In this mycotoxic disease of grazing livestock, the toxic
liver injury commonly results in photodynamic
dermatitis. In sheep, the face is the only site of the body readily exposed to
ultraviolet light, hence the common name. The disease is most common in New Zealand
but also occurs in Australia, France, South Africa, several South American
countries, and probably North America. Sheep, cattle, and farmed deer of all ages
can contract the disease, but it is most severe in young animals.
Etiology and Pathogenesis
are secondary metabolites of the saprophytic fungus Pithomyces
chartarum, which grows on dead pasture litter. The warm ground
temperatures and high humidity required for rapid growth of this fungus restrict
disease occurrence to hot summer and autumn periods shortly after warm rains. By
observing weather conditions and estimating toxic spore numbers on pastures,
danger periods can be predicted and farmers alerted.
The sporidesmins are excreted via the biliary system, in
which they produce severe cholangitis and pericholangitis as a result of tissue
necrosis. Biliary obstruction may be seen, which restricts excretion of bile
pigments and results in jaundice. Similarly, failure to excrete phylloerythrin
in bile leads to photosensitization.
Previous ingestion of toxic spores causes potentiation;
thus, a succession of small intakes of the spores can lead to subsequent severe
Clinical Findings, Lesions, and Diagnosis
Few signs are apparent until photosensitization and
jaundice appear ~10–14 days after intake of the toxins. Animals frantically seek
shade. Even short exposure to the sun rapidly produces the typical erythema and
edema of photodermatitis in nonpigmented skin. The animals suffer considerably,
and deaths occur from one to several weeks after photodermatitis appears.
Characteristic liver and bile duct lesions are seen in all
affected animals whether photosensitized or not. In acute cases showing
photodermatitis, livers are initially enlarged, icteric, and have a marked
lobular pattern. Later, there is atrophy and marked fibrosis. The shape is
distorted, and large nodules of regenerated tissue appear on the surface. In
subclinical cases, livers often develop extensive areas in which the tissue is
depressed and shrunken below the normal contour, which distorts and roughens the
capsule. Generally, these areas are associated with fibrosis and thickening of
corresponding bile ducts. The bladder mucosa commonly shows hemorrhagic or bile
pigment–stained ulcerative erosions with circumscribed edema.
The clinical signs together with characteristic liver
lesions are pathognomonic. In live animals, high levels of hepatic enzymes may
reflect the extensive injury to the liver.
To minimize intake of pasture litter and toxic spores,
short grazing should be avoided. Other feedstuffs should be fed during danger
periods; encouraging clover dominance in pastures helps to provide a milieu
unsuited to growth and sporulation of P chartarum on
The application of benzimidazole fungicides to pastures
considerably restricts the buildup of P chartarum spores and
reduces pasture toxicity. A pasture area calculated at 1 acre (0.45 hectare)/15
cows or 100 sheep should be sprayed in midsummer with a suspension of
thiabendazole. When danger periods of fungal activity are predicted, animals
should be allowed only on the sprayed areas. The fungicide is effective within 4
days after spraying, provided that no more than 1 in. (2.5 cm) of rain falls
within 24 hr during the 4-day period. After this time, heavy rainfall does
little to reduce the effectiveness of spraying, because the thiabendazole
becomes incorporated within the plants. Pastures will then remain safe for ~6
wk, after which spraying should be repeated to ensure protection over the entire
Sheep and cattle can be protected from the effects of
sporidesmin if given adequate amounts of zinc. Zinc may be administered by
drenching with zinc oxide slurry, by spraying pastures with zinc oxide, or by
adding zinc sulfate to drinking water.
Sheep may be selectively bred for natural resistance to
the toxic effects of sporidesmin. The heritable trait for resistance is high.
Ram sires are now being selected in stud and commercial flocks for resistance
either by natural field challenge or by low-level, controlled dosage of ram
lambs with sporidesmin.
Last full review/revision December 2014 by Gary D. Osweiler, DVM, MS, PhD