untitled

spherules then undergo cytoplasmic cleavage to pro-

duce smaller bodies (subspherules), which can repeat

the infection cycle within the lung tissues.

Coccidioides immitishas several interesting features.

First, at common environmental temperatures such

as 25°C it can grow as normal hyphae which produce

arthrospores for dissemination, whereas it produces

spherules at higher temperatures (37– 40°C) typical

of the host environment. Even so, a change of tem-

perature alone is not sufficient to produce spherules,

because in vitrostudies show that these are only pro-

duced in the presence of high CO 2 concentrations and

in specific growth media. C. immitisand the related

species C. posadasii(see later) are the only dimorphic

fungi that produce spherules; all other endemic fungal

pathogens of humans grow in a yeast-like budding phase

in the host tissues. In advanced stages of infection, C.

immitisbecomes disseminated to many of the body

tissues, as shown in Fig. 16.6.

The geographical distribution of coccidioidomycosis

is not fully known, and even within the endemic

regions it shows a patchy occurrence. There are

reports that it is most common near the burrows of

soil-dwelling rodents. In controlled experiments, dogs,

cats, rodents, and other small animals have been

shown to be susceptible to infection (but so are bottle-

nose dolphins and horses!). It has been suggested that

wild desert animals might provide a natural reservoir

for infection of humans. Most recently, a molecular

comparison of microsatellite DNA sequences from

strains isolated from California/Arizona compared

with Texas, Central and South America revealed that

THE MOULDS OF MAN 331

Fig. 16.6(a) A patient showing the disseminated stage of disease (coccidioidomycosis). (b) Spherules. (c) Chains of

arthrospores interspersed with empty cellular compartments. (Reproduced by courtesy of the DoctorFungus website;

http://www.doctorfungus.org/)

(a)

(c)

(b)

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