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Opportunistic mycoses are infections due to fungi with low inherent virulence which means that these pathogens constitute an almost limitless number of fungi. These organisms are common in all environments.
The disease equation:
With opportunistic infections, the
tilted in favor of "disease" because resistance is lowered
when the host is immunocompromised.
In fact, for the immunocompromised host, there is no such thing as a
fungi most frequently isolated from immunocompromised patients are saprophytic
(i.e. from the environment) or endogenous (a commensal).
The most common species are Candida
species, Aspergillus species, and Mucor
upward trend in the diagnoses of opportunistic mycoses reflects increasing
clinical awareness by physicians, improved clinical diagnostic procedures and
better laboratory identification techniques. Another important factor
contributing to the increasing incidence of infections by fungi that have
not been previously known to be pathogenic has been the rise in the number of
immunocompromised patients who are susceptible hosts for the most uncommon
agents. Patients with primary immunodeficiencies are susceptible to mycotic
infections particularly when cell-mediated immunity is compromised. In
addition, several types of secondary immunodeficiencies may be associated with
an increased frequency of fungal infections.
When a fungus is isolated from an immunocompromised patient, the attending physician has to distinguish between:
A great deal of clinical judgment is
required to reach these conclusions, which imply important therapeutic
diagnosis of opportunistic infections requires a high index of suspicion.
Without this curiosity, the clinician may not consider mycotic
infections in the compromised patient because:
Causes of immunodeficiency commonly
Therapeutic procedures can predispose for fungal infections:
Other factors associated with increased frequency of mycotic
Certain fungi may be frequently associated with some of the predisposing factors listed above. However, any one of the ubiquitous saprophytes (most of which do not cause disease in immunocompetent hosts) as well as occasional pathogens may cause disease in these patients.
A biofilm consisting of various bacteria (b) and yeast (y) strains colonizing an indwelling, silicone rubber voice prothesis after being placed for 3 to 4 months in a laryngectomized patient. The image was taken by scanning electron microscopy. Scale bar: 5 µm. © Henny C. van der Mei, E.P.J.M. Everaert, H. J. Busscher. University of Groningen and the MicrobeLibrary
has long been recognized that in patients with a microbial infection, any
artificial device such as an indwelling catheter or prosthetic valve, must be
removed before antibiotic therapy is effective.
The foreign body will act as a nidus, seeding the infection if it
remains present. The exact
mechanism is not clear. A
biofilm is a microcolony of organisms which adhere to a surface (catheter,
implant, or dead tissue) and which resist removal by fluid movement and have a
decreased susceptibility to antimicrobials (figure 1).
This biofilm phenomenon, which occurs on the rocks in a stream, was
first recognized as a public health problem in water pipes and was regarded as
a source of coliform contamination
of drinking water. Recent work in
clinical microbiology has shown that these organisms develop a resistance to
therapy because they are contained in a matrix which acts like a tissue to and
becomes a barrier to antibodies, macrophages and antimicrobial agents.
Candida species readily form biofilms and
are the most prevalent organism isolated from catheters.
Scanning Electron Micrograph of Malassezia furfur
immunosuppressed patients, common fungal infections may have an unusual
presentation because of:
Histopathology of lung shows widened alveolar septum containing a few inflammatory cells and numerous yeasts of Cryptococcus neoformans. The inner layer of
the yeast capsule stains red. CDC/Dr. Edwin P. Ewing, Jr.
Examples of variations from standard fungal clinical presentation, diagnosis and treatment
Studies show that from 10 % to 30 % of AIDS patients have cryptococcal meningitis and they will require maintenance therapy with fluconazole for the remainder of their life. Fluconazole penetrates the cerebro-spinal fluid
Pneumocystis jiroveci is an important cause of opportunistic respiratory tract infections in immunocompromised patients, particularly AIDS patients. This
image depicts P. jiroveci from bronchial washings of an AIDS patient. Mouse monoclonal antibodies against P. jiroveci are labeled with a fluorescent tag. The labeled Pneumocystis organisms fluoresce bright apple green against a red background. © Lewis Tomalty, Gloria J. Delisle Queens University, Ontario and the MicrobeLibrary
the prepared mind can help the impaired host." Dr. Libero Ajello, Opportunistic Fungal Infections.
Proceedings of the Second International Conference. Charles C.Thomas,
1975. P. 31-35.