Invasive fungal infection
Invasive fungal infections are generally uncommon, except in certain groups of patients with immune deficits. They are exceptionally rare in those with apparently normal immune systems. In some people with impaired immune systems, the period of risk is short, such as those with low white cell counts for a few days or weeks following chemotherapy. In other people, the risk period is long and determined by how reversible the immune deficit is. An example of this is renal transplantation, immediately following which the immune system is markedly depressed to prevent rejection of the new kidney. As time passes, anti-rejection drug doses are gradually reduced, thereby reducing the risk of infection. They are increased again if rejection occurs. If a life-threatening fungal infection does occur, one option to preserve life is to completely stop the anti-rejection drugs, sacrificing the new transplanted kidney (rejection is almost inevitable), but enabling the person to survive the fungal infection.
As the whole immune system is complex and functions as a network, few straightforward tests are available to get a precise handle on immune function at any one time. In HIV infection the CD4 cell count is an imprecise but useful general measure of immune status. In leukaemia patients, the neutrophils (granulocyte) count is an approximate measure of risk. In people taking corticosteroids (glucocorticoids), the dose and duration are approximate measures of risk and survivability from an invasive fungal infection. Following a significant bacterial infection, surgery or trauma, the immune system goes naturally from activation to depression (so called immunoparalysis) so that the immune reaction itself is not fatal. This period of immunoparalysis is a high risk period for fungal infections.
In general, an overall assessment of immune status is done by experienced clinicians who mentally ‘calculate’ the ‘net state of immunosuppression’. This imprecise assessment guides diagnostic and empirical antimicrobial choices, along with prior test results, drug tolerability and interactions and environmental factors.
Clinician experience, combined with availability of precise rapid diagnostic testing, is the most important factor determining survival from invasive fungal infection. (review: Hidden Killers: Human Fungal Infections, Brown et al 2012)
The most common invasive fungal infections are: