Pyrrolizidine alkaloids
Many plants throughout the world contain toxic pyrrolizidine alkaloids and they are among the most common poisonous plants that affect horses and other livestock. Most plant species that contain these toxins are bitter and are generally avoided by horses.
The ingestion of hay or other feedstuffs that have been contaminated with these plants is said to be the most common source of pyrrolizidine alkaloid poisoning. During periods of drought horses may consume pyrrolizidine alkaloid plants if little other forage is available.
Toxic seeds of some pyrrolizidine alkaloid-containing plants may contaminate feed grains in some seasons.
The toxicity of pyrrolizidine alkaloid plants varies markedly and some plants may be more toxic in certain seasons or at certain stages of growth. The pyrrolizidine alkaloids themselves are not toxic.
They are absorbed through the gut and are transported via the bloodstream to the liver. The alkaloids react with substances in the liver and both toxic and non-toxic forms of the alkaloids are produced.
The non-toxic chemicals are excreted and the toxic chemicals either remain in the liver where they interfere with vital cell functions leading to the death of liver cells, or they are transported to other parts of the body, for example the lungs and kidneys.
The extent of damage to the liver or to other organs depends upon the particular type of pyrrolizidine alkaloid ingested and the amount of toxin ingested. Most of the pyrrolizidine alkaloid plants that cause illness in horses primarily affect the liver, and affected animals show signs of progressive liver disease.
Pyrrolizidine alkaloid poisoning can be acute or chronic, with the acute form being much less common. Acute poisoning can occur through the accidental ingestion of significant amounts of toxic plants in contaminated hay or feed, or a horse may suffer acute poisoning when environmental conditions have allowed pyrrolizidine alkaloid plants to become the dominant species in pasture.
Chronic poisoning occurs when the horse ingests small amounts of the plants over time.
The onset of signs of chronic pyrrolizidine alkaloid poisoning can be delayed, occurring at a time when the animal no longer has access to the hazardous plants. This can make it difficult to determine when and how a horse was exposed to the toxins.
It is thought that the toxic effects of pyrrolizidine alkaloids are cumulative and that the toxins may stay in the liver and continue to cause damage even when access to the plants has been removed.
A comprehensive liver function test can assist in the diagnosis of pyrrolizidine alkaloid poisoning in a horse showing signs.
Signs of pyrrolizidine alkaloid poisoning
Acute poisoning – Affected horses have rapid, progressive liver failure and may die within days or weeks of ingestion of pyrolizidine alkaloid plants.
Signs of acute poisoning may include:
- Food refusal
- Depression
- Jaundice (characterised by a yellow colouring of the mucous membranes)
- Abdominal swelling
- Signs of colic
Behavioural changes that may include nervousness or excitability Death can occur in severe cases Horses showing advanced signs of acute pyrrolizidine alkaloid poisoning rarely recover. Veterinary attention should be immediately sought if poisoning is suspected in a horse.
Treatment and supportive care may increase the chance of survival and help with the subsequent recovery.
Chronic poisoning –
Signs may not appear for months or even years after the ingestion of pyrrolizidine alkaloid plants. As loss of liver function gradually progresses, there is a corresponding progression in the severity of symptoms. When the liver is damaged to a critical point, liver failure progresses rapidly and death can occur soon after.
- Signs of chronic poisoning may include:
- Food refusal
- Depression
- Increasing signs of jaundice
- Loss of condition
- Secondary photosensitisation – redness and swelling of skin edness the in unpigmented areas. The nose, lips and around the eyes are areas commonly affected. The skin may crack and weep fluid.
- Yawning
- Head pressing - the horse may push its head against a wall or other surface
- Incoordination
- Aimless wandering
Death can occur in severe cases There is no specific treatment for chronic pyrrolizidine alkaloid poisoning. Removal of the plants from the diet can slow the progression of symptoms.
Horses with photosensitisation should be kept out of the sun to help the skin heal. Veterinary attention should be sought if chronic poisoning is suspected in a horse. Horses showing signs of advanced liver disease have little chance of survival.
Paterson’s curse:
(salvation Jane, Lady Campbell weed, purple bugloss, Riverina Echium plantagineum [Boraginaceae] Paterson’s curse is native to the Mediterranean region. It is a major weed of Australia pastures and it is a declared noxious weed in all states except Queensland and the Australian Capital Territory.
In recent times Paterson’s curse has been a particular problem for horse owners in some parts of Australia. The plant is usually unpalatable but horses will eat it if there is little other available forage. Paterson’s curse can dominate pastures and this is especially so in degraded pastures, such as those affected by drought, fire or overgrazing.
Paterson’s curse contains up to ten types of toxic pyrrolizidine alkaloids and the weed has been associated with many horse deaths in recent years. All parts of the plant are toxic and the toxicity is not lost in dried plants.
