Cardiac glycosides are found in several plant families. Cardiac glycosides are extremely toxic but poisoning is relatively uncommon in horses because the plants are generally unpalatable.
Cardiac glycosides disrupt normal heart function by inhibiting an important enzyme called Na+ /K+ ATPase. The enzyme regulates the levels of sodium ions (Na+ ) and Potassium ions (K+ ) in the cells of the body.
Inhibition of the enzyme controlling sodium and potassium levels in cells leads to a build-up of potassium outside cells and a build-up of sodium inside cells.
This triggers a release of calcium into the cells. High levels of calcium in the cells of the heart can cause problems with normal function and can lead to heart failure. The action of cyanogenic glycosides also has effects on the gastrointestinal system and the nervous system. The properties of some cardiac glycosides have made them useful in human medicine.
Note:
Care should be taken if the plants in this group are removed from paddocks or otherwise handled. They are very toxic to humans if ingested and the sap of these plants causes skin irritation. The ingestion of one leaf or flower from the oleander tree can be enough to cause death in a human.
Signs of cardiac glycoside poisoning:
The ingestion of plant material containing cardiac glycosides can rapidly cause death in horses. In many reported cases of acute cardiac glycoside poisoning the horse has been found dead in the paddock. Signs of poisoning appear soon after ingestion of toxic plant material and the symptoms progress rapidly.
- Colic
- Lethargy and weakness
- Diarrhoea
- Abnormal heart beat and weak pulse
- Cold extremities
- Sweating
- Shortness of breath
- Death from heart failure usually occurs from 12 - 48 hours after ingestion of the toxic plant
There is no specific treatment available for cardiac glycoside poisoning. Supportive care and treatments such as activated charcoal administered directly to the stomach, fluids and heart medications have been useful in a small number of cases. Immediate veterinary attention is critical for there to be any chance of survival.
Horses that survive cardiac glycoside poisoning may be left with permanent damage to the heart.
Oleander:
Oleander is native to the Mediterranean region and it is widely grown in Australia as a garden ornamental and street tree.
The plant is extremely toxic to all species, including humans. A study on the toxicity of oleander showed that the ingestion of only 7 leaves could be fatally toxic to an average sized horse.
All parts of oleander trees are toxic and this toxicity is retained in dried leaves, flowers and branches. Oleander contains several cardiac glycosides but the most toxic of these is called oleandrin.
Horses will not usually eat oleander leaves because they have a bitter taste. Cases of oleander poisoning in horses have been reported in Australia and overseas.
Some cases involved the ingestion of fresh oleander leaves from the tree but most documented cases of poisoning have involved the ingestion of plant clippings or fallen and dried leaves. Dried leaves have not lost their toxicity but they are more palatable than fresh leaves.
Growth and identification:
There are many cultivars of oleander grown in Australia and there is much variation between them.
Oleander is an evergreen shrub or tree that can grow from 2 – 5m tall. Some newer dwarf varieties grow to 1.5 m tall.
The plant is upright and has many stems that may start to branch near ground level. The stem and branches exude a thick white sap if damaged. The leaves are narrow and are elliptical or lanceshaped. They are positioned on opposite sides of the stem or in whorls of 3 to 4. The leaves can be from 6 – 20 cm long and up to 3 cm wide.
The tips of the leaves are pointed and the edges are smooth. The leaves have a leathery feel and have a prominent midrib, with side veins that are parallel. The leaves are usually green but can sometimes have a variegated pattern of white or yellow. The lower surface of the leaves is usually paler than the upper surface.
The plant flowers mainly in summer but many cultivars can also flower in spring and autumn. The flowers are usually 3 - 4 cm in diameter. The flowers can have a single or double layer of petals and are most commonly pink but can be white, red, or yellow.
The plant produces a pair of narrow brown pods that can be up to 20 cm long. The pods droop downwards and they contain many silky seeds.
Management notes:
- Oleander is not suitable for planting in paddocks, along paddock fences or in areas where horses are kept.
- Do not put fallen leaves or clippings of oleander in areas where horses could access them.
Yellow oleander:
(Captain Cook tree, Cook tree, lucky nut, Mexican oleander, dicky plant) Yellow oleander is native to South America. The plant contains several cardiac glycosides with the major ones being thevetin A and B and peruvoside.
Yellow oleander is widely grown as a garden ornamental. However, the plant has spread beyond gardens and naturalised populations that are considered weeds occur mostly on the east coast, especially in Queensland and northern New South Wales. Some populations of naturalised yellow oleander are recorded in the Northern Territory and Western Australia. It is a declared noxious weed in Queensland.
