Mucor viewed through a microscope showing dark spherical spores. Photo: T. Rajamani, VINSTROM
Many people recovering from COVID-19 have of late been afflicted by black fungus – or mucormycosis – disease. The fungus invades the sinus and makes its way into the intraorbital and intracranial regions. If its progression is not checked early, 50-80% of patients could die.
Both authors are plant biologists interested in fungi. When we first heard of mucormycosis sometime last year, from reports from Europe, it rang a bell.
People experience fungi most often in their kitchens, when fruits rot or the bread turns moldy. Fungi evolved 400 million years ago and play an important role on Earth. They have helped plants move from their aquatic habitats to land, and still help them obtain minerals from the soil. Fungi decompose organic litter and recycle the nutrients locked up in the leaves and wood.
Some of them have also evolved to become plant pathogens: they infect plants, multiply and disperse to other plants, leaving destruction in their wake. The great Irish famine of 1845 that left a million people dead was the work of the fungus Phytophthora infestans, which wiped out the country’s staple potato crop.
While fungal diseases are common among plants, only a very small fraction of them assail humans. One reason is that animals, including humans, have evolved intricate immune systems.
However, when the immune system has been breached by another illness, fungi that are otherwise harmless take advantage and invade human tissues. These are called opportunistic infections. Even so, unlike their pathogenic bacterial counterparts, fungi rarely cause life-threatening diseases. A few fungi, like the Candida yeast, can sometimes kick off a serious infection. Candida lives on the skin and inside the mouth, throat and vagina of healthy persons without causing any problems. But if the host’s body has been weakened by another disease or drugs, it can cause oral thrush, diaper rash and vaginal infections.
The Mucoralean fungi are even less problematic. They include the genuses of Mucor and Rhizopus. These are ubiquitous molds occurring in the soil, compost, animal dung, rotting wood and plant material. You may have seen them as the black growth on old fruits and bread. Mucoralean fungi are generally the first colonisers of dead or decaying plant material. They rapidly utilise the limited amount of simple carbohydrates available before other fungi show up for the more complex carbohydrates, such as cellulose.
Like most fungi, Mucor produces millions of microscopic spherical, dark-hued structures called spores, which are dispersed in air. When the spores land on moist surfaces, like soil or plant material, they begin to germinate and produce thread like structures called mycelia. The mycelia branch out and feed on sugars in their surroundings and grow.
Fungal spores measure one thousandth to one hundredth of a millimeter. The density of the spores – the number of spores per cubic metre of atmosphere – varies depending on the fungus, the location (vegetation and exposed earth) and season. In tropical areas like in India, spore counts are generally higher during the summer than during the monsoons. But compared to the 1,000-5,000 spores per cubic meter outdoors, the count inside homes is typically 100-250 only. Five to 10 species account for more than 90% of the total spore density in the air.
As it happens, hospitals are not free from these spores. A study in Tehran in 2014 suggested that hospital air could carry many opportunistic pathogenic fungi like Candida, Aspergillus, Penicillium and Rhizopus.
When a patient whose immune system has been compromised inhales Mucor spores, they may develop mucormycosis. This is a rare, non-contagious disease – but it can be debilitating or fatal if not treated quickly. The frequency of mucormycosis infections has increased in the last decade, principally because of the greater number of organ transplants. People who have received transplanted organs depend on immunosuppressant drugs to keep their bodies from rejecting the new organs, but in this state they are also predisposed to infection.
People suffering from COVID-19, HIV/AIDS and other viral diseases, congenital bone marrow disease, severe burns, cancers and untreated or irregularly treated diabetes have reduced immunity and are prone to developing mucormycosis. COVID-19 patients who have received steroids are particularly at risk because steroids suppress the immune system. This is why steroids should not be used unless absolutely necessary.
Experiments with rats and rabbits have found that the inhaled spores in healthy animals are quickly killed by white blood cells. But if the host’s immune response has been suppressed, the body produces fewer white blood cells. In this condition, the spores germinate and grow rapidly as thin, wire-like tubes that branch out and enter the blood vessels and kill them.
When Mucor attacks the sinuses, it spreads to the lungs, the brain and the central nervous system. Common symptoms of the resulting mucormycosis include fever, headache, reddish and swollen skin near the nose or eyes, facial pain, cough producing bloody or dark fluids, and shortness of breath. Doctors can diagnose it through a tissue biopsy and an X-ray scan of the lungs.
The two drugs most effective at treating mucormycosis are amphotericin B and posaconazole – provided the infection is found early. However, the latter is hard to do because we don’t know of a reliable diagnostic feature of mucormycosis.
This said, there are a few simple steps we can follow to lower our risk of contracting mucormycosis. The first is to educate society about the disease. Second: we must periodically sample the air in hospitals, especially in the critical care wards, to check for the presence of spores. Third: we must ensure that the humidifiers used during oxygen therapy are sterile. Fourth: recovering patients should be advised to remain indoors until they regain their natural strength and immunity. Fifth: patients engaged in farming or gardening should be advised to lay off from work until the storm has settled.
T.S. Suryanarayanan is at the Vivekananda Institute of Tropical Mycology (VINSTROM), Chennai. R. Uma Shaanker is with the University of Agricultural Sciences, Bengaluru.