with Phil Rennie
This is a mycotoxic (fungal) 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 here in New Zealand, especially in the North Island.
It also occurs in Australia, France, South Africa, several South American countries and parts of North America.
Sheep, cattle, alpacas and farmed deer of all ages can contract the disease, but it is most severe in young animals.
Cause of disease outbreaks
Sporidesmins are the toxic products of the 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 or heavy dews.
By observing weather conditions and estimating toxic spore numbers on pastures, danger periods can be predicted and owners alerted.
The sporidesmins are excreted via the biliary system, in which they produce severe biliary disease in the liver.
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 can cause potential for a build-up in signs, with a succession of small intakes of the spores leading to subsequent outbreaks.
Clinical presentation and diagnosis
Few signs are apparent until photosensitization and jaundice appear about 10–14 days after intake of the toxins.
Animals regularly seek shade. Even short exposure to the sun rapidly produces the typical reddening and swelling of photodermatitis in non-pigmented (white) 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, yellow and have a marked lobular pattern.
Later, there is shrinking and thickening of the liver. The bladder lining also commonly shows blood or bile pigment staining with focal swelling.
The clinical signs together with characteristic liver lesions are diagnostic. In live animals, high levels of hepatic enzymes also reflect extensive injury to the liver.
Control and prevention
To minimize intake of pasture litter and toxic spores, short grazing should be avoided.
Other feedstuffs should be fed during danger periods. Variation in sward composition can also be useful such as encouraging clover dominance in pastures to reduce facial eczema spore growth in litter.
The application of fungicides to pastures considerably restricts the build-up of fungal spores and reduces pasture toxicity.
When danger periods of fungal activity are predicted, animals should be allowed only on the sprayed areas. The fungicide is effective within four days after spraying, provided that no more than 2.5 cm (1 inch) of rain falls within 24 hr during the four-day period.
After this time, heavy rainfall does little to reduce the effectiveness of spraying, because thiabendazole (the fungicide) becomes incorporated within the plants.
Pastures will then remain safe for about six weeks, after which spraying should be repeated to ensure protection over the entire dangerous season.
Sheep and cattle can be protected from the effects of sporidesmin if given adequate amounts of zinc.
Zinc may be administered individually by drenching regularly with a zinc oxide slurry or administering long acting zinc boluses.
Alternatively for larger groups of stock spraying pastures with zinc oxide may be helpful or adding zinc sulphate 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.