The fungi discussed in this chapter cause a variety of infections, each ranging in severity from subclinical to progressive, debilitating disease. Some of these species are dimorphic, growing in the infectious mold form in the environment but switching to a round, yeast-like form in infected tissues. They differ from the opportunistic fungi in their ability to cause disease in previously healthy persons. However, the most serious infections still occur in patients with compromised immune systems. With the exception of Cryptococcus neoformans, each of these fungi is restricted to geographic niches corresponding to the environmental habitats of the mold form of the species. None of these infections is transmitted from human to human. The major features of the systemic pathogens are summarized in Table 47–1.
TABLE 47–1Features of Systemic Fungal Pathogens |Favorite Table|Download (.pdf) TABLE 47–1 Features of Systemic Fungal Pathogens
|ORGANISM ||CULTURE AT 25°C ||CULTURE AT 37°C ||TISSUE ||SOURCE ||PRIMARYDISEASE ||DISSEMINATED DISEASE |
|Cryptococcus neoformans, C gattii ||Encapsulated yeast ||Encapsulated yeast ||Encapsulated yeast ||Environment, worldwide ||Pneumonia ||Chronic meningitis |
|Histoplasma capsulatum ||Mold, tuberculate macroconidiaa ||Small yeast ||Small intracellular yeastb ||Environment, US Midwestd ||Pneumonia, hilar adenopathy ||RES enlargement |
|Blastomyces dermatitidis ||Molda ||Yeast || ||Environment, US Midwestc ||Pneumonia ||Skin and bone lesions |
|Coccidioides immitis, C posadasii ||Mold, arthroconidia ||(Spherules)e ||Spherules ||Environment, Sonoran desertc,f ||Valley fever ||Pneumonia, meningitis, skin, and bone |
|Paracoccidioides brasiliensis ||Mold ||Yeast, multiple blastoconidia || ||Environment, Latin America ||Pneumonia ||Mucocutaneous, RES |
Cryptococci are yeasts distinguished by a surrounding capsule. The primary disease caused by cryptococci is a chronic meningitis. The clinical onset is slow, even insidious, with low-grade fever and headache progressing to altered mental state and seizures. In the cerebrospinal fluid (CSF) and in tissues, the inflammatory response is often remarkably muted. Most patients who develop this infection have some obvious form of immune compromise, although some show no demonstrable immune defect.
CRYPTOCOCCUS NEOFORMANS AND CRYPTOCOCCUS GATTII
Cryptococcus species were first isolated from environmental sources more than a century ago, and they are now recognized as important human pathogens, especially in the setting of HIV infection. The most important clinical manifestation of cryptococcal disease is a life-threatening meningitis in immunocompromised patients.
Found throughout the world, Cryptococcus species grow primarily as a budding yeast 4 to 6 μm in diameter. The most characteristic feature of these cells is a large polysaccharide capsule...