The Effects of Climate Change on Emerging Infections and the Global Landscape of Neuroinfectious Disease

The Earth’s climate has undergone fluctuations during its existence over billions of years, but there is scientific consensus that the dramatic temperature changes in recent years are attributable to human activity. According to a 2023 Intergovernmental Panel on Climate Change report,¹ the average global surface temperature has risen by 1.1°C from 2011 to 2020 compared with the period from 1850 to 1900, with the rate of temperature rise since the 1970s being faster than at any time over the past 2000 years. These changes have had widespread effects worldwide, including not only warming global temperatures, but also increases in extreme weather and alterations in ecosystems, which affect human society by leading to disruptions in food production, water availability, and infrastructure integrity.¹

To compound these difficulties, the socioeconomic burden of climate change has been unevenly distributed, disproportionately affecting low-resource regions, which often are also the geographic areas that contribute the least to climate change.¹ Despite clear evidence that climate change can lead to increases in both reemergent and new infectious diseases,² relatively little media and societal attention has been paid to this phenomenon. In this review, we shed light on the effects of climate change on neuroinfectious diseases, including vector-borne diseases, waterborne illnesses, and zoonotic diseases.

History of Neuroinfectious Diseases and Climate Change

Infections of the nervous system have been a documented and pervasive part of human history. Archaeologic evidence of spinal tuberculosis dates as far back as 5000 BC, and the writings of Hippocrates vividly describe diseases involving fever and “heaviness of the head and neck.”^3,4 As the scientific method advanced and the ability to identify the pathogenesis of infectious diseases progressed, there was an early and ongoing appreciation of the role the environment and hygiene play in increasing vulnerability to neuroinfectious diseases. In 1805, Gaspard Vieusseux described what were likely cases of meningococcal infections affecting particularly those with fewer resources “in a district inhabited by poor people, dirty, and in whom the manner of life favored the development of every contagious disease.”⁵

With the emergence and identification of climate change, modern scientists have identified and studied the changes in ecologic dynamics that influence the transmission dynamics of pathogens and vectors, leading to shifts in the epidemiology of nearly all infectious organisms, including viruses, parasites, fungi, and bacteria. In addition, climate change increasingly brings humans and animals into closer contact, resulting in more frequent outbreaks of zoonotic and vector-borne diseases, and increasing the risk of new pandemics. The global COVID-19 pandemic, hypothesized to have been secondary to viral transmission from a zoonotic source,⁶ is a notable example of how close contact between humans and animals can lead to new infections with devastating effects on human populations.

Climate Change and the Increase in Vector-Borne Diseases

Vector-borne diseases are infections caused by vectors, such as mosquitoes, ticks, or fleas, that transmit infectious viral, bacterial, or parasitic pathogens to humans and animals. Most of these can affect the nervous system. Changing environmental conditions can create favorable climates for the proliferation of pathogens and their vectors, consequently facilitating the geographic expansion—both latitudinal and altitudinal—of many infections that can affect the nervous system. Some of these vectors and their associated pathogens include mosquitoes (transmitting dengue, malaria, Zika, and chikungunya), ticks (transmitting Lyme disease, tick-borne encephalitis, babesiosis, ehrlichiosis, Rocky Mountain spotted fever, anaplasmosis, and tularemia), triatomines (transmitting Chagas disease), sandflies (transmitting leishmaniasis), snails (transmitting schistosomiasis), and flies (transmitting African trypanosomiasis [ie, sleeping sickness]).⁷

Of these vectors, mosquito-borne illnesses are the most prevalent worldwide. Climate change increases the geographic distribution of mosquito-borne infections through several mechanisms.⁸ Increased rainfall and rising temperatures lead to the geographic spread of mosquito-borne infections by broadening vector habitat areas, speeding vector development and biting frequency, and promoting faster viral replication. Although some of these environmental changes may make previous mosquito habitats inhospitable, it is hypothesized that climate change will likely lead to the net expansion of mosquito habitats overall.^8-11 Rising sea levels also increase areas of brackish and salinated bodies of water, thereby extending breeding grounds for certain salinity-tolerant mosquitos, such as Aedes dorsalis, which can transmit West Nile virus. Although incidence of West Nile virus neuroinvasive disease, which can cause encephalitis and myelitis, has varied over the years, recent spikes in reported cases in North America likely reflect the effects of climate change.^12,13

