Climate Change and Infectious Diseases

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The complex interplay between climate change, ecosystems, and human activities can create conditions that favor the emergence and spread of infectious diseases, posing significant challenges to public health.

A recent NETEC webinar featuring leading experts in the fields of epidemiology, climate science, and health care provides a comprehensive exploration of how climate change and environmental factors affect disease epidemiology and the emergence of vector-borne diseases. The panelists also discussed how climate change can influence clinical reasoning, emergency preparedness, and medical decision-making in the field of emerging pathogens.

The webinar, “Climate Change and Vector-Borne Diseases: Exploring the Impact on Disease Epidemiology, Emerging Pathogens, and Clinical Preparedness,” is now available on NETEC’s YouTube channel. It is also available as an online course for continuing education credit.

Climate-Sensitive Infectious Diseases

Climate change trends such as shifts in seasonal temperature and precipitation patterns, an increase in the frequency and intensity of extreme weather events, and changes in global weather patterns can create conditions conducive to the spread of infectious diseases. Infectious diseases sensitive to climate changes include:

• Waterborne
• Foodborne
• Soil- and dust-associated
• Zoonotic
• Vector-borne

Trends in Vector-Borne Diseases in the U.S.

Vector-borne diseases are transmitted to humans through carriers (vectors) such as mosquitoes or ticks and are usually harbored in wild animals.

Between 2003 and 2023, more than 1 million cases of vector-borne diseases were reported in the U.S. Over the last two decades, the number of annual reported cases approximately doubled. Yet reported cases are just the tip of the iceberg – actual disease occurrence is substantially underestimated. For instance, while around 30,000 cases of Lyme disease are reported to the Centers for Disease Control and Prevention (CDC) each year, recent estimates using other methods such as national insurance databases suggest that approximately 476,000 people are diagnosed and treated for Lyme disease each year in the United States.

Tick-borne diseases account for over 80 percent of all reported vector-borne disease cases in the U.S. And while Lyme disease is the most commonly reported tick-borne illness in the United States, other reportable tick-borne diseases have increased over the last 20 years, as well.

The increasing number of cases of tick-borne diseases in the U.S. corresponds with an expanding area of the country where tick vectors have established a foothold. The blacklegged tick (Ixodes scapularis), a vector for Lyme disease, babesiosis, anaplasmosis, and Powassan virus disease, has greatly expanded its geographic range in the last 25 years. For example, a March 2023 CDC report shows that cases of babesiosis are on the rise and are being reported in new geographic areas in the Northeast.

Tracking New Tick-Borne Diseases

Charting the discovery of tick-borne pathogens as a cause of human diseases along a timeline from 1900 to the present day reveals more frequent discovery of new tick-borne diseases than ever before. While this can be attributed in part to better diagnostic and detection methods that make it possible to identify pathogens, it likely also reflects the fact that more people over a broader area are being exposed to infected ticks.

Global Trends in Tick-Borne Diseases

Cases of tick-borne disease are also on the rise globally and affecting areas traditionally free of these diseases. One such disease, Crimean-Congo Hemorrhagic Fever (CCHF), poses a significant risk due to its high case fatality rate and severe disease manifestations.

CCHF is the second most widespread viral hemorrhagic fever globally and there has been a notable increase in cases over the last two decades, particularly in regions stretching from Turkey to India. CCHF is endemic to areas of Africa, Asia, the Middle East, and Southern Europe, and typically causes sporadic disease in the countries where it is endemic. But in the last 20 years, the tick vector for CCHF, the Hyalomma tick, has established itself in areas north of the 50^th latitude. Learn more.

In 2016, Spain reported its first human cases of CCHF, and in 2023, the CCHF virus was detected for the first time in France in Hyalomma ticks collected from the south of the country. However, as of October 2023, no locally acquired human cases have been reported in France.

What Is Driving the Emergence of Tick-Borne Diseases in the U.S.?

The rapid expansion and exposure to ticks and tick-borne diseases is fueled by several factors. In addition to the effects of climate change, ecological changes including reforestation, abundant deer populations, the expansion of suburbia into wooded areas and the resulting increased exposure of people to ticks all contribute to the rise in tick-borne diseases in the U.S. 

