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Chapter 6

The Mycoses

Diseases Caused by Fungi and Filamentous Bacteria

There are endless varieties and uncountable species of fungi, with estimates varying from 100 000 to 600 000. There is a vast worldwide literature describing the many diseases caused by fungi and the clinical problems of millions of sufferers. Yet, in reality, and fortunately, relatively few, perhaps 150 fungi, are pathogenic to humans and only about 30 are common causes of infection. Of these, about 12 cause serious illness. This chapter is a brief summary of the more clinically and radiologically important varieties and emphasizes those which are common in the tropics and subtropics. Some, such as coccidioidomycosis, are equally or more common in temperate climates. No attempt has been made to provide more than a general description of each entity.

Mycology has been a somewhat neglected subject in radiology, partly because many of the pathogenic fungi have a very provincial geographic distribution ("endemic mycoses"). Examples are Blastomyces dermatitidis, Coccidioides immitis, Histoplasma duboisii, and Paracoccidioides brasiliensis. However, fungi have flourished and become more widespread in the last few years because of mass migration, increased tourism, and the evolution of AIDS, together with the use of immunosuppressive drugs and antibiotics that wipe out preexisting bacterial flora. Previously localized fungi have appeared in patients in geographic regions where they have been unknown. Those who are immunosuppressed may be killed by normally non-aggressive saprophytic fungi, such as Cryptococcus neoformans.

The main responsibility for the diagnosis of mycotic infections lies with the physician caring for the patient, who must include fungal etiologies in almost any differential diagnosis. However, laboratory support is essential; the pathologist who sees the organism in the context of an inflammatory reaction and the mycologist who identifies the culture must decide on cause and effect. The mycologist must try to identify the species and, for certain organisms, determine sensitivity to antifungal agents. Good mycology laboratories are expensive to operate and there are not enough of them, but because of the increasing population of immunosuppressed patients, general diagnostic laboratories are becoming more familiar with the more common pathogenic fungi. Mycology is a very specialized subject and considerable experience is required to identify the many species found in tissue sections, aspirates, and cultures. In most tropical countries, where the necessary laboratory facilities are not always available, imaging can be a very useful initial screening tool leading to more accurate diagnosis and treatment. Imaging can also show the extent and distribution of fungal lesions in soft tissues, and determine whether the infection is superficial or deeply invasive. Imaging allows early recognition of skeletal infection and in both soft tissues and bone can help select the site for biopsy. It is sometimes possible to differentiate mycotic from nonmycotic infections and even to diagnose the actual species of fungus. Furthermore, imaging is a good way to assess the effectiveness of treatment and help in the selection of those patients who may need surgery. Equally important, radiologists must be aware of the specific imaging patterns of fungal infections, because such awareness may not only help to establish the correct diagnosis but also, when the laboratory results and the imaging diagnosis are incompatible, lead to review of all the available information.



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Copyright: Palmer and Reeder