FUNGAL INFECTIONS AND ANTIFUNGAL THERAPY IN THE SURGICAL INTENSIVE CARE UNIT

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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.

Invasive mycoses have emerged as a major cause of morbidity and mortality in hospitalized surgical patients. It is estimated that the incidence of nosocomial candidemia in the United States is about 8 per 100,000 inhabitants. Excess attributable health care costs are approximately $1 billion per year. Average medical costs per episode of candidemia have been estimated at $34,123 for Medicare patients and $44,536 for privately insured patients. In the United States, Candida is the fourth most common cause of catheter-related infection. A recent prospective, observational study reported the incidence of fungemia in the surgical intensive care unit (SICU) to be nearly 10 cases per 1000 admissions with an unadjusted mortality rate of 25%–50%.

Fungemia is the fourth most common type of bloodstream infection in the United States. Outside the United States, several studies have reported a rise in candidemia and other forms of Candida infections. In Canada, there has been an increase in the number of Candida isolates since 1991, where currently it constitutes 6% of all blood isolates. In general, the rates reported from European hospitals are slightly less than those from North America. In a meta-analysis of randomized, placebo-controlled trials with fluconazole prophylaxis, the incidence of fungal infections was significantly reduced; however, there was no survival advantage, raising the issue of the value of prophylaxis.

With the introduction of antibiotics and the subsequent appearance of intensive care units (ICUs), new examples of opportunistic fungal infections have emerged. The use of immunosuppression, organ transplantation, implantable devices, and human immunodeficiency virus infection has also radically changed the spectrum of fungal pathogenicity.

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.25

Organ Transplantation and Immunosuppression

The two most common opportunistic fungal infections in transplant patients are caused by Candida spp. and Aspergillus spp., generally by the inhalation route (Aspergillus) or from gastrointestinal sources (Candida). Interestingly, the risk of fungal infection decreases six months after transplantation, unless a rejection episode requires intensification of the immunosuppression. In the solid organ transplant recipient, the graft itself is often affected. In liver transplantation, the risk of fungemia increases with the duration of the surgery and the number of transfusions. Other risk factors include the type of bile duct anastomosis (Roux-en-Y), the presence of tissue ischemia, infection with cytomegalovirus (CMV), and graft-versus-host disease. The most common place of occurrence for Aspergillus tracheobronchitis in lung transplant patients is at the bronchial anastomosis. Anastomotic colonization is both a risk factor for subsequent disruption or hemorrhage and a predictor for rejection and diminished graft survival. Surveillance bronchoscopies are recommended in this setting. Aspergillus is also the main organism responsible for fungemia after heart transplantation, and second only to CMV as the cause of pneumonia in the first month after operation.

Infectious complications are the main cause of morbidity and mortality in pancreas and kidney–pancreas transplantation. The most common organisms are gram-positive cocci, closely followed by gram-negative bacilli and Candida. Risk factors for fungal infections include bladder drainage (in cases of pancreas transplantation) and use of OKT-3 for rejection treatment. Kidney recipients, of all solid organ transplant recipients, have the lowest incidence of infectious complications. However, the risk is sufficiently high that all solid organ transplant recipients (kidney recipients included) receive fungal prophylaxis with fluconazole.

Long-Term Use of Central Venous Catheters

Numerous studies have shown that many, if not most, episodes of candidemia are catheter-related; one of the largest prospective treatment studies of fungemia implicated a catheter 72% of the time. The isolation of C. parapsilosis from blood cultures is strongly associated with central venous catheter infection, parenteral nutrition, or prosthetic devices. The source of the fungal contaminants is different in neutropenic patients when compared with their non-neutropenic counterparts. In non-neutropenic subjects the most common portals of entry for catheter contamination (and subsequent infection) is the skin during catheter placement, manipulation of an indwelling catheter, and crossinfection among ICU patients attributed to hand carriage of microbial flora from health care workers. Other possible sources for primary catheter colonization include contaminated parenteral nutrition solution, multiple medication administration with repetitive violation of the sterile fluid path, and the presence of other medical devices. The secondary route of contamination for intravascular catheters and other foreign bodies in direct contact with the bloodstream (e.g., pacemakers, cardiac valves, orthopedic joint prostheses) is candidemia originating via translocation from the gastrointestinal tract. Endogenous flora are also the most common source in neutropenic and other immunosuppressed patients. Once the catheter becomes contaminated, a well-studied series of events takes place: The yeast adheres to the surface of the catheter and develops hyphal forms that integrate into a matrix of polysaccharides and proteins (biofilm) that increases in size and tridimensional complexity. This biofilm is the main reservoir for candidemia secondary to contaminated medical devices, as it sequesters the fungi from antimycotic medication and against the protective immune response.

In general, the removal of all central venous catheters is indicated following the diagnosis of systemic fungal infections and fungemia. Removal may not be necessary in neutropenic patients in whom the fungi originated from the GI tract. Antifungals in general are continued after the catheter is removed, and it is recommended that Candida ocular dissemination be ruled out (see following discussion of endophthalmitis).

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