Fungal Pathogens Adapting to Heat

For decades, human body temperature has served as a primary defense mechanism against fungal infections. Most fungi thrive in soil and decaying matter at cool temperatures, unable to survive the 37 degrees Celsius (98.6 degrees Fahrenheit) environment of the human body. However, new research suggests this thermal barrier is breaking down. As global temperatures rise, fungal pathogens like Candida auris are rapidly evolving to tolerate heat, allowing them to bypass our immune defenses and cause severe, drug-resistant infections.

The Breaking of the Thermal Barrier

The fundamental reason healthy humans rarely suffer from systemic fungal infections is thermal exclusion. Of the estimated 1.5 million to 5 million fungal species on Earth, only a few hundred can cause disease in humans. Microbiologist Arturo Casadevall from the Johns Hopkins Bloomberg School of Public Health has long proposed that our high basal body temperature functions as a thermal shield.

This shield is now under siege. As the ambient climate warms, the gap between environmental temperatures and human body temperature narrows. Fungi that adapt to survive heat waves and hotter summers are inadvertently being trained to survive inside the human body.

  • The Adaptation Process: Fungi are typically adapted to temperatures between 12 degrees Celsius and 30 degrees Celsius.
  • The Shift: Global warming selects for fungal lineages that can reproduce at temperatures exceeding 37 degrees Celsius.
  • The Consequence: Once a fungus can tolerate human body heat, the fever response—our body’s attempt to “cook out” invaders—becomes ineffective.

Candida auris: The First Climate Change Pathogen?

The most alarming example of this phenomenon is Candida auris ($C. auris$). First discovered in the ear canal of a patient in Japan in 2009, this yeast seemingly appeared out of nowhere. Unlike other pathogens that spread from a single point of origin, distinct strains of $C. auris$ emerged independently and almost simultaneously on three different continents: Asia, South America, and Africa.

Researchers believe this simultaneous emergence was driven by a common environmental factor: rising temperatures. Because these genetically distinct strains all learned to tolerate heat at the same time, C. auris is widely considered the first pathogenic fungus to emerge specifically due to climate change.

Why C. auris is Dangerous

The Centers for Disease Control and Prevention (CDC) classifies C. auris as an “urgent threat” for several specific reasons:

  1. Multidrug Resistance: Roughly 90% of C. auris strains are resistant to fluconazole, a common antifungal medication. An estimated 30% are resistant to amphotericin B, and some strains are resistant to all three major classes of antifungals (azoles, polyenes, and echinocandins).
  2. High Mortality: Based on limited data, the CDC reports that 30% to 60% of people with C. auris infections impacting the blood, heart, or brain have died.
  3. Persistence: The fungus can live on surfaces for weeks. It is notoriously difficult to eradicate from hospital rooms, requiring specialized high-strength disinfectants.

Research Findings: How Heat Drives Evolution

Recent studies from the Duke University School of Medicine have provided concrete evidence on how heat accelerates fungal mutation. Led by postdoctoral associate Asiya Gusa, researchers grew Cryptococcus deneoformans at human body heat (37 degrees Celsius) rather than its preferred 30 degrees Celsius.

The results showed that heat stress didn’t just kill the weak fungi; it drastically increased the mutation rate of the survivors. Specifically, the team observed the movement of transposons, also known as “jumping genes.”

  • Transposon Movement: At higher temperatures, these DNA segments moved around the fungal genome five times more frequently.
  • Rapid Evolution: This genetic shuffling allows the fungus to turn on genes that help it withstand heat and resist drugs.
  • Permanent Changes: Once these mutations occur, they can be passed down to future generations, creating a lineage of heat-hardy, drug-resistant pathogens.

This mechanism suggests that hotter climates act as an evolutionary accelerator. Fungi are not just surviving the heat; the heat is actively causing them to mutate faster, generating new variants that are harder to kill.

Public Health Implications

The adaptation of fungi to heat presents a complex challenge for modern medicine. Currently, there are no vaccines for fungal infections. Furthermore, because fungal cells are eukaryotic (similar to human cells), developing drugs that kill the fungus without harming the patient is difficult.

The spread is already evident. In the United States, clinical cases of C. auris rose from 476 in 2019 to 1,471 in 2021. Screening cases, where the fungus is found on the skin but is not causing active infection, tripled during the same period to over 4,000.

Hospitals are the primary battleground. Patients with invasive medical devices like breathing tubes, feeding tubes, or central venous catheters are at the highest risk. The fungus colonizes the skin and enters the bloodstream through these lines.

Protecting Against Future Threats

To mitigate this risk, the medical community is focusing on:

  • Enhanced Surveillance: Implementing better diagnostic tools, such as MALDI-TOF mass spectrometry, to identify specific fungal species quickly.
  • Stewardship: Limiting the unnecessary use of broad-spectrum antifungals and antibiotics to prevent resistance.
  • New Therapeutics: Developing novel antifungal classes, such as fosmanogepix and ibrexafungerp, which target different parts of the fungal cell wall or synthesis process.

Frequently Asked Questions

What is Candida auris? Candida auris is a type of yeast that can cause severe infections in hospitalized patients. It is known for being resistant to many antifungal drugs and for spreading easily in healthcare settings.

How does climate change make fungi more dangerous? Most fungi cannot survive at human body temperatures. As the environment warms, fungi evolve to tolerate higher heat. This adaptation allows them to survive inside the human body and resist the fever response that would normally kill them.

What are the symptoms of a C. auris infection? The symptoms depend on the part of the body infected but generally include fever and chills that do not improve after antibiotic treatment. Because it often affects patients who are already sick, symptoms can be hard to distinguish from underlying illnesses.

Is Candida auris contagious? Yes. It can spread from person to person in healthcare settings (like hospitals and nursing homes) through contact with contaminated environmental surfaces or equipment, or from person-to-person contact.

Can healthy people get Candida auris? Healthy people usually do not get sick from C. auris, but they can be colonized by it. This means they carry the fungus on their skin and can spread it to others without knowing it. The risk is highest for people with weakened immune systems or invasive medical devices.