In the 1800s, great swaths of U.S. forests were cleared for fuel, agriculture, and building materials. However, since the 1900s, changing land use patterns and conservation practices across large portions of the eastern U.S. have occurred, leading to extensive reforestation.

During that same period, there was tremendous growth of suburban and urban areas. Suburban expansion into wooded areas creates habitat fragmentation as previously continuous tracts of land are sliced into fragments to make way for human development. As a result, there are more points of contact between human development and animal habitats and increased opportunities for human exposure to animals that play an important role in the life cycle of ticks such as small rodents and deer.

This juxtaposition of human development and animal habitats is paired with an overabundance of deer in the U.S. The density of deer populations is highest in the same areas where there has been an expansion of ticks, and deer are a primary host for adult stage ticks.

Finally, climate change trends such as longer, warmer summers and shorter, milder winters, as well as an increase in severe and unpredictable weather events, regional variations in seasonal trends, and shifts in global weather patterns, can all contribute to the emergence of tick-borne diseases.

Climate Change Impact on Tick-Borne Diseases

The environmental factors influencing the emergence and spread of tick-borne diseases are complex. Environmental change is altering the distribution of disease vectors and reservoirs and the pathogens they transmit. And while it remains uncertain the degree to which vector-borne disease trends can be attributed to climate change versus other factors, it is having an impact. There are three different ways in which climate change might influence tick-borne viruses:

1. Geographic Expansion: Climate change can shift the geographic range of tick vectors, expanding the areas where tick-borne diseases can be transmitted. Warmer temperatures and changes in precipitation patterns can create more favorable habitats for these vectors and facilitate the establishment of non-native ticks in new areas. The Hyalomma tick’s northward expansion beyond the 50th latitude, once considered unlikely, is now a reality in Asia and Europe. Climate change may contribute to the ease with which ticks from warmer regions establish themselves in previously non-endemic areas.
2. Increasing Numbers of Ticks and Subsequent Pathogen Transmission Potential: Climate change will change tick-borne virus transmission cycles. Changes in temperature, humidity, and rainfall patterns can affect the reproduction rates and lifespan of vectors. This can lead to changes in the timing and intensity of disease transmission cycles. For example, longer, warmer seasons contribute to greater tick density and an increase in tick numbers throughout the year. Greater tick numbers and a longer season for tick activity will lead to more human cases. Additionally, warmer temperatures can shorten the incubation period of viruses within ticks, accelerating disease transmission.
3. Adaptation to Changing Climates: Climate change will lead to an emergence of ticks and tick-borne viruses in new geographic areas, particularly northwards—not only in the U.S. but globally. Conversely, some tick species and tick-borne viruses will disappear from areas where the changing climate disrupts transmission cycles. Ticks have demonstrated the ability to adapt to various climates, therefore it is conceivable that they will be able to adapt to future, rapid changes in the environment, including rising temperatures.

Clinical Care for Vector-Borne Diseases

Health care providers and the public health workforce need to know how to diagnose, treat, and prevent emerging vector-borne diseases.

Diagnosis

Clinicians diagnose vector-borne illnesses through a combination of clinical assessment, laboratory testing, and epidemiological considerations.

Evaluate the patient’s medical history, symptoms, and recent travel or outdoor activities. Common symptoms of vector-borne diseases include fever, rash, joint pain, fatigue, and headache. Knowing the patient’s exposure to potential vectors and endemic areas can narrow down the list of possible diseases. However, keep in mind that even virulent tick-borne hemorrhagic fever viruses can present as mild flu-like illness, especially in the beginning. And always include vector-borne diseases in your differential diagnosis, even those not typically reported in the U.S.

A physical examination may reveal specific signs associated with certain vector-borne illnesses, such as the characteristic bull’s-eye rash in Lyme disease or jaundice in cases of severe malaria.

Depending on the suspected pathogen and the patient’s clinical presentation, one or more tests may be used in diagnosing vector-borne diseases, including serological, molecular, blood smear, and culture tests.