Growth and identification:
Yellow oleander is an evergreen shrub or tree that can grow from 2.5 – 3 m tall. Occasionally the plant can grow as high as 10 m.
The plant is upright and has many stems that may start to branch near ground level. The stem and branches exude a thick white sap if damaged. The leaves are lance-shaped and are arranged along the stems in a spiral pattern, often in dense clusters.
The leaves can be from 5 – 15 cm long and up to 1.5 cm wide. The tips of the leaves are pointed and the edges are smooth and may curve downwards. The leaves have a prominent midrib. The leaves are glossy and green on the upper surface and dull and paler green on the lower surface.
The plant flowers mostly in summer and autumn but can flower at any time of the year. The flowers are tubular in shape and are 5 –7 cm long. The flowers are usually bright yellow or occasionally orange. The plant produces a slightly fleshy fruit that is up to 5.5 cm across.
The fruit is roughly triangular and it droops from the branch. The fruit is glossy green turning to black with maturity.
Management notes:
- Yellow oleander is not suitable for planting in paddocks, along paddock fences or in areas where horses are kept.
- Do not put fallen leaves or clippings of yellow oleander in areas where horses can access them.
- Individual plants found as weeds in paddocks should be removed. Small seedling can be removed by hand. Wear thick gloves and take extreme caution to avoid the sap of this plant.
- Larger plants may need to be cut at the base and the stump painted with herbicides. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide.
Rubber vine:
Cryptostegia grandiflora [Asclepiadaceae]
Rubber vine is native to Madagascar. The plant is a notorious invader and it thrives in Australian conditions, particularly in northern Australia.
The plant creates dense thickets and chokes out native species. It is considered one of Australia’s worst weeds and it is on the list of the Weeds of National Significance. It is declared noxious in Queensland, Western Australia, the Northern Territory and South Australia.
Rubber vine contains cardiac glycosides and is very toxic. Reports of poisonings are uncommon but rubber vine was implicated as the cause of poisoning in several horses in Australia in 1990.
Growth and identification:
Rubber vine is a woody shrub or climber. It can grow to 1 – 3 m high without support but it can reach up to 30 m high when growing on vegetation. The plant produces seeds that are spread by wind,
water run-off or through animal and vehicle movements.
The stems of the plant are grey-brown and are covered in raised spots. The stems exude a thick white sap if damaged. Young stems are green and are of two types: leafy and branched or leafless and unbranched ‘whips’. The ‘whip’ stems can grow up to 8 m and may find support for climbing.
The leaves are thick and leathery and are arranged on opposite sides of the stem. The leaves are glossy dark green on the upper surface and they are pale and dull on the lower surface. The leaves are oblong in shape and can be up to 10 cm long by 6 cm wide. The leaves have smooth or slightly wavy edges and they have green or red-purple stalks that can be up to 1.5 cm in length.
Rubber vine flowers mainly in summer. The flowers are white or shades of purple or pink. The usual colour is pale pink with a darker centre. The flowers are tube-shaped and are 4 –6 cm long and 5 – 9 cm wide. The plant produces fruit that can be green or brown in colour. The fruit is produced in pairs and resemble pods. The seeds inside the fruit are brown and flat and are topped with a tuft of white hairs that are up to 4 cm long.
Purple rubber vine:
(rubber vine, Madagascar rubber vine) Cryptostegia madagascariensis [Asclepiadaceae] Purple rubber vine is native to Madagascar. It is a declared noxious weed in Western Australia, the Northern Territory and Queensland.
Growth and identification:
Purple rubber vine is very similar to rubber vine.
The flowers of purple rubber vine are slightly smaller and are a deeper pink colour than those of rubber vine.
Management notes:
- Avoid grazing horses in areas where rubber vine grows.
- Control of these plants, especially large infestations of rubber vine, requires an integrated weed management approach.
- Isolated small plants can be pulled out by hand. Wear thick gloves and take extreme caution to avoid the sap of these plants.
- The strategic use of herbicides is effective in controlling individual plants or small infestations.
- If the stems are greater than 90 mm across, cut the plants at the ground and apply herbicide to the stumps. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
- Fire is often used as a control method for large infestations of rubber vine. Follow-up burns and removal of seedlings will be required. Seek advice from the local council regarding regulations and also seek advice from a local agronomist or weed management agency regarding the best method and timing for this technique.
Mother of millions:
Bryophyllum spp [Crassulaceae]
Most species of Bryophyllum are known as mother of millions. Mother of millions and hybrid mother of millions are native to Africa and Madagascar. The plants are drought tolerant and can quickly dominate pastures that have been affected by dry conditions.
