Opportunistic fungal infections are caused by fungi that are nonpathogenic in the immunocompetent host, many of which are part of the normal upper respiratory tract flora. These organisms may cause pulmonary infection in immunocompromised hosts. Immunocompromised patients and patients with febrile neutropenia with opportunistic fungal infections may have normal chest radiographs. Thus, chest CT should be performed for further evaluation. Imaging abnormalities in this patient population should raise suspicion for opportunistic infection. Neutropenia is the single most important risk factor for Aspergillosis. Aspergillus is the most common opportunistic infection in patients with hematologic malignancy and bone marrow transplantation. Aspergillus spp., Candida spp., and Cryptococcus spp. are the most common fungal infections in patients with solid organ transplantation. These have been a recent increase in uncommon fungal pathogens causing invasive pulmonary disease.

Invasive fungal infections are a significant health problem in immunocompromised patients. The clinical manifestations vary and can range from colonization in allergic bronchopulmonary disease to active infection in local aetiologic agents. Many factors influence the virulence and pathogenic capacity of the microorganisms, such as enzymes including extracellular phospholipases, lipases and proteinases, dimorphic growth in some Candida species, melanin production, mannitol secretion, superoxide dismutase, rapid growth and affinity to the blood stream, heat tolerance and toxin production. Infection is confirmed when histopathologic examination with special stains demonstrates fungal tissue involvement or when the aetiologic agent is isolated from sterile clinical specimens by culture. Both acquired and congenital immunodeficiency may be associated with increased susceptibility to systemic infections. Fungal infection is difficult to treat because antifungal therapy for Candida infections is still controversial and based on clinical grounds, and for molds, the clinician must assume that the species isolated from the culture medium is the pathogen. 

Zygomycosis is the most lethal opportunistic fungal infection particularly among patients with diabetes mellitus, haematological malignancies, and patients receiving deferoxamine treatment. Rhizopus, Mucor, and Rhizomucor species account for up to 75 per cent of mucormycosis cases. Infection with Entomophthora species has been reported in immunocompetent patients. Fusarium species cause a broad spectrum of infections in humans, including superficial and disseminated infections, the latter with a mortality rate that approaches 100 per cent. Members of this genus may also cause allergic diseases and mycotoxicosis following the ingestion of toxin-contaminated food.

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Sophie Kate
Managing Editor
Microbiology: Current Research
Email: aamcr@alliedacademies.org