JAMA | Infectious Diseases in a Changing Climate
Infectious Diseases in a Changing Climate
Matthew C. Phillips, Regina C. LaRocque, George R. Thompson
JAMA. 2024;331(15):1318-1319.
DOI: 10.1001/jama.2023.27724
Climate change is happening now. Nine of the 10 hottest years on record have occurred in the past decade and were accompanied by extreme heat waves, droughts, wildfires, hurricanes, and severe flooding. Primarily due to greenhouse gases released via combustion of fossil fuels, global average temperatures between 2011 and 2020 increased to 1.1 °C (approximately 1.9 °F) above preindustrial levels and are estimated to increase to 1.5 °C (approximately 2.7 °F) by 2040.1
Local manifestations of the warming earth, such as shorter winters, changes in precipitation, and increased frequency of extreme weather events, will cause worldwide changes in pathogens, vectors, and the behavior of animal reservoirs and people. These changes may alter infectious disease epidemiology in the US and internationally as traditionally accepted regions of disease acquisition lose relevance, seasonal associations change, and new pathogens emerge. Awareness of changes in the geographic range, seasonality, and frequency of transmission of infectious diseases because of climate change is important to help clinicians diagnose, treat, and prevent infectious diseases in patients (Table).
Vector-Borne Diseases
In the US, climate change is altering environmental conditions and facilitating proliferation of arthropod disease vectors such as ticks and mosquitoes. Between 2004 and 2018, the number of arthropod-associated illnesses reported to the US Centers for Disease Control and Prevention more than doubled, to a total of more than 760 000 cases. Multiple pathogens, including West Nile, Powassan, and Zika viruses, have been identified that were not previously seen or common in the US.2 This increase may be related to improved awareness, diagnostic methods, and case reporting, but climate-related changes in vector biology are also contributing. For instance, tick survival over winter months increases with shorter, milder winters, leading to larger populations and northern extension of their geographic locations into Canada and the upper Midwest US. Longer summers result in more months per year when ticks bite, creating more opportunities to spread diseases such as babesiosis, anaplasmosis, and Lyme disease. Mosquitoes have a short lifespan and respond rapidly to changes in weather. Mosquitoes require water to breed, and changes in precipitation can significantly alter the size and location of local mosquito populations. Warmer temperatures increase efficiency of the mosquito breeding cycle, leading females to require more blood meals to support multiple gestations over their lifetime.3 Higher temperatures have promoted northern extension of locations of pathogen-carrying mosquito species such as Aedes aegypti, the primary vector for dengue, chikungunya, yellow fever, and Zika. Some of these diseases are now locally transmitted in the US, primarily in southern states such as Florida and Texas.3 Malaria, transmitted by Anopheles mosquitoes, also increased during the summer of 2023 with several locally acquired cases reported in the US.4
Zoonotic Diseases
Zoonotic infections are infections that spread between humans and other vertebrate animals and may be viral, bacterial, or parasitic. Climate change alters population dynamics and behaviors of host animals, which can increase zoonotic disease spread. In the western US, changes in precipitation and temperature affect food availability, population size, and behavior of rodent hosts for plague and hantavirus. These changes result in increased disease incidence and a geographical shift northward and to higher altitudes.5 Models estimate that more than half of the earth’s animal species are experiencing climate-induced alterations of their natural ranges, with many moving northward as habitat suitability declines in the global south.6 Habitat destruction leads to numerous species coming into close contact, increasing the risk of pathogens (particularly viruses) spreading to other species, potentially including humans. Migratory birds are uniquely affected by habitat destruction and have been the primary cause of the current global outbreak of avian influenza (H5N1). Since January 2022, H5N1 has been detected in all 50 US states, in more than 7500 birds, and has promoted interspecies transmission in multiple mammalian species.7 Transmission of zoonotic diseases are increased by globalization; changes in land use, such as deforestation; and human migration, including international travel.
Fungal Diseases
Fungi are soil-dwelling pathogens and, although there are more than 1 million recognized species of fungi, only approximately 300 infect humans. Human body temperature is one of the primary resistance barriers to fungal pathogens, because most are unable to grow at elevated temperatures.8 However, as these fungal organisms develop tolerance to the warming environment, the temperature differential between humans and the environment narrows and the number of fungal infections, and new pathogens, may increase. Candida auris and Sporothrix brasiliensis may be early examples of this phenomenon. C auris was first isolated in 2009 and has emerged as a significant, and often antifungal-resistant, global pathogen. S brasiliensis appears to have transitioned from a saprophytic organism dwelling on plant material to a zoonotic disease supported by growth at higher temperature. Endemic fungi have been located outside of their traditionally described geographic locations. Coccidioides is expanding to northern and eastern locations, with cases recently identified in Nebraska; Histoplasma has been documented in more Northern and Eastern locations, including Alberta, Minnesota, and Wisconsin; and Blastomyces has moved westward, with a novel species (B helicus) now seen in the Rocky Mountain region.9
Waterborne Diseases
Sea level rise is inevitable, directly influencing the approximately 40% of individuals in the US who live in coastal counties. Extreme “once-per-century” sea level events, such as storm surges and coastal flooding, are anticipated to occur 20 to 30 times more frequently by 2050.1 These events, combined with warming, increase contact with coastal pathogens (eg, Vibrio species), which can cause gastroenteritis, soft tissue infections, and sepsis, with mortality rates of up to 50%. Within 2 weeks of Hurricane Ian making landfall in 2022 in Florida, 38 cases of vibriosis had been diagnosed, leading to 36 hospitalizations and 11 deaths.10 While typically associated with southern coastlines, cases of V vulnificus have increased in mid-Atlantic states in association with rising ocean temperatures.3 Other waterborne pathogens, such as Campylobacter, Escherichia coli, and Cryptosporidium, cause diarrheal disease after flooding and are worsening with extreme weather events and warmer climates.3
Conclusions
Thoughtful preparation for current and future changes in infectious disease epidemiology can help decrease the health effects of these diseases. Medical education should train clinicians to anticipate these changes, and an effective surveillance infrastructure is needed to understand, measure, and manage climate-related epidemiologic shifts. Clinicians also have a responsibility to advocate for policies aimed at ending fossil fuel dependence to mitigate the effects of climate change on human health.
