Why scientists are concerned about new plant-to-human fungal transmission

In a rare case, a fungus found only in plants infected a man in India, raising concerns about the growing threat of micro-organisms jumping from one species to the other.

Chondrostereum purpureum, which causes silver leaf disease in plants, made a 61-year-old man from Kolkata sick. He had cough, fatigue and difficulty swallowing food, says a paper published in the peer-reviewed journal Medical Mycology Case Reports. 

The unnamed patient is a mycologist, an expert who works with fungi and mushrooms. Routine lab tests couldn’t tell what had made him sick. It was after samples were sent to a World Health Organization laboratory that doctors were able to confirm the man had contracted a fungal infection.

He was treated with anti-fungal drugs and has since recovered. 

“This was a rare fungus and we haven't seen this before in our practice. Conventional methods like culture and LCB stain (a type of test to detect fungus) were not conclusive. So we had to use help of DNA-sequencing to identify (the pathogen),” Soma Dutta, an Indian microbiologist who co-authored the paper, tells TRT World.

While fungal transmission between plants and humans has happened before, the new case in India is the first to be reported to involve chondrostereum purpureum, which exclusively affects plant species, especially the rose family.  

Some media outlets misreported this to be the first plant-to-human fungus transfer, leading people to paint a doomsday scenario as depicted in the popular TV series The Last of Us, in which a fungal disease turns humans into zombies.

“I don't think there should be any reason to panic. As for this patient, prolonged exposure (to plants) is the probable cause of acquiring this infection,” Dutta says in an email. 

The migration of pathogens from the plant and animal kingdoms has put health experts and officials on edge. The novel Coronavirus, which has killed more than 6 million people in the last three years, is believed to be a zoonotic disease, meaning humans acquired it from an animal source.

In the world of dangerous pathogens, fungal infections pose a serious challenge to global health as they are difficult to identify and only a handful of drugs are available in the market to treat them compared with dozens of therapies for bacteria and viruses. 

Fungi are everywhere - living and breeding in soil, plants, animals and water. They play an important role in decomposing organic material like fallen leaves and feeding nutrients to the trees. Most of them are harmless and cause minor nuisance like dandruff in our hair. 

Among the millions of different fungus species that exist in our environment, around 30 or so, such as aspergillus, cryptococcus and fungal keratitis, which can cause blindness, can make humans really sick. Mortality rate of fungal infections is high especially among people with a weakened immune system. 

“Fungus can be a threat to us anytime. There may be just a few of them but the mortality rate of any invasive fungal infection is 40 percent and that is not good. Forty percent means almost one in two patients will die,” says Dr Arunaloke Chakrabarti, the ex-president of International Society of Animal and Human Mycology and an authority on fungi infections. 

Covid-19’s mortality rate in even one of the most affected countries, such as Peru, has not exceeded 5 percent, according to a Johns Hopkins University analysis. 

The silent killer 

Death due to complications arising from fungal infections largely go unnoticed.  

The Global Action for Fungal Infections, an international organisation made up of leading experts in the field, says that more than 1.5 million people die every year from fungal infections. But cause behind many deaths is under-diagnosed or misreported.

“So why have you not heard about it? I think fungus is not very sexy. I mean Zika virus and Ebola are ‘sexy’. Coronavirus is super interesting for everybody,” says David Denning, a professor of infectious diseases at the University of Manchester.

“If you don’t look for fungus, you don’t diagnose it.”

Hundreds of thousands of people with Human Immunodeficiency Virus (HIV) die every year. HIV weakens the immune system, making patients vulnerable to other diseases, such as tuberculosis (TB). 

But TB kills only a third of HIV patients, while the real cause behind almost half of the deaths is fungal infections. 

“So there’s a lot of misattribution of deaths because of the common association between HIV and TB,” says Denning. 

Without proper training, doctors and nurses can misdiagnose because symptoms of fungal infections, such as fever and cough, are quite common. 

A similar diagnostic challenge occurs in labs. Usual blood culture and even MRI scans are not sufficient to detect fungus in most cases. Newer biomarker testing is expensive and still poses the challenge of false-negative results, says Chakrabarti.

“Generally, our medical curriculum or even the technician course does not encompass fungal diseases in detail. It’s more focused on bacterial diseases,” he says. 

The gaps in diagnostic ability were in stark display in India two years ago, when doctors put Covid-19 patients on powerful steroids for too long without realising that it was exposing them to deadly mucormycosis, also known as black fungus. 

Steroids weaken the immune system, which is the natural defence system of our bodies against pathogens. 

Without proper diagnosis and tests, doctors end up giving antibiotics instead of antifungal drugs, which is leading to antifungal resistance and making fungal pathogens hard to treat, says Denning of University of Manchester. 

“A skin fungus called trichophyton has become resistant to antifungals. We have also come across Candida auris, a new fungus that just didn’t exist 15 years ago.” 

Crossing the kingdoms 

Plant or animal-based fungi infecting humans is a rare phenomenon but not unusual.

Every year, 3 to 5 new fungal infections are found in patients that can be traced to plants or animals, says Denning, adding that this can be because of climate change. 

At any given time, we inhale thousands of fungal particles, or micro-organisms known as spores. Most of them can’t survive at our body’s average temperature of 37 degree celsius and die.

One reason our toes and eyes are more likely to be infected with fungal infections is because they have relatively lower temperatures of 32 and 34 degrees respectively. 

“So our body temperature kills off a lot of fungi. But there’ll be some that, you know, start to edge up the evolutionary drift with a tolerance of high temperature and that probably will lead to some additional human infections and other bird and mammal infections, as well,” says Denning.

A deadly form of fungus has wiped out hundreds of species of toads by attacking their skin and making it difficult for them to breathe and swim. 

For humans, Candida auris, a bloodstream fungal infection, which first emerged in 2007, has become a major threat, says the Centre for Disease Control and Prevention. 

“It has quickly spread across the world and is already a pandemic. It has a mortality rate of 40 percent and to date, we don’t know about its origin,” says Chakrabarti.