Several species of Bryophyllum have been declared noxious weeds in Queensland, New South Wales and Western Australia.
All species of Bryophyllum contain highly toxic cardiac glycosides. There are no confirmed reports of horse poisonings by this plant in Australia. The plants are usually unpalatable but they can heavily invade pastures and they should be considered a risk to horses.
It has been reported that the flowers are the most palatable part of the plant and they may be eaten when other fodder is scarce. Mother of millions can sometimes be found as a contaminant of hay.
All parts of mother of millions plants are toxic but the flowers contain the highest levels of toxins.
Growth and identification:
Mother of millions and its hybrid species are very similar in their growth habit and appearance. The plants are erect perennial succulents that can grow up to 1.8 m but are usually around 1 m in height. Mother of millions produce numerous seeds that can be dispersed by water run-off, as contaminants of hay and grain or through animal and vehicle movements.
The plant also produces numerous tiny plantlets at the edges or tips of the leaves. Each of these plantlets can grow if dislodged from the leaves or dumped as garden waste. This feature of the plant gives it the common name ‘mother of millions” The leaves are arranged on opposite sides of the stem and can be single or compound, with up to 5 leaflets. The leaves vary between species in shape and colour.
The flowers grow in drooping clusters at the top of the upright stems. The flowers are contained within a tube that can be coloured and mottled and can be up to 4 cm long. The petals extend from the tube and are fused for more than half their length. There are 4 petal lobes near the tip of the flower. The flowers can be red, orangered, pink or greenish-yellow. Flowering occurs in winter to early spring.
Management notes:
- Avoid grazing horses on pastures infested with mother of millions. This is especially important when the plant is flowering and when other fodder is scarce, for example during drought conditions.
- Check hay for mother of millions contamination.
- Mother of millions should not be grown as an ornamental plant in areas where horses are kept.
- Do not put clippings of the plant in areas where horses could access them.
- Improve pastures to increase desirable grasses.
- Individual plants can be removed from the paddock by hand. Ensure all plant material is removed and dispose of plants by burning or place them in bags. Wear thick gloves and take care to avoid the sap of these plants.
- The use of herbicides can be an effective control measure for larger infestations. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
Cape tulips:
Cape tulips are native to Africa. There are two species of cape tulip and both are declared noxious weeds in all states except Queensland, the Northern Territory and the Australian Capital Territory.
Cape tulips are a common contaminant of hay that has been cut in areas where the weed grows.
Growth and identification:
One leaf cape tulip is an upright plant that usually grows to 60 cm tall but can occasionally grow to 75 cm. The plant produces perennial underground ‘bulbs’ (called corms). The leaves and stems die back annually. The plant spreads mostly by seed. It can also spread in its immediate area by the production of the underground corms. These corms can be spread to other areas by cultivation or other practices that move soil.
The rounded stems may appear to grow in a zigzag manner and the stems may branch near the top of the plant. Each plant has 1 leaf that sheaths the lower part of the plant.
This leaf is folded when young and matures to be strap-like and slightly ribbed. The plant flowers in late winter to early spring. The flowers are usually pink or orange with a yellow centre. The flowers appear at the end of the branches and each flower has 6 elongated petals that can be up to 4 cm long.
The plant produces a green fruit capsule that turns brown as it matures. The fruit capsule is elongated and is around 5.5 cm long, with a short ‘beak’ at its tip. Each fruit capsule contains up to 150 small seeds.
Management notes:
- Avoid grazing horses on pastures infested with cape tulips.
- Check hay for cape tulip contamination.
- Reducing the chance of introducing cape tulips to a property is important because, once established, the weeds are very hard to control.
- Check hay that is to be fed on the ground for cape tulip contamination.
- Seed can be carried onto a property in mud on vehicles. Clean vehicles after visiting cape tulip infested areas.
- Control of established infestations is a difficult and ongoing process because of the high dormancy rate of underground corms of both species and the cormils of two leaf cape tulip.
- Individual plants found growing as weeds can be removed by hand or spot sprayed with herbicide.
- Dig around the base of the plants to ensure that all of the underground corms are removed.
- The use of herbicides is the main control technique for managing cape tulip infestations. The timing and type of herbicide used are critical to the success of a control program. Seek advice from a local agronomist or weed management agency on the best method and timing for herbicide control of cape tulip
Cotton bushes:
Gomphocarpus spp and Asclepias spp [Apocynaceae] Cotton bushes are those plants in the general Gomphocarpus and Asclepias. Cotton bushes are also known as milkweeds and some are known as swan plants.
They were introduced to Australia as garden plants. Some are still grown in gardens today and some have become important agricultural weeds. Narrow leaf cotton bush (Gomphocarpus fruticosus) is a declared noxious weed in Western Australia.
