Pithomyces is a genus of saprophytic fungi that feeds on dead organic material, making it common on decaying plant matter within pastures. This widespread presence means it is a consistent feature of grazing environments for livestock like sheep and cattle. While numerous species exist, one has drawn significant attention in agriculture for its impact on animal health. This fungus thrives in conditions often found in managed pastures, creating a health challenge for grazing animals.
The Link Between Pithomyces and Animal Health
The primary species of concern for livestock health was identified as Pithomyces chartarum, but recent research has distinguished a more toxic species, Pseudopithomyces toxicarius, as the main culprit. This fungus flourishes in the dead litter at the base of pasture grasses, especially during the high humidity and warm temperatures of late summer and autumn. These conditions promote the rapid growth of the fungus and the production of its spores.
Grazing animals, such as sheep, cattle, and deer, ingest these spores along with the grass. The danger lies in a mycotoxin called sporidesmin, which is concentrated in its spores. Once ingested, sporidesmin is absorbed from the digestive tract and travels to the liver. There, it inflicts severe damage by targeting the cells of the bile ducts, impairing the organ’s ability to process waste.
Clinical Signs of Pithomyces Toxicity
The disease caused by sporidesmin is known as facial eczema. It is not a direct skin infection but a form of photosensitization that results from severe liver injury. A healthy liver breaks down chlorophyll from digested plants into a compound called phylloerythrin, which is then excreted in bile. When sporidesmin damages the bile ducts, this process is disrupted, and phylloerythrin accumulates in the bloodstream.
This buildup of phylloerythrin makes the animal’s skin highly sensitive to ultraviolet (UV) light from the sun. The most visible clinical signs appear on non-pigmented or lightly-furred areas of skin, such as the face, ears, and eyelids. Exposure to sunlight triggers an inflammatory reaction, causing the skin to redden, swell, and become itchy. In advanced cases, the skin may thicken, peel away, and develop weeping sores.
Beyond the visible skin lesions, animals with sporidesmin toxicity exhibit other signs of liver failure. These can include jaundice, identifiable by a yellowing of the mucous membranes in the eyes and gums, as well as significant weight loss and lethargy. Clinical signs typically become apparent 10 to 14 days after animals have grazed on a pasture with high spore counts.
Management and Prevention Strategies
Preventing facial eczema revolves around managing both the pasture and the animals. A primary strategy is to avoid grazing practices that force animals to consume the dead plant litter at the base of the sward, where spore concentrations are highest. This can be achieved by maintaining a higher pasture cover and rotating livestock before they graze the pasture down too low.
Monitoring pasture spore counts is a practice used to assess risk. By regularly collecting and analyzing grass samples, farmers can determine the concentration of spores, measured per gram of grass. A count of 100,000 or more spores per gram is generally considered dangerous, signaling a need to move animals to safer pastures or implement other protective measures.
Direct animal treatments are also effective. The administration of zinc salts, often as an oral drench, can help protect liver cells from the damaging effects of sporidesmin and is most effective when started before animals are exposed to high-risk pastures. Additionally, mycotoxin binders mixed with feed can help reduce the absorption of sporidesmin from the gut. Over the long term, selecting for animal breeds with greater natural tolerance to the toxin can also reduce the disease’s incidence.