When we think of climate change, we often picture melting glaciers, raging wildfires and soaring temperatures. Rarely do we think about its effect on the spread of infectious diseases and the downstream effects on human health.
But the truth is that warmer, shorter winters; changes in precipitation patterns; extreme weather events and habitat encroachment have created ideal conditions for the spread of new pathogens.
Infectious disease specialists from Massachusetts General Hospital and the University of California have authored a new review in JAMA that highlights the impact of this growing threat and calls for more training and awareness among the medical community.
Matthew Phillips, MD, PhD
“Climate change is an all-encompassing problem that affects so much of human health,” says Matthew Phillips, MD, PhD, a fellow in the Division of Infectious Diseases at Mass General and the first author of the paper. “It’s not just infectious disease specialists that will have to deal with these challenges—they will affect all healthcare providers.”
“So, the goal of our review was really to create a primer that would be accessible to even a general practitioner, informing and teaching about the kind of changes that can be expected.”
Climate Change and Infectious Diseases
.Infectious diseases are caused by pathogens (such as bacteria, viruses and fungi) that enter the body, multiply and ultimately cause an infection.
While certain diseases are communicable, spreading from person to person, others can transmit from animals to humans or even spread through contaminated food or water.
Vector Borne Diseases
Bloodsucking insects such as mosquitos and ticks often serve as vectors capable of transmitting infectious pathogens to humans or from animals to humans.
Climate change has led to shorter, warmer winters and wetter springs and summers that have enabled mosquitos and ticks to thrive and multiply.
Between 2004 and 2018, the number of arthropod (mosquito and tick) associated illnesses reported to the CDC more than doubled.
Tick-borne diseases such as babesiosis and Lyme disease are becoming increasingly prevalent even during winter months (given Lyme disease has been known to be a ”summer disease”).
Additionally, researchers observed that these diseases are expanding their geographical reach, manifesting in regions further west and northward compared to their previously known patterns.
“With longer summers, less freezes, and shorter winters, the time that ticks and other vectors can stay alive is longer. So there are more opportunities for these ticks to interact with people and potentially spread disease,” says Phillips.
“We were taught in medical school that these are summertime diseases. But as the climate changes, these are diseases that we're going to start seeing in the wintertime as well because we're not getting those intense freezes right away."
Another concern underscored by the researchers is the spread of malaria–a disease caused by a parasite that is spread to humans through the bites of infected mosquitoes.
Historically, malaria has remained uncommon in temperate climates such as those found across much of the United States.
It does, however, remain prevalent in tropical and subtropical countries.
Phillips and colleagues note the mosquitoes responsible for transmitting malaria are now moving northward due to climate-driven shifts.
Changes in rainfall patterns have resulted in increased mosquito populations and an increased rate of disease transmission.
“As an infectious disease clinician, one of the scariest things that happened last summer was the locally acquired cases of malaria.” said lead author George R. Thompson, a professor at the UC Davis School of Medicine. “They happened to people who didn't travel outside of the U.S.”
Zoonotic Diseases
Zoonotic diseases are diseases that can spread between humans and other vertebrate animals, including viruses, bacteria, and parasites.
Climate change plays a significant role in altering the dynamics and behaviors of these host animals.
In the Western U.S., shifts in precipitation and temperature have impacted the availability of food, population sizes, and behaviors of rodent hosts responsible for transmitting diseases such as plague and hantavirus (a disease transmitted through contact with rodent excretions).
These changes have led to higher rates and a greater northward spread of zoonotic diseases.
Furthermore, habitat destruction worsens the situation, bringing various species into closer contact and increasing the risk of pathogen transmission between species, including humans.
Fungal Infections
Fungal infections are diseases caused by fungus (yeast or mold); they can range from mild fungal skin infections, like ringworm, to more severe lung infections from breathing in fungal spores, like Cocciodioides (also known as Valley fever).
Cases of Valley fever have been limited to hot, dry areas in California and Arizona until recently, when it was diagnosed as far north as Washington state, the authors note.
While fungal infections in humans are rare because the temperature of the human body is typically too warm to support fungal growth, the rising temperatures could be helping to acclimate fungi to higher temperatures, which could lead to higher rates of infection in the future.
"Due to climate changes, fungi that initially weren't pathogens for people become more tolerant of higher temperatures, and this can lead to them expanding into becoming a pathogen in a way that they weren't maybe 50 years ago,’’ adds Phillips.
Climate Change and the Medical Community
The researchers emphasize that the medical community can play a crucial role in advocacy and awareness of the role of climate change in the spread of infectious diseases.
Regina Larocque, MD, MPH
‘’Physicians are very trusted people, scientists as well, they both rank in the top five kinds of professions that people trust,’’ says Regina Larocque, MD, MPH, a physician investigator in the Division of Infectious Diseases at Mass General and a co-author of the JAMA paper.
‘’Similar to how physicians can advocate for vaccines to prevent future illnesses, physicians advocating for policies to help curb the greenhouse gas emissions and curb future climate change is kind of the same thing. It's preventing these changes from getting worse down the line,’’ she adds.
Larocque, Phillips and the rest of the research team view this review paper as an opportunity—an urgent call to action—for healthcare professionals worldwide to address this critical issue. Hoping to remind everyone that we only have one planet Earth, and it is imperative to act now.
‘’Even though we're not climate scientists, we're not meteorologists– we're still very much involved in the day-to-day, very personal effects that climate change has on our patients. ’’
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