Sheep and Cattle Disease
Facial eczema is a disease of sheep and cattle which occurs in warmer districts of the North Island during late summer and autumn and is responsible for serious production losses in some years. It is caused by a fungus, Pithomyces chartarum, which proliferates on dead plant material in pasture under warm, humid conditions. The minute spores of this fungus contain a substance, sporidesmin, which produces severe toxic effects in the liver. The appearance of livers of affected animals varies, according to the severity of the damage, from slight mottling with light patches to gross discoloration, distortion, and atrophy of large areas. Frequently the severely damaged portions are surrounded with new liver tissue. As a result of this damage the functions of the liver are impaired. Blockage of bile ducts may prevent the excretion of waste substances in the bile; for example, accumulation in the fat and skin of bile pigments, derived from the normal breakdown of old red corpuscles, produces the jaundice or yellow staining commonly seen in the carcasses of affected sheep. Of particular importance is the loss of ability to excrete the substance phylloerythrin. This is formed in the digestive tract of ruminants through the degradation of chlorophyll and is absorbed from the intestine and carried to the liver, where it is normally excreted in the bile. If this excretory mechanism is upset, phylloerythrin passes into the bloodstream which supplies the whole of the body. Phylloerythrin belongs to a class of flourescent pigments which are capable of making the skin sensitive to sunlight, causing reddening, intense itching, swelling, and scab formation. It is these effects, generally showing on the face of affected animals but also on other unpigmented skin exposed to light, such as the teats and udders of cows, which give rise to the popular name “facial eczema”. These skin effects are, however, secondary to the much more serious impairment of liver function.
The fungus, Pithomyces chartarum, grows only on dead or dying plant tissues, not on the living leaf. Hence the amount of the fungus in a pasture is related to some extent to the amount of this dead material, or litter, present. Growth of the fungus, and its production of spores, is strongly influenced by climate and environmental factors. Temperature, humidity, and the time during which the litter remains wet appear to be particularly important. This explains the typical, although not invariable, association of the disease with a period of warm, wet weather, often following a dry spell during which grass growth has ceased and litter has accumulated in the herbage.
The toxic substance, sporidesmin, has been isolated from cultures of the fungus and its chemical structure determined. A single dose of one-thousandth of an ounce is sufficient to kill a lamb of about 60 lb live weight. Sporidesmin itself does not appear to accumulate in the liver, but its effects are cumulative, so that repeated small doses are as effective as a single large dose. Even with a single dose, the full sequence of changes takes some time to develop. Hence photosensitisation usually does not occur until 10 to 14 days after the animal received the toxin, and it may be even further delayed. Both the chemical nature of sporidesmin and its effects on tissues present unusual features which have not yet been fully studied.