Zygomycosis

What is zygomycosis?

Zygomycosis is a rare infection caused by a class of fungi called Zygomycetes. They are a relatively primitive class of fungi and live on decaying organic matter.

What causes zygomycosis?

There are 2 orders of Zygomycetes:

• Mucorales
• Entomophthorales

Mucorales

Mucorales are rapidly growing fungi including two families, the Mucoraceae and Cunninghamellaceae. Mucorales usually causes infection in individuals with a compromised immune system due to drugs such as systemic steroids, and diseases such as lymphoma and poorly controlled diabetes mellitus. The fungi invade blood vessels and cause mucormycosis, an acute, rapidly spreading and fulminant systemic mycosis. Rhino-cerebral (nose and brain), lung, gastrointestinal and abdomino-pelvic, cutaneous and widespread forms have been reported. The mortality rate is very high.

COVID-19-associated mucormycosis has emerged particularly in diabetic males with COVID-19 treated with systemic corticosteroids. Many cases have been reported in India as an 'epidemic of black fungus' presenting most commonly as the rhino-orbital form.

Cutaneous lesions from Mucorales are due to traumatic implantation or secondary to spread via the bloodstream to the skin. Mucorales infection may result in plaques, pustules and abscesses or necrotic, ulcerated lesions.

The most common species to cause zygomycosis is Rhizopus arrhizus (Rhizopus oryzae).

Entomophthorales

Entomophthorales produces slowly progressive chronic disease. There is no vascular invasion and the infection is generally restricted to subcutaneous tissue (subcutaneous zygomycosis).

There are species of 2 different genera:

• Conidiobolus coronatus/incongruus
• Basidiobolus ranarum

Zygomycosis

Zygomycosis

What are the clinical features of subcutaneous zygomycosis?

Subcutaneous zygomycosis starts as a slowly progressive, painless, subcutaneous swelling. A single lesion or multiple satellite lesions may arise. There may be a history of preceding trauma. 

On palpation there is a uniform, disc shaped, movable lump. Characteristically it is non-pitting and hard in consistency. The overlying skin is normal in most cases. However, it can sometimes be tense, swollen, peeling or hyperpigmented but not ulcerated.

Clinical features show slight variations according to the species involved.

How is the diagnosis of zygomycosis made?

Diagnosis of zygomycosis requires deep incisional skin biopsy from the subcutaneous mass for histopathology and for microscopy and fungal culture.

• Culture reveals fast growing fungi characterised by primitive mostly aseptate hyphae.
• Culture at 30C confirms which organism is involved: Conidiobolus sp. will grow white to grey waxy colonies and Basidiobolus sp. will grow cream or yellow waxy colonies.

Care needs to be taken during the biopsy not to damage the fungi because non-viable organisms result in a negative culture result.

What is the treatment for zygomycosis?

Treatment is difficult and prolonged for both types of Zygomycosis and the results for Conidiobolus infections are particularly disappointing.

Therapies may include:

• Itraconozole
• Fluconozole
• Amphotericin B
• Voriconazole

Unfortunately even surgical resection is rarely curative.