Growth and identification:
Paterson’s curse is a short-lived winter annual or biennial that grows to 90 cm in height. The plant produces numerous seeds that can accumulate in the soil and remain dormant for many years. Seeds can be easily spread to new areas through water run-off, as contaminants of transported hay and grain or through animal and vehicle movements. The seeds usually germinate following rain in summer or autumn.
During winter Paterson’s curse grows as rosettes . The leaves of the rosettes are hairy and oval or oblong in shape. The leaves can grow to 30 cm in length and they have distinct branched veins. Flowering occurs in early spring with the appearance of several upright flowering stems that grow from the middle of the rosette.
The branched stems are covered in stiff hairs. The leaves on the stems are hairy and are smaller and narrower than the leaves of the rosette. The leaves are arranged alternately along the stem and they decrease in size towards the flower. The leaves are held close to the stem and they do not have stalks.
The flowers are tube-shaped and are usually purple or blue, but white and pink flowers are occasionally seen. Each flower has 5 fused petals and is 2 – 3 cm long. The flowers have 5 stamens and 2 of these extend beyond the flower tube. Paterson’s curse seeds are around 3 mm long and are brown, grey or black. They are three-angled and have a wrinkled and pitted surface.
Management notes:
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Avoid grazing horses on pastures infested with Paterson’s curse.
- Check hay for Paterson’s curse contamination.
- Individual plants can be pulled out by hand or spot sprayed with herbicide. Remove plants at rosette stage for the best results. Ensure all parts of the plant are removed from the ground.
- The use of herbicides for larger infestations is most effective when the plant is in the rosette stage. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
- The establishment of competitive pastures can control Paterson’s curse.
- Minimise summer and autumn grazing pressure on lightly infested pastures as this will maintain a good ground cover and will prevent Paterson’s curse from establishing in bare patches.
- Check for Paterson’s curse seed in seed batches bought for sowing crops and pasture.
- Graze adult non-pregnant goats or sheep on heavily infested pastures as they are more tolerant of pyrrolizidine alkaloids and will readily eat the weed. Heavy grazing by these species in spring and early summer can suppress seed set in Paterson’s curse. Do not leave these animals on the infested pasture continuously as toxicity problems can occur.
Crotalarias:
Plants in the Crotalaria genus are known as the ‘rattlepods’ because the seeds rattle inside the dried inflated seedpod when shaken. Some crotalarias are known to contain pyrrolizidine alkaloids and several species have caused cases of poisoning in Australian horses.
A horse poisoned by the pyrrolizidine alkaloids in crotalarias is said to be suffering ‘crotalism’, ‘walkabout disease’ or ‘Kimberly horse disease’. Two species of crotalaria contain an unknown toxin that causes a disease known as ‘Chillagoe horse disease’ and this is discussed on page 75. The incidence of horse poisoning cases caused by the ingestion of crotalarias has decreased over the past 50 years.
This is due to an increased awareness of the potential toxicity of the plants and the subsequent implementation of horse management techniques that reduce horses’ exposure to crotalarias. Poisoning cases are still occasionally reported and crotalaria poisoning can be a significant problem for horse owners in northern regions of Australia where crotalarias are common, particularly when those areas are affected by drought.
Crotalarias that contain pyrrolizidine alkaloids are usually unpalatable but horses will eat the plants if other forage is scarce. Some crotalarias can become the dominant species in pastures that are degraded by drought, fire or overgrazing. Crotalarias may also be found as contaminants of hay. The seeds of some crotalaria can contaminate feed grains, particularly sorghum.
Not all species of Crotalaria contain toxic pyrrolizidine alkaloids and the amount of toxins in crotalarias varies between species. More than forty species of crotalaria grow in Australia and around eighteen of these are native Australian species. Not all crotalarias have been tested for alkaloid content and although all species should be regarded as potentially toxic to horses, it is advisable to seek a positive identification of the species before removing crotalarias from pastures.
The crotalarias most commonly associated with poisoning in horses is C. crispata and C. ramosissima, both commonly called Kimberly horse poison. Other species that have been associated with horse poisoning cases, or are known to contain toxic alkaloids are listed below.
Growth and identification:
Identification note: distinguishing between the different species of Crotalaria can be extremely difficult and only a very basic description is given here. See page 5 for more information on obtaining a precise identification of a plant species. Two general characteristics of plants in the genus Crotalaria are their pea-shaped flowers that are yellow in all but one species and their seedpods that are usually inflated when dried.
Identification note: Crotalaria crispata and Crotalaria ramosissima are very difficult to tell apart, with Crotalaria crispata having the larger calyx lobes (the bracts that hold the flower).
Kimberly horse poison:
Crotalaria crispata and Crotalaria ramosissima [Fabaceae] Annual or short-lived perennials that grow to 50 cm. The stems lie on the ground and grow upwards at the tips (decumbent). The oval to wedge-shaped, grey-green leaves can be up to 2.5 cm long. Flower spikes appear at the tips of branches from spring to late summer. Numerous yellow pea-shaped flowers. Produces hairy pods.