Tick-borne illnesses that can lead to neurologic disease are particularly relevant in North America and Europe. Lyme disease, caused by the spirochete Borrelia burgdorferi, has expanded its geographic range in both continents because of warming temperatures, leading to an increased incidence of neuroborreliosis, characterized by neurologic symptoms such as meningitis, encephalitis, and facial nerve palsy (see article by Harrold on Neurologic Complications of Tick-Borne Illness elsewhere in this issue, for more details regarding tick-borne neurologic diseases).

The burden of vector-borne diseases is highest in tropical and subtropical regions, especially in Africa, Asia, and South America. Although the extent of how climate change will affect vector-borne diseases in these regions is uncertain, early observations suggest they will expand into new geographic areas. For example, leishmaniasis, caused by a protozoan Leishmania spp, is transmitted through the bite of a sandfly and can produce cutaneous, visceral, or mucocutaneous manifestations. Leishmaniasis has historically been most prevalent in northern Africa and southern Europe but is expanding into Italy, Germany, and Central Europe as warming climate has allowed sandflies to survive in new regions.¹¹ Similarly, schistosomiasis—endemic to certain parts of Africa, South America, and Southeast Asia—is also expected to spread into nonendemic areas as the warming climate increases snail populations.¹¹ Overall, climate change will increase outbreaks of vector-borne diseases globally as more areas become hospitable to vectors and the pathogens they carry.^8,11

Climate Change and Waterborne Diseases

Through its effect on sea levels and water distribution, climate change also influences the prevalence of waterborne diseases, which can produce neurologic symptoms. Heavy rainfall and flooding attributable to changing weather patterns, especially in low-resource settings without water or sanitation infrastructure, can result in contamination of water sources with fecal pathogens that can produce meningitis or encephalitis if ingested, such as Escherichia coli, Cryptosporidium spp, and, perhaps most prominently, Vibrio cholerae. Known primarily as a diarrheal disease, cholera is caused by the toxin produced by the O1 and O139 subtypes of V. cholerae. Higher temperatures have accelerated the growth and spread of V. cholerae, particularly in Africa.¹⁴ The most common neurologic symptoms secondary to cholera are attributable to metabolic abnormalities caused by diarrhea, which can cause seizures and encephalopathy. Although rare, direct nervous system infection by V. cholerae leading to cholera meningitis has been described, most often in high-risk populations, such as neonates, infants, and immunocompromised adults, who have been exposed to contaminated water. Most of these cholera meningitis cases have been attributed to V. cholerae subtypes that do not produce cholera toxins.¹⁵

In addition, rising lake temperatures increase growth of cyanobacteria (leading to algae blooms), which can profoundly affect the nervous system through the production of neurotoxins, including microcystins, saxitoxins, anatoxins, cylindrospermopsins, and BMAA (β-methylamino-L-alanine).^16-19 These toxins can enter the human or animal body through ingestion, inhalation, or dermal contact. Microcystin can cross the blood–brain barrier, producing symptoms such as headache, dizziness, confusion, seizures, and, in severe cases, coma or death. Anatoxins, including anatoxin-a and homoanatoxin-a, are potent nicotinic agonists, with exposure leading to neuromuscular blockade and symptoms of muscle twitching, paralysis, convulsions, and respiratory failure. Saxitoxins, which block voltage-gated sodium channels, cause symptoms ranging from numbness and tingling to seizures, weakness, respiratory muscle paralysis, and death. Chronic exposure may result in neurologic symptoms resembling Guillain-Barré syndrome. BMAA has gained attention because of its potential association with neurodegenerative processes, such as amyotrophic lateral sclerosis and Alzheimer disease.²⁰

