The genus Candida encompasses more than 150 species, only a few of which cause disease in humans.With rare exceptions, the human pathogens are C. albicans, C. guilliermondii, C. krusei, C. parapsilosis, C. tropicalis, C. kefyr, C. lusitaniae, C. dubliniensis, and C. glabrata. Ubiquitous in nature, these organisms are found on inanimate objects, in foods, and on animals and are normal commensals of humans. They inhabit the gastrointestinal tract (including the mouth and oropharynx), the female genital tract, and the skin. Although cases of candidiasis have been described since antiquity in debilitated patients, the advent of Candida species as common human pathogens dates to the introduction of modern therapeutic approaches that suppress normal host defense mechanisms.
Of these relatively recent advances, the most important is the use of antibacterial agents that alter the normal human microbial flora and allow nonbacterial species to become more prevalent in the commensal flora.With the introduction of antifungal agents, the causes of Candida infections shifted from an almost complete dominance of C. albicans to the common involvement of C. glabrata and the other species listed above. The non-albicans species now account for approximately half of all cases of candidemia and hematogenously disseminated candidiasis. Recognition of this change is clinically important, since the various species differ in susceptibility to the newer antifungal agents. In developed countries, where medical therapeutics are commonly used, Candida species are now among the most common nosocomial pathogens. In the United States, these species are the fourth most common isolates from the blood of hospitalized patients. Candida is a small, thin-walled, ovoid yeast that measures 4–6 μm in diameter and reproduces by budding. Organisms of this genus occur in three forms in tissue: blastospores, pseudohyphae, and hyphae. Candida grows readily on simple medium; lysis centrifugation enhances its recovery from blood. Species are identified by biochemical testing (currently with automated devices) or on special agar.
In the most serious form of Candida infection, the organisms disseminate hematogenously and form microabscesses and small macroabscesses in major organs. Although the exact mechanism is not known, Candida probably enters the bloodstream from mucosal surfaces after growing to large numbers as a consequence of bacterial suppression by antibacterial drugs; alternatively, in some instances, the organism may enter from the skin. A change from the blastospore stage to the pseudohyphal and hyphal stages is generally considered integral to the organism’s penetration into tissue. However, C. glabrata can cause extensive infection even though it does not transform into pseudohyphae or hyphae. Numerous reviews of cases of hematogenously disseminated candidiasis have identified the following predisposing factors or conditions: antibacterial agents, indwelling intravascular catheters, hyperalimentation fluids, indwelling urinary catheters, parenteral glucocorticoids, respirators, neutropenia, abdominal and thoracic surgery, cytotoxic chemotherapy, and immunosuppressive agents for organ transplantation. Patients with severe burns, lowbirth- weight neonates, and persons using illicit IV drugs are also susceptible. HIV-infected patients with low CD4+ T cell counts and patients with diabetes are susceptible to mucocutaneous infection, which may eventually develop into the disseminated form when other predisposing factors are encountered.Women who receive antibacterial agents may develop vaginal candidiasis. Innate immunity is the most important defense mechanism against hematogenously disseminated candidiasis, and the neutrophil is the most important component of this defense. Although many immunocompetent individuals have antibodies to Candida, the role of these antibodies in defense against the organism is not clear.
Mucocutaneous Candidiasis Thrush is characterized by white, adherent, painless, discrete or confluent patches in the mouth, tongue, or esophagus, occasionally with fissuring at the corners of the mouth. This form of Candida disease may also occur at points of contact with dentures. Organisms are identifiable in gram-stained scrapings from lesions.The occurrence of thrush in a young, otherwise healthy-appearing person should prompt an investigation for underlying HIV infection. More commonly, thrush is seen as a nonspecific manifestation of severe debilitating illness. Vulvovaginal candidiasis is accompanied by pruritus, pain, and vaginal discharge that is usually thin but may contain whitish “curds” in severe cases. Other Candida skin infections include paronychia, a painful swelling at the nail-skin interface; onychomycosis, a fungal nail infection rarely caused by this genus; intertrigo, an erythematous irritation with redness and pustules in the skin folds; balanitis, an erythematous-pustular infection of the glans penis; erosio interdigitalis blastomycetica, an infection between the digits of the hands or toes; folliculitis, with pustules developing most frequently in the area of the beard; perianal candidiasis, a pruritic, erythematous, pustular infection surrounding the anus; and diaper rash, a common erythematous-pustular perineal infection in infants.
