Cyanogenic glycosides are responsible for plant-related cyanide poisoning. The toxins are present in varying amounts in many plants and under certain conditions some plants can accumulate the toxins.
Plants containing cyanogenic glycosides are usually unpalatable but cases of poisoning in horses have been reported in Australia.
Cyanogenic glycosides can convert to the deadly toxin cyanide (also known as prussic acid). This conversion takes place under the influence of certain plant enzymes.
The enzymes and cyanogenic glycosides are both present in the plant material but they are found in separate cell compartments. Chewing, crushing, trampling or grinding the plant material exposes the cyanogenic glycosides to the enzymes, leading to the production of cyanide.
Cyanide is quickly absorbed from the gastrointestinal tract into the bloodstream. Once in the blood it travels all over the body and inhibits blood cells from delivering oxygen to tissue cells. The blood becomes supersaturated with oxygen and appears bright red. However tissue cells are simultaneously starved of this oxygen.
Factors that affect the toxicity of plants containing cyanogenic glycosides:
- A high nitrogen to phosphorus ratio in the soil favours cyanogenic glycoside formation in plants.
- Plant species containing cyanogenic glycosides have different levels of the toxins depending on the stage of growth.
- In general the highest toxin levels are present when the plant is young and/or rapidly growing.
- Wet leaves, such as from dew or rain, are considered more toxic than dry leaves.
- Some herbicides can increase both the toxicity and the palatability of cyanogenic glycoside plants.
- Environmental stresses such as drought or frost can promote the formation of cyanogenic glycosides.
Signs of cyanogenic glycoside poisoning:
Acute poisoning - signs develop rapidly after ingestion of the toxic plant material, with the first signs appearing within 10 minutes to an hour. Cases of acute cyanogenic glycoside poisoning are more commonly seen in cattle than in horses.
The signs that may be seen with acute cyanide poisoning in horses include:
- Rapid breathing (hyperventilation) Low blood pressure
- Mucous membranes are characteristically bright red Convulsions
- Coma, respiratory failure and death
In the most severe cases it may only be a matter of minutes between the onset of symptoms and death and in other cases there may be 1-2 hours before death occurs. In many reported cases of acute cyanogenic glycoside poisoning the horse has been found dead in the paddock.
If a veterinarian attends the horse in the early stage of acute poisoning there are steps that can be taken that may save the horse. Among the treatment options is the administration of either sodium nitrate or methylene blue.
These compounds can remove cyanide from the blood to allow the delivery of oxygen to the tissues. If either of these compounds is rapidly administered there is a chance the horse will recover. Once the horse shows signs of severe breathing difficulties it is unlikely to survive.
Chronic poisoning -
chronic cyanide poisoning can occur if a horse consumes small amounts of cyanogenic glycoside plants over time. It is thought that low levels of cyanide damage parts of the spinal cord and long nerves.
The signs that may be seen with chronic cyanide poisoning include:
- Hindlimb incoordination that is more noticeable when the horse is backed or turned
- Urinary incontinence
- Constipation
- Weight loss
The horse may develop urinary tract infections that can be severe Animals affected by chronic cyanide poisoning usually make a slow recovery once access to cyanogenic glycoside plants has been removed.
The prognosis may not be so good if hindlimb incoordination is advanced and/or serious urinary tract/kidney problems have developed.
Prunus:
includes plums, peaches, apricots, cherries, nectarines and almonds) Prunus spp [Amygdalaceae] Many species of Prunus are grown in Australia, either as orchard fruits or as garden ornamentals. The ingestion of only small quantities of prunus leaves can be enough to cause poisoning in horses.
Estimates on exactly how much is too much varies but the consensus in the literature is that horses should be restricted from ingesting any amount of prunus leaves. Cyanogenic glycoside levels are highest in prunus leaves when they are damaged or stressed from frost, storms, drought or seasonal wilting.
Growth and identification:
The Prunus genus contains trees or shrubs that are mostly deciduous. The plants produce blossoms that are usually pink or white. The blossoms appear in spring before the appearance of new leaves. The plants produce fleshy fruit that is usually indented on one side and many of these fruits are edible.
Management notes:
- Prunus trees or shrubs are not suitable for planting in paddocks, along paddock fences or in areas where horses are kept.
- Do not put fallen leaves or clippings of prunus in areas where horses could access them.
All sorghums contain cyanogenic glycosides but there is much variation between species and varieties. Sorghums that have been associated with poisoning in horses either in Australia or overseas include the forage or weed sorghums (Johnson grass, Sudan grass, Columbus grass) and grain sorghum.
There is some confusion over sorghum names and for the purposes of this book we will refer to sorghums by their common names and note the species name by which they are usually known. A taxonomic review of sorghums in 2003 showed that there are many hybrids and types of sorghum that are difficult to classify. There may be further changes to the naming of these plants in the future.
Horses may be at risk of chronic cyanide poisoning if they are grazed on sorghumdominant pastures or if they are fed hay containing sorghum species. Sorghum grain has very low levels of cyanide and is considered safe.
Johnson grass:
Usually known as Sorghum halepense [Poaceae] Johnson grass is native to the Mediterranean region, Europe, Asia and Africa. It is a major weed in pastures and crops and it is declared noxious weed in New South Wales, the Northern Territory and Western Australia.
Growth and identification:
Johnson grass is a long-lived perennial grass that competes vigorously with crop and pasture species. It reproduces by seed and by creeping underground stems. The seeds can spread by wind, water
run-off, as contaminants of hay and grain or through animal and vehicle movements.