Cyanobacteria also serve as a nutritional source for Naegleria fowleri, a free-living amoeba found in warmer climates and the pathogenic agent responsible for primary amebic meningoencephalitis. The recent increase in the reported cases of primary amebic meningoencephalitis—especially in areas previously thought inhospitable for N. fowleri growth—is a clear example of the effect of warming temperatures on the spread of neuroinfectious diseases into new geographic areas.²¹ Public health interventions that reduce the levels of cyanobacteria and associated toxins and pathogens, such as regular monitoring of water bodies for cyanobacterial blooms and toxin levels; public awareness and educational campaigns; and necessary restrictions on recreational swimming in affected waters will be critical to reduce the emergence of waterborne neuroinfectious diseases.²²

Human Responses to Climate Change and Neuroinfectious Diseases

Besides the ability of climate change to promote the geographic expansion of neuroinfectious diseases through the environment, many of its effects can also be attributed to human responses to climate change. Because of rising temperatures and variable weather patterns, climate change affects human food production, food distribution, and water security. These disruptions in infrastructure tend to disproportionally affect low-resource populations, leading to exacerbation of societal and geopolitical instability and subsequent mass migrations and population displacements.¹ The Internal Displacement Monitoring Centre estimates that in 2019, ~23.9 million people were displaced from their homes because of weather-related events.²³ The number of climate refugees is expected to increase to 1 billion by 2050 because of rising sea levels, water shortages, and decreased crop yield.^23,24 Climate refugees often live in suboptimal conditions, with limited or absent sanitation and overcrowding, which can lead to the rapid spread of neuroinfectious diseases, particularly those secondary to bacterial infections.

Numerous studies have reported a close link between climate change and meningitis, as extreme heat and weather patterns promote overpopulation, which in turn leads to more rapid spread of diseases among populations living in close contact.²⁵ Although many studies have focused on the sub-Saharan Africa “meningitis belt,” recent data suggest that climate change has also increased rates of meningitis in Australasia and North America.²⁵ Higher temperatures and altered precipitation can lead to increased bacterial reproduction rates, horizontal transfer of resistance genes between bacteria, and more frequent and severe outbreaks, with subsequent antibiotic resistance. Recent studies suggest greater antimicrobial resistance in common bacterial pathogens, such as E. coli and Klebsiella pneumoniae,²⁶ among others. With increasing transmission of bacterial meningitis along with increased antibacterial resistance, there is the risk of a marked increase in morbidity and mortality from previously treatable infections in the years to come.

Secondary effects of climate change, including increased migration and food insecurity, can also lead to increased spread of sexually transmitted diseases through expanded sexual networks and riskier sexual behaviors.^24,27 Most notably, climate change may increase the transmission of human immunodeficiency virus (HIV) and secondary opportunistic infections, which have a multitude of neurologic manifestations, detailed in the article on the neurologic complications of HIV and other opportunistic infections by Ocampo appearing elsewhere in this issue. This will particularly affect areas of high human immunodeficiency virus prevalence, such as sub-Saharan Africa and Southeast Asia, which are also areas most vulnerable to the adverse effects of climate change.

In addition, food insecurity, inadequate nutrition, and overpopulation secondary to climate displacement can lead to increased transmission of highly contagious diseases, such as tuberculosis, whose neurologic manifestations are detailed in the article on this topic by Wahed and Marshall appearing elsewhere in this issue.^24,27 Given that efforts to control sexually transmitted and highly transmissible infections require robust public health systems for surveillance and interventions, especially in low-resource areas, additional resources should be put toward these efforts in anticipation of increased burden from neuroinfectious diseases.

Climate Change, Ecosystem Destruction, and Geographic Disease Expansion

Increases in the human population, which is projected to reach 10 billion by 2100,²⁴ along with climate change, will compound the destruction of natural ecosystems. Habitat destruction will lead to greater human contact with wildlife and subsequent increases in zoonotic infections, which are predicted to make up the majority of new, emerging infectious diseases.²⁸ Most notably, the COVID-19 pandemic likely began as a zoonotic infection⁶ and has led to the deaths of ~7 million people worldwide to date.