Generalized disseminated cutaneous candidiasis, another form of infection that occurs primarily in infants, is characterized by widespread eruptions over the trunk, thorax, and extremities.The diagnostic macronodular lesions of hematogenously disseminated candidiasis (Fig. 107-1) indicate a high probability for dissemination to multiple organs as well as the skin.While the lesions are seen predominantly in immunocompromised patients treated with cytotoxic drugs, they may also develop in patients without neutropenia. Chronic mucocutaneous candidiasis is a heterogeneous infection of the hair, nails, skin, and mucous membranes that persists despite intermittent therapy. The onset of disease usually comes in infancy or within the first two decades of life but in rare cases can come in later life.
The condition may be mild and limited to a specific area of the skin or nails, or it may take a severely disfiguring form (Candida granuloma) characterized by exophytic outgrowths on the skin.The condition is usually associated with specific immunologic dysfunction; most frequently reported is a failure of T lymphocytes to proliferate or to stimulate cytokines in response to stimulation by Candida antigens in vitro. Approximately half of patients have associated endocrine abnormalities that together are designated the autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy (APECED) syndrome. This syndrome is due to mutations in the autoimmune regulator (AIRE) gene and is most prevalent among Finns, Iranian Jews, Sardinians, northern Italians, and Swedes. Conditions that usually follow the onset of the disease include hypoparathyroidism, adrenal insufficiency, autoimmune thyroiditis, Graves’ disease, chronic active hepatitis, alopecia, juvenile- onset pernicious anemia, malabsorption, and primary hypogonadism. In addition, dental enamel dysplasia, vitiligo, pitted nail dystrophy, and calcification of the tympanic membranes may occur. Patients with chronic mucocutaneous candidiasis rarely develop hematogenously disseminated candidiasis, probably because their neutrophil function remains intact.
Deeply Invasive Candidiasis Deeply invasive Candida infections may or may not be due to hematogenous seeding. Deep esophageal infection may result from penetration by organisms from superficial esophageal erosions; joint or deep wound infection from contiguous spread of organisms from the skin; kidney infection from catheter-initiated spread of organisms through the urinary tract; infection of intraabdominal organs and the peritoneum from perforation of the gastrointestinal tract; and gallbladder infection from retrograde migration of organisms from the gastrointestinal tract into the biliary drainage system. However, far more commonly, deeply invasive candidiasis is a result of hematogenous seeding of various organs as a complication of candidemia. Once the organism gains access to the intravascular compartment (either from the gastrointestinal tract or, less often, from the skin through the site of an indwelling intravascular catheter), it may spread hematogenously to a variety of deep organs. The brain, chorioretina (Fig. 107-2), heart, and kidneys are most commonly infected and the liver and spleen less commonly so (most often in neutropenic patients). In fact, nearly any organ can become involved, including the endocrine glands, pancreas, heart valves (native or prosthetic), skeletal muscle, joints (native or prosthetic), bone, and meninges. Candida organisms may also spread hematogenously to the skin and cause classic macronodular lesions (Fig. 107-1).
Frequently, painful muscular involvement is also evident beneath the area of affected skin. Chorioretinal involvement and skin involvement are highly significant, since both findings are associated with a very high probability of abscess formation in multiple deep organs as a result of generalized hematogenous seeding. Ocular involvement (Fig. 107-2) may require specific treatment, such as partial vitrectomy to prevent permanent blindness. An ocular examination is indicated for all patients with candidemia, whether or not they have ocular manifestations.
The diagnosis of Candida infection is established by visualization of pseudohyphae or hyphae on wet mount (saline and 10% KOH), tissue Gram’s stain, periodic acid–Schiff stain, or methenamine silver stain in the presence of inflammation. Absence of organisms on hematoxylineosin staining does not reliably exclude Candida infection. The most challenging aspect of diagnosis is determining which patients with Candida isolates have hematogenously disseminated candidiasis. For instance, recovery of Candida from sputum, urine, or peritoneal catheters may indicate mere colonization rather than deep-seated infection, and Candida isolation from the blood of patients with indwelling intravascular catheters may reflect inconsequential seeding of the blood from or growth of the organisms on the catheter. Despite extensive research into both antigen and antibody detection systems, there is currently no widely available and validated diagnostic test to distinguish patients with inconsequential seeding of the blood from those whose positive blood cultures represent hematogenous dissemination to multiple organs. Many studies are under way to establish the utility of the â-glucan test. Meanwhile, the presence of ocular or macronodular skin lesions is highly suggestive of widespread infection of multiple deep organs.
