Climate Change and the Epidemiology of Infectious Diseases in the United States

Abstract

The earth is rapidly warming, driven by increasing atmospheric carbon dioxide and other gases that result primarily from fossil fuel combustion. In addition to causing arctic ice melting and extreme weather events, climatologic factors are linked strongly to the transmission of many infectious diseases. Changes in the prevalence of infectious diseases not only reflect the impacts of temperature, humidity, and other weather-related phenomena on pathogens, vectors, and animal hosts but are also part of a complex of social and environmental factors that will be affected by climate change, including land use, migration, and vector control. Vector- and waterborne diseases and coccidioidomycosis are all likely to be affected by a warming planet; there is also potential for climate-driven impacts on emerging infectious diseases and antimicrobial resistance. Additional resources for surveillance and public health activities are urgently needed, as well as systematic education of clinicians on the health impacts of climate change.

Climate change is altering the planet. In 2020, the global surface temperature was 2.14˚F warmer than during the preindustrial period (1880–1900), and the 10 warmest years on record have occurred since 2005 [1]. This rise in global temperature results from human activity, including the burning of fossil fuels and deforestation (Figure 1), and a further 1.5˚C –2˚C rise is anticipated by 2100 without deep reductions in greenhouse gases [2].

In North America, rising surface temperatures have led to decreased snowpack and seasonal runoff from mountains, glacial retreat, sea ice reduction, severe weather and flooding, droughts in western and central plains regions, sea level rise, and acidification of coastal waters. These changes have degraded marine, freshwater, and terrestrial ecosystems; impaired freshwater resources; and caused damage to infrastructure, livelihoods, and well-being. Alterations of the hydrologic cycle are predicted to bring increased precipitation and flooding in eastern regions and the northern half of North America and decreases in the western and central plains, worsening aridity and drought. Changing weather patterns will have profound impacts on food and nutritional security, physical and mental health, and the epidemiology of infectious diseases [3, 4].

Global warming is leading to changes in the density and range of vector populations, as well as the migration patterns of animals and birds [4, 5]. Avian influenza in waterfowl and shore birds, Lyme disease, and West Nile virus (WNV) have all been shown to be spread by migratory animals. Changes in ecosystems from drought, fires, and other extreme weather may force animal species and humans into closer proximity. Changes in the incidence of human and animal diseases may be an early sign that geographic ranges of intermediary hosts and vector species are evolving [5].

Climate-related migration of human populations may also drive susceptible people into areas with endemic disease and push those with infection into areas where the disease is not present. The impact of large-scale human migration prompted by severe economic and political dislocation can be seen in the recent experience with malaria among Venezuelan refugee populations resettling in Colombia [6, 7].

Climate change–driven extreme weather events will also directly impact infectious diseases by a variety of mechanisms, including dispersing pathogens, such as the spread of waterborne bacteria by flooding [8] and fungi due to drought [9], and optimizing conditions for pathogens, such as allowing for the geographic expansion of Vibrio species due to changing sea surface temperature and salinity [10].

To understand the complex interplay among changing climate, land use, human and animal migration, environmental impacts, and disease epidemiology in human and animal populations, an expansion of public health surveillance and environmental monitoring is essential, particularly in areas with vulnerable human, animal, and plant populations and where current systems are inadequate. A One Health perspective, defined as using “a collaborative, multisectoral, and transdisciplinary approach—working at the local, regional, national, and global levels—with the goal of achieving optimal health outcomes recognizing the interconnection between people, animals, plants, and their shared environment,” offers a framework to adapt to changing environmental, social, developmental, and health-related terrains [11, 12].

Climate change poses a grave and multifaceted threat to human health. Improving surveillance for emerging and established pathogens and expanding monitoring and analysis of climatic and environmental changes as they relate to infectious disease epidemiology can help public health professionals anticipate and respond to the infectious disease impacts of climate change.

