Many poisonous plants are bitter and unpalatable while they are growing and will not be eaten by livestock under normal circumstances.
Therefore, when plant ingestion and poisoning occur there is usually something else happening, says Alistair Moffat from Synergy Farm Health.
This might be a lack of available food, or the plant may have died or been dying back.
Instances can also occur when plants have been chopped and mixed into forage and feed, making them more palatable for livestock.
However, there is no accounting for individual animal behaviour so it pays to know the plants that can be a concern.
Below, four vets from the XLVet community of independent veterinary practices from across the UK, give advice on the more common instances of poisoning they have come across.
Lee-Anne Oliver, Scott Mitchell Associates, Hexham
Although oak trees are commonly found in grazed pastures, cases of oak poisoning are actually quite rare.
Oak poisoning is more likely after stormy weather when leaves and acorns fall and are eaten by animals sheltering under the tree.
Cause: Oak leaves and acorns contain tannins. Young, immature leaves contain more tannins than older leaves.
At a low level of ingestion, the rumen microbes detoxify the breakdown products of the tannins and they don’t cause a problem.
But when the rumen microbes are overwhelmed, the breakdown products of the tannins, mainly gallic acid, cause damage to the cells of the kidneys.
Symptoms: The initial signs exhibited by an animal suffering from oak poisoning are:
- Abdominal pain and distension with constipation
- They are quite often lying down (recumbent).
- If a large quantity of leaves and or acorns have been eaten, death can occur within just a few hours.
Treatment: There is no specific treatment for oak poisoning.
Animals should be removed from the high-risk pasture and fed normal forage. Fluids by the intravenous route or orally (if the rumen is still functioning), may help support the kidneys.
Prevention: Fence off oak trees within grazed pasture.
Ed Hill, Thrums Veterinary Group, Angus
Bracken poisoning often occurs when alternative forage is in short supply and animals have little else to eat, additionally some animals develop a taste for the young shoots in the spring.
Cause: Ingestion of the toxins cause bone marrow suppression and depletion of white blood cells.
Symptoms: The signs of bracken poisoning will vary depending on the amount eaten, over what duration and the species of animal affected.
Weakness, haemorrhages and sudden death characterise acute toxicity.
A diagnosis is usually made by clinical signs and grazing history.
Where animals are exposed to lower doses of bracken over a longer period of time, this may result in the growth of bladder tumours. Signs of this include:
- blood in the urine
- straining to urinate
- weight loss
Bladder tumours in cattle can be diagnosed by rectal palpation and ultrasound scanning.
Treatment: When signs of acute toxicity are seen, the course of disease is almost certainly fatal, and any treatment is likely to be futile.
Again, any form of treatment for bladder tumours is likely to be unsuccessful and these animals should be culled or destroyed before their welfare becomes compromised.
Prevention: Reduce exposure of animals to bracken by fencing, topping, burning or applying an herbicide treatment, though this is likely to be laborious and expensive. Alternatively, ensure animals have adequate nutrition at all times, so they are not forced to eat bracken as an alternative.
Kaz Strycharczyk, Black Sheep Farm Health, Northumberland
A Victorian introduction, the rhododendron has become both a common garden plant and a persistent invasive species. Thriving in poor acidic soils and dappled sunlight, the evergreen shrub is found across the UK.
Cause: Rhododendron contains a poison that slows the heart and lowers blood pressure when ingested.
Symptoms: Affected stock are weak and often unable to rise. Unusually for ruminants they vomit, with stomach contents apparent around the mouth. They may also bloat. Animals can die quickly. Poisoning is confirmed by examining the rumen contents post-mortem.
Treatment: If you suspect rhododendron poisoning, seek veterinary advice. Treatment includes anti-inflammatories and oral rehydration, which incorporates cooled tea to provide tannins that may help bind the rhododendron toxin.
Prevention: Rhododendron toxicity occurs more often when grazing becomes scarce: in deep snow and during droughts, for example.
Ensure stock have access to adequate silage or hay at these times to reduce the likelihood of livestock browsing. Pastures bordering woodland should be inspected for rhododendron and fenced accordingly.
Gardeners may tip clippings into fields as well – gentle polite education may be enough for good neighbours, while fencing off a buffer zone with electric wire may be necessary for bad ones.
Anne Abbs, Paragon Veterinary Group, Cumbria
Pieris is an ornamental shrub noted for the vibrant colour of its foliage in spring and racemes of dangling white flowers.
Poisoning is most likely when hungry livestock, commonly sheep, break into gardens or nibble at foliage through a fence. Clippings can also be toxic, but it isn’t a plant that is regularly pruned, so is less of a risk than other more vigorous garden shrubs.
Symptoms: The signs of poisoning relate to:
- abdominal discomfort
- groaning and teeth grinding
- followed by staggering, recumbency and death.
A presumption of poisoning can be made from the symptoms and a history of exposure to the plant.
It may be necessary to walk the field carefully, particularly the edges, to determine the source of the exposure. A careful check of fences for breakouts into gardens and a check of field edges, particularly those that back up to gardens or are along roads, may reveal signs of tipping of garden waste.
This is particularly relevant at the moment when many councils have suspended garden bin collections and people may be tempted to fly tip garden waste.
The suspicion of poisoning can be strengthened by a post-mortem examination as the leaves and flowers are fairly distinctive and tough, so tend to survive in the rumen for some time following ingestion.
Contact your vet immediately for advice including confirmation of a suspect diagnosis.
Treatment: There is no antidote. Supportive treatment may help some animals but unfortunately for many it is fatal.
Prevention: Remove any risk of exposure for the rest of the group as soon as possible and they should be monitored closely for several days.