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140 CHAPTER 7

Sporidesmin

The mitosporic fungus Pithomyces chartarumis a sapro-

troph that grows on the accumulated dead leaf

sheaths at the bases of pasture grasses. It is common

in parts of New Zealand, Australia, and South Africa,

where it causes a condition called facial eczemain

sheep and cattle. The conspicuous symptoms are seen

as blistering sores on parts of the body such as the face

and udders that are exposed to sunlight. However this

is a secondary symptom, and the primary cause is a

mycotoxin, sporidesmin, which is present only in the

spores of Pithomyces, not in the mycelium. The toxin is

ingested when the animals graze on pastures where the

fungus is sporulating, leading to necrosis of the liver, and

scarring and partial blockage of the bile duct, so that

the partial breakdown products of chlorophyll accumu-

late in the blood. They are photoactive compounds so

they cause photosensitization of the skin where it is

not protected by a covering of hair (Fig. 7.20).

P. chartarumrequires quite specific conditions for

sporulation – a combination of relatively high tem-

perature and high humidity over a period of days. So,

in countries where facial eczema is common there is

a forecasting system so that farmers can bring the

animals into enclosures when the risk of exposure

to spores is high. Fungicides can also been used to con-

trol this problem.

Endnotes: some further toxins

More than 300 mycotoxins have been identified to date

and, although many of them might not be a serious

Fig. 7.20(a) Spores of Pithomyces chartarumcollected in a spore sampling device. The darkly pigmented spores are

multicellular and shaped like hand-grenades. They contain the toxin sporidesmin. (b) Comparison of a healthy liver

(lower) and two damaged livers of animals suffering from facial eczema. (c) Blistering lesions on the nose and snout of

a sheep suffering from facial eczema. (d) Photosensitization of the udder of a cow affected by the Pithomycestoxin.

(Courtesy of E. McKenzie.)

(a) (b)

(c) (d)