Production Animal Clinical Toxicology

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

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Plants

Sorghum spp. - grain and forage

Sorghum halepense - Johnson grass

Cynodon spp. - blue couch

Brachyachne spp. - native couches

Eremophila maculata - native fuschia

Some acacias - e.g. Acacia glaucescens

linseed meal/cake - esp. immature seeds

Heterodendrum oleifolium - rosewood

Epidemiology

animal factors

• aggravated if animals hungry, resulting in rapid ingestion, or under stress of droving or mustering
• animals recently introduced to area may be more susceptible than indigenous populations
• more common in ruminants than monogastrics - related to low activity of beta-glycosidase at low pH.

plant/environmental factors

• 2 enzymes required; both usually in same plant
• may also be in ruminal microbes
• glycoside --->(beta-glycosidase)--->(lyase)--->HCN
• glycoside levels most important:
• high in young plants, rapidly growing green shoots
• plants wet with dew/light rain
• glycoside levels can increase transiently:
• frosted/wilted plants
• plants treated with herbicides
• in addition, in these plants there is an increased spontaneous breakdown to HCN

Pathogenesis

• HCN released either enzymatically or spontaneously
• rapid absorption in rumen
• can be metabolised by liver, but if release excessive cytochrome oxidase can be inhibited
• blocks cellular respiration - histotoxic anoxia
• myocardium tissue most severely affected; cardiac failure and cerebral anoxia
• toxic dose 2 mg/kg bodyweight - large amount of toxic plant needs to be ingested

Clinical Signs

• sudden death
• bright red mucosae and blood from high levels of oxyHb
• respiratory distress
• muscular twitching; staggering gait
• spasm
• dilated pupils; pronounced bloat
• coma, death

Clinical Pathology

• generally nonspecific

Necropsy

• blood may be brighter in colour; may clot poorly
• generally nonspecific

Diagnosis

• history; red venous blood
• picrate paper test - plant material or ruminal contents
• some labs may do muscle levels but these levels rapidly decline

Treatment

• Na nitrite/Na thiosulphate: CN- binds preferentially to MetHb; CN- slowly released from cyano MetHb to combine with thiosulphate.

Control

• grazing management
• test graze