ASSESSING IMPACTS OF GLOBAL WARMING ON INFECTIOUS DISEASES

Climate-related changes in the geographic and seasonal range of pathogens and vectors and in the prevalence and emergence of vector- and waterborne infectious diseases have been documented across many global regions [13]. Surveillance studies and mathematical models are the 2 scientific approaches used to assess the impact of climate change on infectious diseases [14]. Surveillance studies correlate precipitation, temperature, humidity, and other climatic factors with changes in the epidemiology of disease. A principal challenge to surveillance studies is that meteorologic measurements and data collection methods may not always be consonant with epidemiologic data systems, including differences in areas covered and in timing of observations. Short-term changes in disease epidemiology may reflect climate-related factors, such as land use, human migration, and distribution of animal and bird vectors, as well as changes in disease ascertainment and surveillance measures. Mathematical modeling studies offer another approach to estimating the effects of climate change on infectious diseases [15, 16] and can be useful for directing surveillance activities to emerging geographic and spatial areas [17]. However, mathematical models depend on the accuracy and completeness of the underlying data, as well as assumptions used in the models [18].

To continue to assess the interplay between climate change and emerging diseases and to meaningfully compare trends globally with these methods, strategies for the collection of epidemiological and environmental data and their analysis will require the cooperative work of varied disciplines of the physical, biological, and social sciences [19, 20]. Such “climate epidemiology” will play a powerful role in studying, planning for, and mitigating the health impacts of climate change [21].

VECTOR-BORNE INFECTIONS

Vector-borne infections in North America are principally caused by pathogens carried by mosquitoes and ticks. The biology of these 2 vectors differ, and their adaptations to climate change are likely to be distinct [22]. The lifetime of a mosquito is brief, typically just days to weeks, while ticks survive for 2–3 years. In addition, mosquito life cycles depend on the availability of freshwater pools for the development of juvenile stages.

There are no simple relationships between temperature, mosquito density, and disease incidence. Temperature may influence mosquito density only within certain limits, while mosquito-biting behavior can vary depending on whether the species tends to invade homes or remains outdoors and what time of day it feeds [23, 24]. Mosquito-biting behavior is also influenced by temperature, with higher mean temperatures correlating with higher biting rates for some species [25].

In the United States, the principal pathogen carried by mosquitoes is WNV, primarily vectored by Culex species mosquitoes, which has spread dramatically across the United States since its introduction in 1999 [26, 27]. Climate change could cause fundamental changes in the epidemiology of mosquito-borne diseases in the United States, including sustained localized transmission of Zika and chikungunya, the spread of dengue [28] from localized areas in Texas and Florida, and the reintroduction of yellow fever [29], which has not been seen in the United States for more than a century. Aedes aegypti, the primary vector for each of these, is limited to areas along the US Gulf Coast and has low enough density that only very limited transmission from infected travelers has been expected [30]. However, the Aedes albopictus mosquito, an important vector for chikungunya, dengue, and, possibly, Zika, has a far broader geographic range in the United States, extending through all southeastern and mid-Atlantic states and across the Midwest, and is predicted to reach Canada's Hudson Bay and southern Greenland [3, 31]. Global heating may extend the northern limits of endemic mosquito-borne diseases, while making the current southern-most tropical limits unacceptably warm for mosquito survival [32]. Higher temperatures may also expand the spatial distribution of mosquito-borne diseases to include higher-altitude areas [33].

Mosquito-borne diseases are sensitive to both atmospheric temperature and rainfall, and changes in the density of insect vectors can occur relatively rapidly, over a few months. Rising temperatures can speed the development of both vectors and their pathogens, leading to a longer lifetime for the vectors and an increased risk of human infection. Importantly, reproduction numbers and vectorial capacity relate to temperature-sensitive factors, such as mosquito-biting rates, in a nonlinear fashion; small changes in temperature can therefore have large effects on disease dynamics. The rapid spread of WNV into the United States, as well as the introduction of Zika virus to the Caribbean and Central and South America, reflects the speed with which mosquito-borne viruses may become established in new areas [25].

The most common tick-borne disease in the United States is Lyme disease, while anaplasmosis, babesiosis, ehrlichiosis, Powassan virus disease, Rocky Mountain spotted fever, tick-borne encephalitis and tularemia occur less commonly [34]. Ticks burrow into the soil, shielding themselves from high surface temperatures and some effects of severe rainfall [22]. Unlike mosquitoes, whose short life cycles and rapid response to increased temperatures can allow population expansion in a matter of weeks, tick populations can take years to expand, as increased temperatures mostly affect tick survival by shortening the maturation time. Warming winter temperatures can also enable more ticks to overwinter, further increasing tick populations [35].