Native Australian species.
Grey rattlepod
C. dissitiflora [Fabaceae]
Mostly upright annual that grows 30 - 40 cm. Hairy stems. The leaves have 3 oval leaflets up to 3.5 cm long. Flower spikes appear at the tips of branches in summer to early winter. Numerous yellow pea-shaped flowers. Produces hairy pods.
Native Australian species.
Narrowleaf rattlepod
Crotalaria linifolia [Fabaceae]
Erect annual that grows 50 cm –1 m. Hairy stems and leaves. The leaves are narrow and grow 2 – 3 cm long by 3 mm wide. Flower spikes appear at the tips of the branches. Numerous yellow pea-shaped flowers. Produces egg-shaped black pods. Native Australian species.
Yellow rattlepod
Crotalaria mitchellii [Fabaceae]
Upright perennial shrub that grows to 1 m. The oval leaves are hairy on the lower surface and can grow to 8.5 cm long by 4 cm wide. Flower spikes appear at the tips of the branches. Numerous yellow pea-shaped flowers. Produces hairless greenish-grey pods. Native Australian species.
Showy rattlepod
Crotalaria spectabilis [Fabaceae]
Upright annual that grows 1 2 m. Stems and – leaves are hairless. The lower surface of the leaves is sometimes hairy. The leaves are oval and are up to 10 cm long by 4 cm wide. Flower spikes appear at the tips of the branches in summer and autumn. Numerous yellow peashaped flowers. Produces a black pod that can be up to 6.5 cm long. Introduced species.
Purple-flowered rattlepod
Crotalaria verrucosa [Fabaceae]
Annual that grows to 80 cm in height. The leaves are broadly oval and are up to 7 cm long by 2 cm wide. Flower spikes appear at the tips of the branches. Numerous blue or purple pea-shaped flowers. Produces oblong-shaped pods that have brownish hairs. Native species.
Streaked rattlepod
Crotalaria pallida [Fabaceae]
Upright annual or short-lived perennial that grows to 3 m. Leaves have 3 egg-shaped leaflets up to 7.5 cm long. Flower spikes appear at the tips of the branches from autumn to spring. Numerous yellow pea-shaped flowers with reddish-brown streaks. Produces hairy pods that are brown or mottled yellow and dark green.
Introduced species.
Sunhemp
Crotalaria juncea [Fabaceae]
Upright annual that grows to 2 m. Leaves are long and narrow and grow to 6.5 cm long by 0.5 cm wide. Flower spikes appear at the tips of the branches in autumn and winter. Numerous yellow pea-shaped flowers that have a faint purplish blush. Produces black pods. Introduced species.
Wedge-leaf rattlepod
Crotalaria retusa [Fabaceae]
Flower spikes appear at the tips of the branches. Numerous yellow pea-shaped flowers. Produces purple-black pods. Native Australian species. Upright annual that grows to 1 m. Leaves can be up to 8 cm long and they are dark green on top and paler below. The leaves are wedge-shaped.
Crotalaria montana [Fabaceae]
This variable species can be annual or perennial and prostrate (creeping) or upright. Grows to 1 m when upright. Leaves can be oblong to elliptical and can be up to 4.5 cm long by 0.8 cm wide. Flower spikes appear at the tips of the branches from late summer to winter. Numerous pale yellow pea-shaped flowers. Produces dark grey to blackish pods.
Native Australian species.
New Holland rattlepod
Crotalaria novae-hollandiae [Fabaceae]
Upright or spreading perennial that grows to 1.5 m. Stems covered in long silky hairs. Oval or triangular hairy leaves up to 12 cm long and 4.5 cm wide. Flower spikes appear at the tips of the branches for most of the year but mainly in autumn to spring. The numerous pea-shaped flowers are yellow with darker streaks. Produces hairy pods up to 4.5 cm.
Crotalaria brevis
[Fabaceae]
Annual stems and perennial rootstock. Prostrate (creeping), growing to 30 cm in height. The stems and leaves have long white-reddish coloured hairs. The leaves are lance-shaped and can be up to 2.5 cm long by 0.5 cm wide. Flower spikes appear at the tips of the branches from summer to winter. Numerous crowded yellow pea flowers that have darker yellow markings. Produces brown hairless pods.
Native Australian species.
Bluebush pea
Crotalaria eremaea
Upright perennial that grows to 2 m. The leaves have 1 – 3 oblong leaflets that can be up to 4 cm long. Flower spikes appear at the tips of the branches in summer and autumn. Numerous yellow pea flowers. Produces yellow pods.
Native Australian species.
Management notes:
- Avoid grazing horses on pastures infested with crotalaria.
- Improve pastures to increase desirable grasses.
- Isolated small plants can be pulled out by hand or spot sprayed with herbicide.
- 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.
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