Production Animal Clinical Toxicology

Aetiology | Epidemiology | Pathogenesis | Clinical Signs | Clinical Pathology | Necropsy | Diagnosis | Treatment | Control

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Aetiology

Pithomyces chartarum

Epidemiology

animal factors

• sheep, cattle, deer susceptible
• horses resistant
• evidence for genetic resistance in sheep (link to OMIA database for citations)

plant/environmental factors

• fungus grows on the dead leaf litter of pasture
• most frequent pasture is perennial rye grass, but can occur on other species
• requires warmth and humidity to promote rapid fungal growth and sporulation
• typical weather conditions involve autumn break rains after dry summer, several days of consistent warmth (TºC>15.5ºC) and high humidity (>80%)
• fungus concentrates toxin in spores which may be distributed throughout whole pasture sward
• most toxic part of pasture is base of sward

Pathogenesis

• sporidesmin toxin concentrated in spores that are ingested by animal
• sporidesmin absorbed, removed by liver and concentrated into biliary system
• toxin participates in reduction/autoxidation processes to form superoxide radicals
• these radicals destroy membrane integrity and induce a necrotising obliterative cholangitis - obstructive jaundice develops
• accumulation of phylloerythrin results in photosensitisation

Clinical Signs

• initial dullness, lethargy and anorexia
• variable onset of jaundice and photosensitisation
.
• some animals may die without either being observed
• photosensitisation:
• sheep - non wool skin including muzzle, ears, face, escutcheon
• cattle - non black pigmented areas including teats
• deer - generalised
• some animals develop chronic ill-thrift
• some progress to a hepatic encephalopathy
• dullness, depression
• tremor, recumbency

Clinical Pathology

• early stages serum gamma glutamyl transferase - serum bilirubin

Necropsy

• skin lesions - photosensitisation
• acute - swollen mottled liver
• chronic - severe hepatic fibrosis, nodular surface
• left lobe often atrophied
• marked fibrosis of bile duct

Diagnosis

• history
• clinical signs
• clinical pathology
• necropsy

Treatment

• supportive therapy
• removal from sunlight

Control

• meteorological data allows prediction of potentially dangerous periods
• regional spore counting supplements above data
• on farm spore counting to identify dangerous pastures
• graze
• pastures with low spore counts
• long pastures for short periods
• alternative feeds - hay, silage, crops
• zinc oxide as a prophylactic
• benzimidazole fungicides to inhibit fungal growth
• breed for resistance
• vaccine development in progress