Changes in average temperature in the United States could result in the introduction of new vector-borne diseases and increases in incidence of endemic diseases. Given the rapidity with which mosquitoes can extend their ranges once the environment becomes more accommodating, this might become evident sooner with mosquito-borne diseases than with tick-borne diseases. Gradual northern spread of the Ixodes scapularis tick from the United States into Canada has taken place, and climate warming may have been a key factor [3].

WATERBORNE DISEASES

The principal waterborne diseases in the United States are the bacterial diarrheal diseases salmonellosis, shigellosis, Campylobacter disease, diarrheagenic Escherichia coli, and yersiniosis, as well as the viral diseases caused by norovirus and rotavirus [36]. These diseases are generally spread through contaminated food and water and from person to person because of unhygienic practices, and they are also sensitive to climate warming. Flooding from extreme precipitation may contaminate water supplies with animal waste, leading to outbreaks of waterborne infections [37]. Severe storms may also cause the overflow of wastewater, threatening drinking water supplies, as exemplified by an outbreak of E.coli gastroenteritis in Ontario in May 2000 [38]. Heavy rainwater runoff has been associated with outbreaks of giardiasis in Montana [39], cryptosporidiosis in Milwaukee, Wisconsin [40], and E. coli and Campylobacter at a New York State fairground [41].

Nontoxigenic vibrioses, generally limited to warmwater coasts, are another cause of waterborne disease. Recently, cases of non-cholera Vibrio infection have been newly reported in Alaska [42, 43] and increasingly in the mid-Atlantic region [44]. These emerging Vibrio risks at higher latitudes are likely a result of ocean warming and changes in ocean salinity, allowing an expansion in this pathogen’s range [45].

COCCIDIOIDOMYCOSIS

Coccidioidomycosis in the United States occurs principally in southern California and Arizona, where the etiologic agents are found in soil. Two epidemiologic trends in coccidioidomycosis have been seen over the last 30 years: its incidence has increased in endemic areas, and the regions where it has been found to cause disease have expanded [46, 47]. Case reports have steadily been increasing in Arizona, California, Nevada, New Mexico, and Utah [48]. Between 2000 and 2018, the coccidioidomycosis incidence in California increased nearly 800%, from 2.4 cases per 100 000 population to 18.8 per 100 000 [49, 50]. While changes in testing and surveillance practices may account for some of this increase, exceptionally warm and dry weather is likely a contributing factor. It has long been noted that a period of high rainfall, presumably promoting soil hyphae growth, followed by a dry period, which increases the risk of dust storms with aerosolized arthroconidia, leads to increased disease incidence [51]. Coccidioidomycosis has also been identified in more northern areas in the last 2 decades, including northern Utah and eastern Washington State. Coccidioides immitis was subsequently identified in soil samples in both regions [52, 53]. Conditions that promote the growth of Coccidioides and aid in the dispersal of the organism are likely to be more common in North America as the earth heats.

EMERGING INFECTIOUS DISEASES

Since December 2019, the world has faced the challenge of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) global pandemic. While the origin of SARS-CoV-2 is still under investigation, it is presumed to have been transmitted to humans from a bat species and possibly through an intermediary mammalian host [54]. This would be the third such spillover of an animal coronavirus to humans in the last 2 decades, including notably severe acute respiratory syndrome coronavirus from civet cats and Middle East respiratory syndrome coronavirus from camels [55, 56]. Climate change may be a driver of spillover events by altering habitats and pushing the geographic distribution of animal and human populations into closer proximity, as well as by creating ecological and evolutionary pressures that lead to genetic changes in pathogens that may affect the fitness of animal-to-human transmission [57–60]. For example, the Hendra virus, similar to Nipah virus, is a bat-borne virus associated with spillover events into horses and humans; it has caused outbreaks with a fatality rate in humans of 50%–75%. Models suggest that under the pressures of climate change, the geographic range of risk to humans of spillover epidemics of Hendra virus will expand [61]. The risk of emergence and reemergence of infections from animal hosts to human populations necessitates increased surveillance and modeling of the influences of climatic changes on animal behaviors, breeding, migration, and pathogen shedding, as well as enhanced epidemiological monitoring of at-risk human populations [62].