CANDIDEMIA AND SUSPECTED HEMATOGENOUSLY DISSEMINATED CANDIDIASIS
All patients with candidemia are now treated with a systemic antifungal agent. A certain percentage of patients, including many of those who have candidemia associated with an indwelling intravascular catheter, probably have “benign” candidemia rather than deep-organ seeding. However, because there is no reliable way to distinguish benign candidemia from deep-organ infection, and because antifungal drugs less toxic than amphotericin B are available, it has become the standard of practice to treat all patients with candidemia, whether or not there is clinical evidence of deep-organ involvement. In addition, if an indwelling intravascular catheter may be involved, it is best to remove or replace the device whenever possible. The drugs used for the treatment of candidemia and suspected disseminated candidiasis are listed in Table 107-2. Various lipid formulations of amphotericin B, three echinocandins, and the azoles fluconazole and voriconazole are used; no agent within a given class has been clearly identified as superior to the others.
Most institutions choose an agent from each class on the basis of their own specific microbial epidemiology, strategies to minimize toxicities, and cost considerations. Unless azole resistance is considered likely, fluconazole is the agent of choice for the treatment of candidemia and suspected disseminated candidiasis in nonneutropenic, hemodynamically stable patients. Initial treatment in the context of likely azole resistance depends, as mentioned above, on the epidemiology of the individual hospital. For example, certain hospitals have a high rate of recovery of C. glabrata, while others do not. For hemodynamically unstable or neutropenic patients, initial treatment with broader-spectrum agents is desirable; these drugs include polyenes, echinocandins, or later-generation azoles such as voriconazole. Once the clinical response has been assessed and the pathogen specifically identified, the regimen can be altered accordingly. At present, the vast majority of C. albicans isolates are sensitive to fluconazole. Isolates of C. glabrata and C. krusei are less sensitive to fluconazole and more sensitive to polyenes and echinocandins.
C. parapsilosis is less sensitive to echinocandins in vitro, although the clinical significance of this finding is not known. Some generalizations about the management of specific Candida infections are possible. Recovery of Candida from sputum is almost never indicative of underlying pulmonary candidiasis and does not by itself warrant antifungal treatment. Similarly, Candida in the urine of a patient with an indwelling bladder catheter may represent colonization only rather than bladder or kidney infection; however, the threshold for systemic treatment is lower in severely ill patients in this category since it is not possible to distinguish colonization from lower or upper urinary tract infection. If the isolate is C. albicans, most clinicians use oral fluconazole rather than a bladder washout with amphotericin, which was more commonly used in the past. The significance of the recovery of Candida from abdominal drains in postoperative patients is also unclear, but again, the threshold for treatment is generally low because most of the affected patients have been subjected to factors predisposing to disseminated candidiasis. Removal of the infected valve and long-term antifungal therapy constitute appropriate treatment for Candida endocarditis.
Although definitive studies are not available, patients usually are treated for weeks with a systemic antifungal agent and then given chronic suppressive therapy for months or years (and sometimes indefinitely) with an oral azole. Hematogenous Candida endophthalmitis is a special problem requiring ophthalmologic consultation. In lesions that are expanding or that threaten the macula, an IV polyene combined with flucytosine has been the regimen of choice. However, as more data on the azoles and echinocandins become available, new strategies 1021 may evolve.Of paramount importance is the decision to perform a partial vitrectomy.This procedure debulks the infection and can preserve sight, which may otherwise be lost as a result of vitreal scarring. All patients with candidemia should undergo ophthalmologic examination because of the relatively high frequency of this ocular complication. Not only can this examination detect a developing eye lesion early in its course; in addition, identification of a lesion signifies a probability of ∼90% of deep-organ abscesses and may prompt prolongation of therapy for candidemia beyond the recommended 2 weeks after the last positive blood culture. Although the basis for the consensus is a very small data set, the recommended treatment for Candida meningitis is a polyene plus flucytosine. Successful treatment of Candida-infected prosthetic material (e.g., an artificial joint) nearly always requires removal of the infected material followed by long-term administration of an antifungal agent selected on the basis of the isolate’s sensitivity and the logistics of administration.
The use of antifungal agents to prevent Candida infections has been controversial, but some general principles have emerged. Most centers administer prophylactic fluconazole (400 mg/d) to recipients of allogeneic stem cell transplants. High-risk liver transplant recipients are also given fluconazole prophylaxis in most centers. The use of prophylaxis for neutropenic patients has varied considerably from center to center; most centers that elect to give prophylaxis to this population use either fluconazole or a comparatively low dose of an IV polyene— either amphotericin B deoxycholate or a lipid formulation of this agent. Some centers have used itraconazole suspension. Prophylaxis is sometimes given to surgical patients at very high risk. The widespread use of prophylaxis in general surgical or medical intensive care units is not—and should not be—a common practice for three reasons: (1) the incidence of disseminated candidiasis is relatively low, (2) the cost-benefit ratio is suboptimal, and (3) increased resistance with widespread prophylaxis is a valid concern. Prophylaxis for oropharyngeal or esophageal candidiasis in HIV-infected patients is not recommended unless there are frequent recurrences.