Cotton bushes contain highly toxic cardiac glycosides but there are no confirmed reports of horse poisoning cases caused by the plants in Australia. The plants are usually unpalatable to horses but they may be ingested if other forage is scarce.
Cotton bushes are sometimes found as contaminants of hay. There are reports from other countries of horse deaths caused by the ingestion of cotton bushes.
Growth and identification:
Cotton bushes are perennial upright plants that are usually 1 – 2 m tall. The stems are usually thin, stiff and woody. The leaves are usually lance-shaped and they are arranged on opposite sides of the stem. All parts of the plant release a milky sap when damaged.
The flowers are white in most species and they are produced in clusters of 3 – 10. Red headed cotton bush has red and orange flowers. The flower clusters droop on a stalk from the forks of the leaves. Flowering can occur from spring to autumn.
The plants produce distinctive balloon-like green fruit that has thin walls. The fruits are covered in soft bristles and they are usually rounded or slightly oval. The fruits of some species may taper to a short curved projection at one end.
The seeds inside the fruit are brown or black and are topped with numerous white silky hairs.
Management notes:
- Avoid grazing horses on pastures infested with cotton bushes.
- Cotton bush species should not be grown as ornamental plants in areas where horses are kept.
- Do not put clippings from ornamental cotton bush species in areas where horses could access them.
- Check hay for cotton bush contamination.
- Individual plants can be easily removed from the paddock by hand as they have shallow root systems. Ensure all root material is removed from the ground. Wear thick gloves and take care to avoid the sap of this plant.
- The use of herbicides can be an effective control measure for larger infestations. The plant should be sprayed while actively growing. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
Foxglove:
Digitalis purpurea [Scrophulariaceae]
Foxglove is native to Europe and is widely grown in Australian gardens. In some areas of Australia the plant grows as a weed in pastures and it has the potential to contaminate hay and other feedstuffs.
glycosides and it has been reported to cause poisoning in horses in some countries, although there has been no confirmed horse poisoning cases in Australia.
Growth and identification:
Foxglove is an erect perennial or biennial plant that grows to around 1.5 m. The plant produces a large number of seeds that can be spread by wind or water run-off.
The stem and leaves of the plant are covered in dense hairs. The leaves are egg-shaped and form a rosette. A showy flower spike grows from the centre of the rosette in spring to early summer.
The flowers are produced at the top of the spike and flowers hang from all sides. The flowers are tubular and bell shaped and can be white, purple or pink with mottled patterns.
Management notes:
- Avoid grazing horses on pastures infested with foxgloves.
- Foxgloves should not be grown as an ornamental plant in areas where horses are kept.
- Do not put clippings from foxgloves in areas where horses could access them.
- Individual plants found growing as weeds can be removed by hand. Wear thick gloves and take care to avoid the sap of this plant.
- The use of herbicides can be an effective control measure for larger infestations. The plant should be sprayed while actively growing. Seek advice from local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
Ornithogalums:
Ornithogalum spp [Hyacinthaceae]
Some species of Ornithogalum are considered weeds in some areas of Australia where they invade crops and pastures, for example Ornithogalum umbellatum (star of Bethlehem), Ornithogalum longibracteatum (pregnant onion) and Ornithogalum thyrsoides (Chinkerichee).
Ornithogalums are cultivated as garden plants throughout most of Australia. The plants are usually unpalatable to horses but they can sometimes be found as contaminants of hay.
Growth and identification:
Ornithogalums are erect annual plants. The plants grow from bulbs, around which smaller bulbs are produced. These bulbs can be spread though machinery movements or cultivation and other practices that move soil. The plant can also reproduce by seed.
The leaves are usually dark green and shiny and some species have leaves with a distinctive white midrib. The leaves are strap-like. The flowers grow on a long stem that is hollow.
There can be many flowers on the branches of each stem. Each white flower has 6 ‘petals’(not all are true petals) and there is usually a green midvein on the outer surface of the petals.
Flowering occurs mostly in spring but can occur at other times of the year.
Management notes:
- Avoid grazing horses on pastures infested with ornithogalums.
- Ornithogalums should not be grown as ornamental plants in areas where horses are kept.
- Do not put clippings from ornithogalums in areas where horses could access them.
- Individual plants found growing as weeds can be removed by hand or spot sprayed with herbicide.
- Dig around the base of the plants to ensure that all of the bulbs are removed. Wear thick gloves and take care to avoid the sap of this plant.
- The use of herbicides can be an effective control measure for larger infestations. The plant should be sprayed while actively growing. Seek advice from local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
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