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How do fungi cause disease? – Part 2

How do fungi cause disease? Continuations from previous article explaining methods used by fungi to cause disease.

Subcutaneous mycoses

Subcutaneous fungal infections extend from the lower skin layers down to the bone. They almost always result from inoculation via a break in the skin surface (trauma). Inoculation usually arises from contamination with soil or vegetation. They are more common in tropical areas. Examples include: mycetoma and sporotrichosis.

Madurella mycetomatis (brown) in tissue (purple) The ankle of a patient suffering from severe mycetoma

Systemic mycoses

Systemic mycoses are fungal infections of the normally sterile tissues and organs of the body. These are most often seen in moderately or severely immunocompromised or critically ill patients. The main systemic fungal infections are:

  • Candidaemia and metastatic Candida infections (endocarditis, renal candidosis)
  • Pulmonary and invasive aspergillosis
  • Pneumocystis jirovecii pneumonia
  • Cryptococcal meningitis.
  • Other invasive mould infections (e.g. mucormycosis)

These systemic infections are medical emergencies with high mortality particularly if treatment is delayed.

The source of infection for candidaemia is predominantly the patient’s own colonising flora (mainly GI tract or skin). Human to human transmission is mainly seen in neonates (staff to pre-term neonates). Candida gains access into the blood stream through vascular lines, deep wounds or damaged GI tract mucosa. Good hand hygiene and line care is essential in prevention.

image of a neonate on a neonatal ICU, with multiple lines which can be a portal of entry for Candida Above is an image of a neonate on a neonatal ICU, with multiple lines which can be a portal of entry for Candida

Cryptococcus species can be found on the oral mucosae of healthy individuals and its portal of entry is typically the airways. Pigeon and bat droppings have been identified as the main environmental reservoirs. This has implications on hospital air filtration requirements as part of infection prevention.

birds at an air duct opening

Spores of Aspergillus and other environmental moulds are commonly present in both outdoor and indoor air. The portal of entry for these moulds is inhalation. This has implications for hospital air (HEPA) filtration requirements as part of infection prevention in areas with patients at risk (haematology and transplant units).

Pneumocystis jirovecii is a strict human pathogen with no known environmental reservoir. Some cases of human to human transmission have been reported in immunocompromised patients but low level colonisation is the most likely source of infection in most cases.

Some systemic mycoses are caused by endemic primary pathogens. Although they can also infect immunocompetent individuals, they are clinically relevant in immunocompromised patients as well. The most common endemic mycoses are blastomycosis, coccidioidomycosis, histoplasmosis and paracoccidioidomycosis. The severity of these mycoses can vary from a brief, self-limiting disease of the lung (e.g. coccidioidomycosis in an immunocompetent individual) to a life-threatening disseminated disease in an immunocompromised patient. Although rare, these mycoses should be considered in every patient with relevant travel history.

world map showing the location of common endemic mycoses

Above is a world map showing the location of common endemic mycoses

(Click to expand image)

Image source

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The Role of Antifungal Stewardship

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