COVID-19 and black fungus: what is mucormycosis?

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Slime viewed through a microscope showing dark spherical spores. Photo: T. Rajamani, VINSTROM

Many people who are recovering from COVID-19 have recently had black fungal disease or mucormycosis. The fungus penetrates the sinus and reaches the intraorbital and intracranial regions. If progression is not checked early, 50-80% of patients can die.

Both authors are plant biologists interested in fungi. When we heard of mucormycosis for the first time last year, according to reports from Europe, the doorbell rang.

People most often experience mushrooms in their kitchens when fruits rot or the bread becomes moldy. Fungi evolved 400 million years ago and play an important role on earth. They have helped plants move onto land from their aquatic habitats and have helped them extract minerals from the soil. Mushrooms break down organic waste and recycle the nutrients trapped in the leaves and wood.

Some of them have also evolved into plant pathogens: they infect plants, reproduce, and disperse into other plants, leaving behind the destruction. The great Irish famine of 1845, which killed a million people, was the work of the fungus Phytophthora infestans, which wiped out the country’s staple crop.

While fungal diseases are common in plants, only a very small proportion of them affect humans. One reason is that animals, including humans, have developed complex immune systems.

However, when the immune system has been injured by another disease, fungi, which are otherwise harmless, take advantage and invade human tissues. These are known as opportunistic infections. Even so, unlike their pathogenic bacterial counterparts, fungi rarely cause life-threatening diseases. Some fungi, like candida yeast, can sometimes cause serious infections. Candida lives on the skin and in the mouth, throat, and vagina of healthy individuals without causing any problems. However, if the host’s body has been weakened by another disease or medication, it can lead to oral thrush, diaper rash, and vaginal infections.

The mucous membrane fungi are even less of a problem. These include the genera Mucor and Rhizopus. These are ubiquitous molds that can be found in soil, compost, animal dung, rotting wood, and plant material. You may have seen them as the black growth on old fruit and bread. Mucous membrane fungi are generally the first to colonize dead or rotting plant material. They quickly take advantage of the limited amount of simple carbohydrates that are available before other mushrooms show up for the more complex carbohydrates like cellulose.

Like most mushrooms, Mucor produces millions of microscopic, spherical, dark-colored structures called spores that are dispersed in the air. When the spores land on moist surfaces such as soil or plant matter, they begin to germinate and produce thread-like structures known as mycelium. The mycelia branch out and feed on the sugars around them and grow.

Fungal spores measure a thousandth to a hundredth of a millimeter. The density of the spores – the number of spores per cubic meter of atmosphere – varies depending on the fungus, location (vegetation and exposed soil) and time of year. In tropical areas like India, spore numbers are generally higher in summer than during monsoons. But when compared to the 1,000 to 5,000 spores per cubic meter outdoors, the number in homes is usually only 100 to 250. Five to 10 species make up more than 90% of the total spore density in the air.

Hospitals are not free from these spores. A 2014 study in Tehran suggested that the hospital air could carry many opportunistic pathogenic fungi such as Candida, Aspergillus, Penicillium, and Rhizopus.

If a patient whose immune system is weakened inhales mucus spores, they can develop mucormycosis. This is a rare, non-contagious disease – but it can be debilitating or fatal if not treated quickly. The incidence of mucormycosis infections has increased over the past decade, mainly due to the greater number of organ transplants. People who have received transplanted organs rely on immunosuppressive drugs to keep their bodies from rejecting the new organs. However, in this state they are also predisposed to infection.

People with COVID-19, HIV / AIDS and other viral diseases, congenital bone marrow diseases, severe burns, cancer, and untreated or infrequently treated diabetes have decreased immunity and are prone to developing mucormycosis. COVID-19 patients who have received steroids are particularly at risk because steroids suppress the immune system. For this reason, steroids should only be used when absolutely necessary.

Experiments with rats and rabbits have shown that the inhaled spores are quickly killed by white blood cells in healthy animals. However, if the host’s immune response has been suppressed, the body will produce fewer white blood cells. In this state, the spores germinate and grow quickly as thin, wire-like tubes that branch and enter and kill blood vessels.

When Mucor attacks the sinuses, it spreads to the lungs, brain, and central nervous system. Common symptoms of the resulting mucormycosis are fever, headache, reddish and swollen skin near the nose or eyes, facial pain, cough that creates bloody or dark fluids, and shortness of breath. Doctors can diagnose this through a tissue biopsy and an X-ray of the lungs.

The two drugs that are most effective in treating mucormycosis are amphotericin B and posaconazole – provided the infection is detected early. However, the latter is difficult as we do not know a reliable diagnostic feature of mucormycosis.

However, there are some simple steps we can follow to lower our risk of developing mucormycosis. The first is to educate society about the disease. Second, we need to regularly examine the air in hospitals, especially in intensive care units, to see if there are any spores. Third, we need to ensure that the humidifiers used during oxygen therapy are sterile. Fourth, patients who are recovering should be instructed to stay indoors until they have regained their natural strength and immunity. Fifth, patients who farm or garden should be instructed to stop working until the storm subsides.

TS Suryanarayanan is at the Vivekananda Institute for Tropical Mycology (VINSTROM) in Chennai. R. Uma Shaanker is at the University of Agricultural Sciences in Bengaluru.



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