Microbiology

Aspergillus fumigatus

Aspergillus fumigatus

Also known as: Aspergillosis

Industry of interest: Healthcare

Classification: Fungi

Microbiology: Aspergilli are saprophytic fungi that are ubiquitous in nature. They are commonly found in soil and decaying vegetative material (Dagenais, and Keller, 2009). Aspergillus fumigatus is the primary cause of aspergillosis infection in humans. Although aspergillosis can be acquired in the community a large proportion of cases emerge from the hospital environment.
 

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Biology

Habitat and transmission: A. fumigatus produces small, hydrophobic conidia that are easily dispersed in air currents. The A. fumigatus conidia are hardy and can survive in a wide range of environmental conditions. A. fumigatus causes infection in most cases through inhalation of conidia from the environment.

Treatment and antimycotic resistance: There are currently four classes of antifungal drugs with activity against Aspergillus species. They are the polyenes (amphotericin B), the triazoles (itraconazole, voriconazole and posaconazole), the echinocandins (caspofungin, micafungin and ondulafungin) and the allylamines (terbafine) (Qiao et al., 2008).  Repeated exposure to these antimycotics, especially the azoles, can lead to drug resistance in A. fumigatus (Qiao et al., 2008). Indeed, there have been reports of increasing drug resistance in A. fumigatus (Qiao et al., 2008).

Prevention and control: Some have suggested the use of antimycotic prophylaxis in at risk groups, however there is potential to cause unnecessary side-effects in the patient and a risk of causing drug resistance (Humphreys, 2004). Others suggest the use of positive pressure rooms to minimise at risk patient contact with potentially contaminated air. Construction and associated dusts on hospital wards are a particular issue for immunocompromised patients. If construction is necessary then patients should be placed as far away from site as possible in rooms with sealed windows. Classic infection control procedures such as hand hygiene, environmental cleaning/decontamination, minimising traffic in isolation rooms and considerations of room design should all reduce the likelihood of invasive aspergillosis in immunocompromised patients (Humphreys, 2004).

Symptoms/Effects

Aspergilli such as A. fumigatus are particularly problematic in asthma and cystic fibrosis sufferers as inhalation of conidia can cause allergic bronchopulmonary aspergillosis, which is a hypersensitivity reaction (Dagenais, and Keller, 2009). Non-invasive aspergillomas can form as a result of repeated exposure to Aspergillus conidia. Invasive aspergillosis targets the immunocompromised causing severe and life-threatening disease. Patients with haematological malignancy, chronic granulomatous disease, stem cell patients, organ transplant patients and HIV positive individuals are particularly at risk from this organism. The mortality rates from invasive aspergillosis are between 40-90% in these high risk groups (Dagenais, and Keller, 2009).

Technical

References:

Dagenais T.R. and Keller N.P. (2009) Pathogenesis of Aspergillus fumigatus in invasive aspergillosis. 22(3): 447-465.

Humphreys H. (2004) Positive-pressure isolation and the prevention of invasive aspergillosis. What is the difference? 56(2): 93-100.

Qiao J., Liu W. And Li R. (2008) Antifungal resistance mechanisms of Aspergillus. Jpn J Med Mycol. 49(3): 157-163.
 

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