ANTIMICROBIAL RESISTANCE

Recent evidence links global temperature changes to expanding antimicrobial resistance. The immediate temperature surroundings of bacteria directly affect bacterial growth, survival, and adaptation. Temperature trends in local geographic areas, in addition to selective pressure from antibiotics, have been associated with antimicrobial resistance [63]. MacFadden et al found that a 10°C difference in temperature across regions of the United States was associated with increases in antibiotic resistance of 4.2% for E. coli, 2.2% for Klebsiella pneumoniae, and 2.7% for Staphylococcus aureus [64]. A subsequent ecological study replicated these results across Europe and also found that rates of antibiotic resistance correlated with ambient temperature [65]. Climate change may also affect fungal evolution, including contributing to the thermotolerance that led to the expansion of Candida auris into the human population. Antifungal resistance was already in existence at the time this organism was first identified in humans [66]. These findings suggest that efforts to manage the public health concern of antimicrobial resistance may need to incorporate climate factors.

FUTURE SURVEILLANCE AND DISEASE CONTROL ACTIVITIES

The urgency of a global response to climate warming has been highlighted by scientists, clinicians, and public health experts who continue to identify and investigate climate-related threats to health [67–69]. Surveillance of the effects of climate change will, to a large degree, be reflected in the work public health agencies do every day in tracking diseases. However, some special approaches will need to be integrated with the activities already being carried out, such as focused, enhanced surveillance for certain diseases in certain regions. For example, current limited mosquito surveillance could be augmented to facilitate the identification of novel pathogens in vector populations, perhaps using next-generation sequencing techniques [70]. A One Health approach, with attention to the epidemiology of animal and bird infections, will also be needed to help anticipate and respond to outbreaks and spillover events [8].

Large-scale human population migrations due to crop failures, severe flooding and storm damage, and environmental deterioration may generate vector-borne and diarrheal disease outbreaks. As an example, severe drought in East Africa led to overcrowding at the Dadaab refugee camp in Kenya, and a cholera outbreak occurred at the camp [71]. Scenarios such as this will increase in frequency as climatic changes, such as a devastating drought, multiply the existing threats of poverty, political instability, and inadequate infrastructure.

There are no effective treatments or vaccines for many infectious diseases that will be impacted by climate change. For some, such as dengue, vaccines are currently under development or in the process of introduction. It is worth recalling that diseases such as malaria and yellow fever were eradicated in the United States before the availability of antibiotics and antivirals by the application of vector control measures; many of those programs have been reduced or eliminated.

A CALL TO ACTION

Climate change is the preeminent health emergency of our time [70, 72]. As both the direct and indirect effects of climate change increase, the medical and public health communities will have more responsibilities to recognize and respond to changes in the epidemiology of infectious diseases. Increasing attention to vectors, more intensive surveillance for selected pathogens, integration of detailed meteorological data with human and animal epidemiologic research, and recognition of new pathogens will require a commensurate increase in funding and in recruitment and training of skilled personnel. In addition, clinicians will need to be prepared to encounter infections that may not have been previously endemic to a region and may occur outside of expected seasonal times. Medical schools, residencies, fellowship programs, and continuing medical education should offer training for clinicians to help anticipate and respond to these changes (Table 1).

The time to act is now. The Sixth Assessment Report of the Intergovernmental Panel on Climate Change emphasizes that the window to secure a livable future through concerted global action is brief and closing [12]. A dramatic reduction in the use of fossil fuels that increase global temperature is essential to mitigate serious consequences for public health. Fortunately, climate change mitigation efforts may also generate immediate “health co-benefits” by decreasing air pollution, improving diet, and encouraging more active lifestyles [73].

Changes in the global climate have already irreversibly occurred, and we can expect continued changes in the climate to affect the epidemiology of both common and uncommon infectious diseases in the years to come. Collaborations with veterinary, environmental, and physical sciences will be necessary to build systems to predict and adapt to short- and long-term, climate change–driven health effects. The infectious diseases community must be prepared to recognize and respond to changes in the diseases we treat and to advocate for a stronger national response, including robust public health measures to lessen the health harms of climate change on our patients.

References

Author notes