CHAPTER 98 FUNGAL INFECTIONS AND ANTIFUNGAL THERAPY IN THE SURGICAL INTENSIVE CARE UNIT
The first clinical description of Candida infection can be traced to Hippocrates, with Parrot recognizing a link to severe illness. Langenbeck implicated fungus as a source of infection, and Berg established causality between this organism and thrush by inoculating healthy babies with aphthous “membrane material.” The first description of a deep infection caused by Candida albicans was made by Zenker in 1861, even though it was not named until 1923 by Berkout. On the other hand, the genus Aspergillus was first described in 1729 by Michaeli, and the first human cases of aspergillosis were described in the mid-1800s.
Fungi are ubiquitous heterotrophic eukaryotes, quite resilient to environmental stress and able to thrive in numerous environments. They may belong to the Chromista or Eumycota kingdom.1 For identification purposes, the separation of taxa is based on the method of spore production, assisted by molecular biology techniques (rRNA and rDNA) that further refine fungal phylogeny and establish new relationships between groups. The most important human pathogens are the yeasts and the molds (from the Norse mowlde, meaning fuzzy). The dual modality of fungal propagation (sexual/teleomorph and asexual/anamorph states) has meant that since the last century there has been a dual nomenclature.
PREDICTORS OF FUNGAL INFECTIONS
The National Nosocomial Infection Surveillance program (NNIS) of the U.S. Centers for Disease Control and Prevention (CDC) has reported that whereas the rate of hospital-acquired fungal infections nearly doubled in the past decade compared with the previous decade, the greatest increase occurred in critically ill surgical patients, making the surgical population in the ICU an extremely high risk group.2 Several conditions (both patient-dependent and disease-specific) have been recognized as independent predictors for invasive fungal complications during critical illness. ICU length of stay was associated with Candida infection as were the degrees of morbidity, alterations of immune response, and the number of medical devices involved. Neutropenia, diabetes mellitus, newonset hemodialysis, total parenteral nutrition, broad-spectrum antibiotic administration, bladder catheterization, azotemia, diarrhea, use of corticosteroids, and cytotoxic drug utilization are also associated with candidemia.2–5
Candida Colonization
Because colonization with Candida spp. is not benign in the context of critical illness, it is desirable to identify and characterize patients further in terms of risk for invasive candidiasis. Screening techniques include routine surveillance cultures in ICU patients. The method proposed by Pittet et al., the colonization index, has been validated in surgical patients. A threshold index of 0.5 has been proposed for the initiation of empiric antifungal therapy in critically ill patients (see following treatment section), although some authorities suggest that the presence of multiple Candida isolates is an epiphenomenon.6
Use of Broad-Spectrum Antibiotics
The use of broad-spectrum antibiotics is one of the bestdocumented risk factors for fungal overgrowth and invasive infections, but the precise mechanism is not understood completely. In evaluating the effect of antibiotic use, one must consider first the complex interrelations between bacteria and fungi in human disease. At least three experimental models have been created to investigate and characterize possible interactions between bacterial and fungal pathogens. In murine models, ticarcillin-clavulanic acid and ceftriaxone (both of which have some antianaerobic therapy) are associated with substantial increases in colony counts of yeast flora of the gut. On the other hand, antibiotics with poor anaerobic activity are less likely to produce this effect (examples are ceftazidime and aztreonam). This observation was validated in a clinical review of the quantitative colonization of stool in immunocompromised patients treated with those antibiotics. However, this interaction between fungi overgrowth and anaerobic suppression is different from the well-studied model of Escherichia coli and Bacteroides fragilis in intra-abdominal abscess formation. The work of Sawyer et al. showed that C. albicans induces bacterial translocation into abscesses, but the relationship is one of direct competency, rather than synergy or cooperation.7,8 This is different than the cooperation between C. albicans and Staphylococcus aureus, Serratia marcescens, and Enterococcus faecalis, where an amplification-type interaction has been documented. A number of immunomodulatory and immunosuppressive viruses have been shown to facilitate superinfections with opportunistic fungi, the most notable examples being CMV and human herpes virus (HHV)-6, because they induce the production of immunosuppressive cytokines. It seems that C. albicans thrives in situations where immunocompromise is present and adds virulence and mortality to existent bacterial infections in a species-specific manner. This hypothesis has been validated from clinical observations, where antifungal treatment adds little to the therapeutic effect of antibacterial agents alone. Thus, the use of antibiotics (three or more), especially those with anti-anaerobic properties, constitute a risk factor for fungal colonization and overgrowth, which in turn is a predictor for systemic fungal infections. The precise mechanism of action for this observation is unknown but is probably related to fungi-to-microbe competence and growth suppression. Candida may enhance the pathogenicity of certain bacteria, but not others, and this interaction remains to be elucidated.7