Fungal diseases: An emerging threat?

The unfolding of the zombie virus in The Last of Us to cause a pandemic could be a dystopian science fiction. But is the world facing the threat of a fungal infection pandemic?

Every living organism on earth is prone to fungal infection. Over 300 million people around the globe suffer from it. There are about 12 million species of fungi thriving in marine, freshwater, desert, forests and almost all other ecosystems on the earth. Of these, a fraction cause infections in humans. Yet, about 1.5 million die each year of fungal infections—more than people killed by malaria and TB.

These are mostly people from an ‘at-risk’ group—old or suffering from health problems with compromised immune systems. A healthy individual is unlikely to fall to a fungus.

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But, recently, a 61-year-old man from Kolkata who works on plant fungi was diagnosed with a fungus that commonly causes a disease called ‘Silver Leaf’ in rose plants.

Incidences of plant fungi infecting humans are a cause for concern. The emergence of new fungal species capable of infecting humans, fungi becoming resistant to a handful of available treatments, overuse of fungicides in agricultural settings and climate change pose serious human health concerns.

Modernisation of the healthcare industry has enabled people to live longer with chronic illness which is one of the many factors that are contributing to increasing fungal threats. “About 30 years ago, fewer people were under medical treatment such as surgery or organ transplant and required immunosuppressive drugs to keep their immune system at a low,” says Vikas Yadav, Postdoctoral Associate, Duke University Medical Center, USA.

Mucormycosis caused by the fungus is a rare infection which challenged the healthcare facilities in India during Covid. This fungus is commonly found within our close environments like soil, plants, manure, decaying fruits and vegetables and even in the air. During the second wave of the pandemic, when the hospitals were crammed with severely immunocompromised individuals, the conditions were perfect for black fungus. The infection cases surged unexpectedly.

Climate change and fungi

Global warming can stir things up on the fungi front. “We have a body temperature of around 37°C. Fungi usually grow at an optimal temperature of 25 °C to 30 °C. Fungi have the potential to tolerate and adapt to increasing temperatures which could be one reason which increases the likelihood of fungal pathogens in the soil to adapt to different hosts," says Amey Redkar, Assistant Professor, National Centre for Biological Sciences, Bengaluru.

Studies have shown that most fungal diseases are acquired from the environment. Climate change models have predicted that disruptions caused by increased tropical cyclones, droughts and floods can aid soil-borne fungi to travel long distances and cause infections in new areas leading to emerging diseases.

Candida is a fungus that causes healthcare-associated infections such as urinary tract infections, pneumonia, and surgery-related infections. Like many other species, Candida auris was present in the environment. But suddenly, it became pathogenic to humans. It was first found infecting a woman in Japan in 2009 and was reported in multiple regions across the world. It is speculated that the ability of Candida to cross the human body temperature filter might have emerged due to climate change.

“We are now discovering more species that could cause human infections and are also resistant to multiple anti-fungal drugs. Candida auris is one of them. Most fungal species that cause human infections have the ability to become drug-resistant,” says Vikas.

In a study published earlier this year, researchers from Vallabhbhai Patel Chest Institute, University of Delhi, screened 62 apples from various states and found Candida on the surfaces of eight cold storage apples. They say that Candida on these apples was genetically similar to other Candida strains found in hospitals around the world.

On WHO list

Candida, along with Cryptococcus and Aspergillus, tops the World Health Organisation's list of most dangerous fungi to humans released in 2019. Until recently, microbial diseases dominated research focus across the globe. Fungal infections were largely neglected and excluded from microbial surveillance programmes.

Fungal pathogens are an under-recognised emerging global health concern and their increased resistance to anti-fungal drugs complicates it further. The WHO priority list hopes to bring more attention and resources to fungal pathogens.

Fungal resistance is a natural process. However, excessive use of anti-fungal drugs and fungicides facilitates fungi to rapidly acquire resistance outpacing the development of new treatments. According to Vikas, the availability of over-the-counter anti-fungal creams for cosmetic purposes and their prescription without proper diagnosis contributes to the development of drug resistance in fungi.

For example, Azole is one class of antifungals being used since the early 1980s. Studies around the world have mapped an increase in clinical resistance of fungi towards azoles which is driven by agricultural use.

"There are very few classes of fungicides. Whether pesticides or antifungal drugs used in clinical treatments, they target limited fungal cellular processes. These fungi are exposed to chemical pesticides and at a much higher concentration in the field than during the clinical treatments," adds Amey.

Removing fungicides from agriculture is neither trivial nor practical. Panama Wilt, a fungal disease that infects cultivated varieties of bananas, can stay in the soil for up to 20 years. Amey says fumigating fields with fungicides destroys important soil microbes and makes the soil infertile. Farmers who realise this often perform an incomplete treatment, exposing fungi in the soil to different concentrations of fungicides, which can lead to the emergence of fungicide resistance.

There is a reservoir of new pathogens in the environment that can potentially adapt when exposed to antifungal chemicals causing pandemics. Tackling antifungal infections and resistance requires coordinated efforts. Currently, the lack of infrastructure and funding for research globally limits the understanding of the world of fungi.