The grass can grow to 1.5 m. It has stout upright stems with alternately arranged leaves that are glossy in appearance. The leaves have a prominent midrib that is almost white and the edges of the leaves are rough. The flowering stems have joints that may have tufts of downy hairs.
The flowering seed heads can be pale green, red or purple in colour and they appear at the top of a flowering stem. The seeds are borne on many slender branches that divide into finer branches. The seed heads can be up to 30 cm long and 30 cm wide.
Johnson grass has underground stems (rhizomes) that are white or reddish-purple in colour and are covered in brown scaly sheaths. These stems have nodes from which new growth can occur.
Sudan grass:
Known a Sorghum sudanense or Sorghum drummondii [Poaceae] Sudan grass is very similar to Johnson grass. The main differences are that Sudan grass does not produce underground shoots (rhizomes), it is an annual and the seed head is longer.
Columbus grass:
May be a hybrid of Sorghum halepense x almum [Poaceae] Columbus grass is very similar to Johnson grass. The main difference is that the sections between the nodes of the underground stems (rhizomes) are thicker and shorter in Columbus grass.
Declared a noxious weed in New South Wales and Western Australia.
Grain sorghum:
Grain sorghum is similar to Johnson grass but it does not have underground stems. The flower head of grain sorghum is denser than that of Johnson grass. It can be an annual or a shortlived perennial.
Management notes:
- Young sorghum pastures are not safe for grazing, especially if damaged by frosts, drought or storms. Pasture over 50 cm high is usually considered safe for grazing.
- If feeding sorghum hay to horses, ensure that it is properly cured. Fresh or green sorghum hay has the potential to contain high levels of cyanogenic glycosides.
- Select low-risk cultivars if planting sorghums as pasture grasses.
Other plants that can accumulate cyanogenic glycosides:
The following plants may, under certain conditions, contain high levels of cyanogenic glycosides.
Spotted fuchsia
(native fuchsia, spotted emu bush)
Eremophila maculata [Myoporaceae] Densely branched perennial shrub growing to 2 m. Alternately arranged, narrow leaves that can be up 5 cm long by 1 cm wide. The leaves are dark green and glossy. Flowers in winter and spring. The tubular flowers appear in the forks of the leaves on S-shaped stalks. Flowers can be shades of yellow or red. The inside surface of the flowers is spotted.
Birdsfoot trefoil:
Lotus corniculatus [Fabaceae]
Annual or perennial plant growing to 40 cm. The stems lie along the ground and grow upwards at the tips (decumbent). The alternately arranged leaves have 5 egg-shaped leaflets that are each up to 1.5 cm long. The 2 lower leaflets are sometimes very small. The pea-shaped flowers appear in a group of 2 – 8 and the flower stalks of each group arise from a single point on the stem. The spring - summer flowers are usually yellow with red veins. Produces brown pods.
Wattles:
Acacia spp [Fabaceae]
Acacia is the largest genus of flowering plants in Australia with 960 species. Acacias can be trees or shrubs. Young leaves are bi-pinnate, which means that the main leaf stem has opposite pairs of stems and these smaller stems have opposite pairs of leaflets. Older leaves might retain this form, be cylindrical or be a flattened, leaf-like stem. Acacia flowers are small and round, oval or cylindrical. The flowers are usually yellow. Pods are usually leathery or woody.
Couch grasses:
Cynodon spp [Poaceae]
Particularly African star grass (Coondai couch) Cynodon nlemfuensis and Cynodon plectostachyus Cynodons are perennials. They produce underground (rhizomes) and aboveground (stolons), root-like stems that can take root and produce new growth. The leaves are sheathed. The inner junction of the leaf sheath and leaf blade is thin and translucent.
The flowers have 2 – 6 narrow spikes spreading from the top of the flowering stem and all of the spikes arise from the same point. The small spikelets appear in 2 rows on the underside of the flower stems.
Native couch:
(spider grass)
Brachyachne convergens (also known as Cynodon convergens) [Poaceae] Upright or semi-upright annual that grows to 50 cm. The branched stems are bent (like a knee or elbow joint) at the base. The leaves have a loose overlapping sheath. The flower has 3 – 5 narrow spikes spreading from the top of the flowering stem. All of the spikes arise from the same point and they can be 3 – 5 cm long.
Linseed (flax):
Linum usitatissimum [Linaceae]
Linseed meal was once a common protein supplement for horses but it is not as commonly used today. Heat pressed linseed meal does not contain cyanogenic glycosides and is not a risk to horses. Cold pressed linseed meal that has not been treated with heat contains cyanogenic glycosides and poses some risk to horses. Whole linseed seed is sometimes fed to horses as a coat conditioner and laxative. Whole linseed seed is safe to feed dry and untreated if using small amounts in the ration (less than half a cup, or 75 g). Larger amounts of whole linseed seed should be heat treated to remove cyanogenic glycosides before being fed to horses. Method:
- add the seeds to boiling water and continue boiling for at least 5 minutes.
- Do not soak linseed seeds in cold water as this will increase the amount of cyanide in the seeds, making them more toxic.
Management factors:
- Some of the plants on the above list are important pasture species and are safe under most circumstances. An observance of the risk factors listed at the start of this chapter will reduce the risk of poisoning by pasture species that can accumulate cyanogenic glycosides.
- The ornamental plant species on the above list are usually unpalatable but horses may eat them if other forage is scarce. In some circumstances these ornamentals may accumulate large amounts of cyanogenic glycosides and the ingestion of only small amounts may be enough to cause poisoning.
- These plants should be considered unsuitable for planting in paddocks, along paddock fences or in areas where horses are kept.
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