Fungal infections are becoming more common in the United States, but unlike illnesses caused by bacteria or viruses, there's no vaccine to protect against a fungal threat.
While scientists aren't worried that a fungal infection like the one seen in HBO's "The Last of Us" will wipe out humanity, the infections are certainly a cause for concern.
Fungi cause a wide range of illnesses in people, from irritating athlete's foot to life-threatening bloodstream infections.
In the U.S., fungal infections are responsible for more than 75,000 hospitalizations and nearly 9 million outpatient visits each year, according to the Centers for Disease Control and Prevention. In 2021, around 7,200 people died from fungal diseases. These numbers, the CDC said, are likely an underestimate.
One type of fungus, Candida auris, can be resistant to all of the drugs used to treat it, and is particularly dangerous for hospitalized and nursing home patients. The fungus was first identified in Japan in 2009 and has since been found in over 30 countries, including the U.S., the CDC said.
Climate change also threatens to make several infection-causing fungi more widespread: The fungus that causes Valley fever thrives in hot, dry soil, and the fungus that causes an illness called histoplasmosis prefers high humidity.
Despite the growing threat, there are currently no licensed vaccines — in the U.S. or abroad — to prevent fungal infections.
"These are the most important infectious diseases that you have not heard of," said Karen Norris, an immunologist and vaccine expert at the University of Georgia. "A vaccine has the potential to move forward and protect a large swath of individuals."
Fatal fungal infections
Norris said that the ultimate goal would be to develop a single vaccine that protects against all fungal infections.
But a "pan-fungal" vaccine is incredibly challenging to make.
That's because, she said, unlike the Covid vaccines, which target a single pathogen — the SARS-CoV-2 virus — a fungal vaccine would ideally protect against the wide spectrum of fungi in existence, each biologically different from the next.
For now, Norris and her team have decided to focus on the three fungi responsible for the vast majority of fatal fungal infections in the U.S.:
- Aspergillus, a common mold that can cause an infection in the lungs and sinuses that can later spread to other parts of the body.
- Candida, particularly Candida auris, a type of yeast that can cause serious blood infections, particularly in people in health care settings.
- Pneumocystis, which can cause pneumonia.
In preclinical trials, the experimental vaccine developed by Norris and her team was shown to generate antifungal antibodies in animals, including rhesus macaques. With funding support, the researchers could start and finish the human vaccine trials within the next five years, she said.
In Arizona, researchers are focused on a vaccine to prevent Valley fever, a lung infection caused by the fungus Coccidioides. The fungus, typically found in the hot, dry soils of the Southwest, is an "emerging threat," Norris said, because climate change is expanding its range.
So far, the vaccine has been shown to be effective in dogs, said John Galgiani, the director of the Valley Fever Center for Excellence at the University of Arizona College of Medicine.
Little urgency, lack of funding
While experts know which fungi are best to target, vaccine development has been slow, mostly due to a lack of funding, said Galgiani, who is working to start a trial in humans for the Valley fever vaccine.
Many in public and private spaces don't see fungal vaccines as a "critical unmet need," he said. Respiratory viruses, such as the ones that cause Covid, the flu or measles, infect millions of people and lead to thousands of hospitalizations worldwide each year, he said. The viruses can be deadly for anyone, in any part of the world, he said, illustrating the need for vaccines to prevent those diseases.
By comparison, hundreds of species of fungi can cause illness in people, but the most common ones — such as those that infect the skin and nails, or cause vaginal yeast infections or athlete's foot — are non-life-threatening, according to Galgiani.
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Additionally, severe cases are sporadic across the U.S., he said.
Valley fever, for example, is usually limited to the Southern and Western regions of the U.S. and are usually serious for people with weakened immune systems. Most people breathe in Aspergillus every day without getting sick, but it can be life-threatening for people with cystic fibrosis or asthma. Candida auris infections have been mostly limited to health care settings, and pose the biggest threat to very sick patients.
"As a risk-benefit investment proposal, it fails," Galgiani said of developing a vaccine. "You would not put your retirement investment into this." He said it could take eight years before a fungal vaccine is made available in the U.S.
But as awareness of climate change's impact on fungal infections grows, funding support could grow and there could be a fungal vaccine developed sooner, Norris said.
In response to growing public health concerns about severe and life-threatening fungal diseases, the National Institutes of Health in September released a framework for how the U.S. could create a vaccine for Valley fever in the next 10 years.
Last October, the World Health Organization released its first-ever list of fungi that pose the greatest threat to public health, calling for more research into 19 fungal diseases.
Dr. Andrew Limper, a pulmonologist at the Mayo Clinic in Rochester, Minnesota, said that there are a handful of oral treatments for most mild to moderate fungal infections. Depending on the fungus, he said, people may need to take the medications for three to six months to clear the infection from their system. The drugs can come with side effects, including headache, stomach pain, vomiting and diarrhea.
People with strong immune systems oftentimes will recover with medication, but fungal infections, particularly those that affect lungs, can leave scarring, he said.
In severe cases, some people may need to take intravenous medications, such as Amphotericin B, he said.