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Livestock producers have several names for prussic acid poisoning, including simply prussic acid and geilsiekte. No matter the name, they all refer to cyanide poisoning that can lead to sudden mortalities among cattle and small stock.
Graaff-Reinet based veterinarian, Dr Mackie Hobson, says prussic acid poisoning typically occurs when green pastures wither amid mountain wind conditions or frost, or when it rains after an extended period of drought, after which plants sprout and then wither. It also occurs when plants have regrown following tissue damage from livestock trampling or hay cutting, or after extremely hot conditions during which the vegetation was subject to water stress.
Limp green or partially wilted yellowed leaves are the most dangerous, as they can taste sweeter and are more digestible. Once the plants store more moisture or dry out completely, they lose their toxicity.
Why plants turn poisonous
Wilting occurs when the plants lose moisture faster than the roots can replace it. Similarly, frost damages the plant cells, and the moisture is no longer available to the leaves. Under such conditions, the plant forms cyanogenetic glucoside, which the micro-organisms in the rumen convert into cyanide. A rumen pH of 6,5 to 7 favours this conversion. Cyanide is extremely toxic because it releases prussic acid or cyanic acid – a deadly gas.
Once prussic acid has formed in the animal’s rumen, the blood rapidly absorbs it and transports it throughout the body. It affects the brain, nervous system and rumen, and poisons the red blood cells, resulting in almost all organs being deprived of oxygen.
There are certain plants that contain varying amounts of glucoside under specific environmental conditions. These include cultivated grass, oats, lucerne, redgrass, bristle grass, certain marigold species, sorghum, wheat, maize, certain acacia species, Sudan and Johnson grass. There are approximately 1 000 plant species worldwide that can cause prussic acid poisoning.
Prussic acid poisoning symptoms
Symptoms of prussic acid poisoning typically occur within 15 to 20 minutes after animals ingested wilted plant material. Animals rarely live longer than a few hours after the first clinical signs appear.
Symptoms include initial tension, rapid and deep respiration, high heart rate, bright red mucous membranes that in time turn blue, a characteristic bitter almond smell on the animals’ breath, salivation and excessive tears, and trembling and shaking while standing before they finally collapse. The blood of animals that die suddenly has a typical bright red colour. Animals with a large amount of prussic acid in their stomachs will collapse and succumb without warning.
Animals must be treated immediately after being poisoned to avoid death. A veterinarian can administer an intravenous injection of 1,2% sodium nitrite and 25% sodium thiosulfate (commonly known as ‘Hypo’) – results are typically favourable. It is advised to also administer Hypo orally at a dosage rate of 30g in 100mℓ of water.
Dr Hobson says producers can administer activated charcoal at a rate of 2g/kg live weight. A large dose is essential to effectively absorb and bind the excess plant toxins in the rumen. Vitamin B12 is also used as a cyanide antidote. It binds to the cyanide and is excreted in the urine.
To prevent prussic acid poisoning, Hypo can be administered in the animals’ drinking water at 1 to 2g/ℓ water. It is also advisable to provide starchy feed such as grain concentrate to reduce the potential for poisoning.
Hungry animals should be fed before being put on pastures that can cause prussic acid poisoning. Hungry animals are greedy feeders, increasing the likelihood of prussic acid poisoning. In areas that receive a lot of frost, which experience mountain wind conditions or are unusually hot, animals should not graze wilted pastures, especially where there are plants known for prussic acid poisoning, Dr Hobson concludes. – Andries Gouws, Stockfarm
For more information, contact Dr Mackie Hobson on 082 860 0406 or firstname.lastname@example.org.