Know the vector-borne diseases in your region, but also consider new and unfamiliar pathogens. You may see new and uncommon tick- and mosquito-borne illnesses, as well as fungal, food, and waterborne illnesses in your area.

Climate-Sensitive Infectious Diseases Reported in the U.S.

Tick-Borne	Mosquito-Borne	Soil-Associated
Lyme Disease	West Nile	Fungal Infections
Rocky Mountain Spotted Fever	Dengue	Food- and Water-Borne
Anaplasmosis	Malaria
Ehrlichiosis
Powassan virus
Babesiosis

Track trends in vector-borne diseases through reliable sources of information including:

• Centers for Disease Control and Prevention (CDC) Tick-Borne Pathogen Surveillance: Realtime tracking and mapping of tick-borne diseases.
• Food and Agriculture Organization of the United Nations: Track vector-borne diseases affecting animals.
• Local Public Health Department: Track which diseases have been reported.
• U.S. Department of Agriculture
• Additional real-time tracking and mapping tools are under development.

Treatment and Care

Treatment for vector-borne diseases varies depending on the specific pathogen involved, the severity of the illness, and individual patient factors. Here’s an overview of common approaches to treating vector-borne diseases:

1. Antibiotics: For bacterial infections transmitted by vectors, such as Lyme disease, antibiotics are often prescribed. The choice of antibiotic and duration of treatment depend on the stage of the disease and the presence of complications. Commonly used antibiotics include doxycycline, amoxicillin, and ceftriaxone.
2. Antiviral Medications: In cases of viral infections like dengue fever, Zika virus infection, and chikungunya, treatment focuses on relieving symptoms, as there are no specific antiviral drugs available for most of these diseases. Supportive care, such as hydration and pain management, is essential to help patients recover.
3. Antimalarial Drugs: Malaria, a parasitic disease transmitted by mosquitoes, is treated with antimalarial medications. The choice of drug depends on the species of Plasmodium causing the infection and the severity of the illness. Commonly used antimalarial drugs include chloroquine, artemisinin-based combination therapies (ACTs), and mefloquine.
4. Antiparasitic Medications: Parasitic infections such as babesiosis and trypanosomiasis may require treatment with antiparasitic drugs. For example, babesiosis is often treated with a combination of atovaquone and azithromycin, while trypanosomiasis may require medications like suramin or pentamidine.
5. Supportive Care: Supportive care plays a crucial role in managing vector-borne diseases, particularly in cases of severe illness. This may include hydration, pain management, fever reduction, and management of complications such as organ failure or hemorrhage.
6. Long-Term Management: Some vector-borne diseases may require long-term management to prevent recurrence or complications. For example, patients with Lyme disease may require extended courses of antibiotics or treatment for persistent symptoms.

Do not delay lifesaving treatments, even if the suspected pathogen is not typically found in your area.

Prevention

Educate your patients on risk reduction techniques such as wearing long clothing, performing tick checks, using insect repellant, and eliminating standing water. Advise patients of changing patterns of risk where they live and travel, and if remind them to inform their health care provider if they get a fever or rash.

Share resources with your patients on how to prevent tick-borne and mosquito-borne illness.

Resources for Clinicians

Watch the webinar, download the presentation slides, or register for the online course.

Dengue Resources for Health Care Providers

West Nile Resources for Health Care Providers

Alpha-gal Resources for Public Health Officials

Rocky Mountain Spotted Fever (RMSF) Diagnosis and Testing

Lyme Disease Resources for Health Care Providers

Vector-Borne Disease Outlook

Humans are at an increasing risk for vector-borne diseases. More people are being exposed to the bites of potentially infected vectors for longer periods each year. In the absence of effective protection tools and implementation, current trends will likely worsen in the future.

The interplay between climate change and the spread of vector-borne viruses poses a significant challenge to global health. The emergence of these viruses in new geographic areas, coupled with increased human cases, highlights the urgent need for proactive measures. Clinicians and public health officials must remain vigilant, considering the evolving landscape of vector-borne illnesses and adapting diagnostic and preventive strategies accordingly.

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