Climate and human-driven changes in ecosystems have also been linked to the emergence of other diseases, such as Nipah virus encephalitis. Discovered in 1999, Nipah virus can be transmitted through direct contact with body fluids of an infected animal (eg, fruit bat, pig, human), or through contact with fruit contaminated by an infected bat. Migrant and low-income communities often lack access to clean water and sanitation infrastructure, increasing their likelihood of acquiring waterborne zoonoses, such as leptospirosis, which can cause meningitis.

With changing ecosystems, the geographic distribution of fungal infections that can produce meningitis is also expanding. Meningitis caused by Coccidioides immitis or Coccidioides posadasii, commonly known as “coccidioidomycosis” or “valley fever”, historically has been most abundant in the American Southwest. Coccidioides spp proliferate during wet periods, then break apart into spore-containing fragments during dry periods, with aerosolized spores leading to human disease. Increasing temperatures and shifting precipitation patterns are predicted to increase the geographic distribution of coccidioidomycoses to 17 states across the Great Plains region of the United States.²⁹ Similarly, the geographic spread of both histoplasmosis and blastomycosis have spread northward with increasing temperatures in North America.³⁰

Deforestation by human populations may affect pathogen spread both directly and indirectly by exacerbating climate change. Cryptococcus gattii, a soil-dwelling fungus that produces a pneumonia-like illness or meningitis in both immunocompetent and immunocompromised individuals, has spread from British Columbia to the Pacific Northwest and American Southeast in recent years.³¹ Although the reasons for the geographic spread of C. gattii in the United States are not fully understood, recent studies indicate that tree cutting leads to aerosolization of pathogens and promotes spread of the fungus.³² Deforestation has also been correlated with increased geographic distribution of Paracoccidioides brasiliensis.³³

Destruction of the natural ecosystem may also lead to the emergence of as yet unidentified neuroinfectious diseases. With the warming climate, it is estimated that nearly two-thirds of the Arctic’s surface permafrost may disappear by 2100, and there has been considerable interest in understanding how permafrost thawing could lead to new diseases emerging from previously unknown pathogens.³⁴ Although no pathogen capable of infecting the human nervous system has been identified, early studies have found antibiotic resistance in bacteria isolated from deep permafrost even though they were never exposed to antibiotics.³⁴ Cases of an anthrax outbreak among reindeer and people during an unusually hot summer in Siberia, as well as the emergence of a new pox virus from the Alaskan Arctic (named “Alaskapox”), highlight how new infectious diseases could spread through changing climates, and suggest the need for increased vigilance for potential emergence of new neuroinfectious diseases.

Conclusions

Changes in temperature, precipitation patterns, and ecologic dynamics influence the transmission of pathogens and vectors, leading to shifts in the epidemiology of neuroinfectious diseases (Figure). Vector-borne, waterborne, and emerging infectious diseases with neurologic manifestations are particularly affected by climate change, posing substantial challenges to public health systems worldwide. The human response to climate change, including increased migration and displacement, changes in human behavior, and human-made alterations to the geographic landscape, have also affected the transmission and spread of neuroinfectious diseases.

As the landscape of “expected” neuroinfectious diseases continues to change, clinicians will need to pay close attention to each individual’s history, particularly with regard to travel, exposure to potential vectors, and recent weather changes. As the effects of climate change continue to expand, neurologists will need to maintain a low threshold to consider neuroinfectious pathogens not previously present in their geographic region within their differential diagnoses.

Addressing the challenges associated with climate change requires an interdisciplinary approach that integrates neuroscience, climatology, epidemiology, veterinary science, and public health. Strategies to mitigate the effects of climate change on neurologic infections and safeguard human health through enhanced surveillance for emergence of human and animal diseases as well as implementation of efficient and targeted public health responses will be crucial elements in the identification, management, and prevention of the often-devastating effects of neuroinfectious diseases, and perhaps the next global pandemic.