Biomedical Engineering Reference
In-Depth Information
meningitis or meningo-encephalitis in immune-deficient patients. Occasional
cases of cryptococcal meningitis are caused by Cryptococcus laurentii and
Cryptococcus albidus.
Cryptococcus gattii produces highly virulent infections that involve the
lungs, the meninges, or the brain. Until recently, it was confined to tropical
or subtropical climates, and was particularly frequent in Papua New Guinea
and northern Australia. In recent years, a few hundred cases have occurred
in the northwest United States and in Vancouver. Unlike Cryptococcus neo-
formans, which causes disease almost exclusively in immune compromised
individuals, Cryptococcus gattii occurs in immune competent individuals.
Malassezia species produce a type of ringworm called tinea versicolor
(alternately known as pityriasis versicolor), and a folliculitis called pityros-
porum folliculitis (Malassezia was formerly known as Pityrosporum).
Mallasezia species are part of normal skin flora. Active infections, when
they occur, arise from endogenous skin organisms, not through contagion
with infected humans. Tinea versicolor occurs in up to 8% of the general
population, most frequently in adolescents. It consists of a round itchy macu-
lar rash with a sharply demarcated circumference; hence, falling into the
broad clinical category of so-called ringworm fungal infections. In biopsies
of infected skin, yeast forms admixed with hyphae are seen, producing a his-
tologic appearance likened to spaghetti and meatballs. Most cases of tinea
versicolor are caused by Malassezia globosa. In pityrosporum folliculitis,
organisms descend into hair follicles, producing inflammation, with papule
formation.
The clinical term “ringworm” is irreconcilable with taxonomic nomencla-
ture, and a few words of explanation are required. Ringworm is synonymous
with dermatophytosis, skin infections caused by one of several fungi that
live in the top, keratin layer of the epidermis, producing round macules or
plaques. Within Class Ascomycota (Chapter 36), there is a subclass called
Arthrodermataceae that contains the traditional dermatophytic genera:
Epidermophyton, Microsporum, and Trichophyton. Nonetheless, additional
fungal genera, outside Class Arthrodermataceae may produce ringworm
infections: Hortaea (an ascomycote), and Malassezia (a basidiomycote).
Malassezia species are ubiquitous and grow, in yeast form, as commen-
sals on normal keratinized skin. They require fatty acids, and are thus found
in highest concentrations in sebum-rich areas, including the face. They have
been found in a high percentage of cases of several common and mild skin
disorders, including dandruff, seborrheic dermatitis, and even hyperhidrosis.
The pathogenic role of Malassezia species in these diseases is obscure.
As with most fungal infections, otherwise mild conditions can progress
into life-threatening diseases in individuals who are malnourished or
immune-deficient. Malassezia species have been involved in serious fungal
infections arising in low-birth-weight infants. Additional cases have occurred
in adults who receive intravenous parenteral nutrition, presumably through
