doi:10.1038/nclimate3352.

Disclosure: Ernst reports no relevant financial disclosures.