Poisonous plants for horses: Ragwort
A poison is a compound that irritates, damages body tissue or alters the metabolism. As horses are trickle feeders and graze for most of the time, on rare occasions they can eat something poisonous. Some of the common plants poisonous to horses include acorns, sycamore seedlings, yew and ragwort. In this article, we will focus on Ragwort, which is part of the Senecio plant family.
There are more than 37 species of Senecio but 2 main species found in our pastures are:
–Tansy ragwort (Senecio jacobaea)
-Narrow-leaved ragwort (Senecio inaequidens)
These plants contain toxic substances called Pyrrolizidine alkaloids (PA) which are, for them, a way to protect themselves from herbivores. They are biennial plants. In the first year, a flattish crown of branched leaves is formed. This flat crown is fairly resistant to mowing and is often not noticed. In the second year, yellow flowers are produced on long stems. Ragwort proliferates easily as the wind carries the seeds over long distances.
Ingested Pyrrolizidine alkaloids cause irreversible liver damage. However, acute toxicity is very rare due to the plants unpalatability. Toxicity more commonly occurs due to chronic ingestion of Ragwort over time, resulting in a cumulative effect of Pyrrolizidine alkaloids in the liver. In these cases symptoms are usually delayed and do not become apparent until 4 weeks to 6 months after the plant is eaten. Consequently, diagnosis and treatment will also be delayed.
The whole plant is poisonous whether it is standing or dried. However, horses are more likely to eat Ragwort when it is dried as the plant loses its anti-nutritional factors (bitterness). The amount of PA in the dry plant is between 0.2 and 0.3% of its weight. The lethal dose is reached after chronic ingestion of an amount of Ragwort (dry matter) of 3% to 5% of bodyweight (i.e. about 300g / day for 50 days for a 500kg horse). Therefore, as well as ensuring that your paddocks are Ragwort free it is also important to carefully check for the presence of ragwort in any conserved forage fed (e.g. hay or haylage). It is important to be vigilant with young horses and horses on a forage diet.
Symptoms of ragwort poisoning in horses
Clinical signs appear after a silent phase. Their number, intensity and rate of onset vary according to the individual and the degree of intoxication. Weight loss and lethargy are the most commonly observed clinical signs followed by anorexia and jaundice. Signs of photosensitization may appear in summer. In severe cases behavioral abnormalities such as compulsive walking, head pressing or apparent blindness can occur due to the toxins effect on the horse’s brain (hepatic encephalosis). Unfortunately, severe cases of Ragwort poisoning are often fatal.
The diagnosis is determined by observation of the clinical signs, the animal’s history and its environment. Laboratory tests, including the measurement of liver enzymes, bile acids and bilirubin levels in the horse’s blood are also important. However, a definite diagnosis will only be made by undertaking a liver biopsy to enable histopathological analysis of liver tissue.
The best way to prevent Ragwort toxicity is to regularly check and remove Ragwort plants from your horse’s environment. Plants should be dug or pulled up by their roots and disposed of away from livestock. If pulling Ragwort by hand it is recommended to cover your skin and wear a facemask to avoid contact with the plant or the inhalation of its pollen. It is also important to always ensure that your horse has adequate access to grazing, or if grazing is limited provide hay or haylage in the paddock, this will help to ensure that your horse is not tempted to eat any ragwort which may have been missed. There are several weed-killing sprays that are reputed to be effective against Ragwort. However, before using it is always best to check with the manufacturer to ensure that they are suitable for use near horses. Finally, not use pasture which is contaminated with Ragwort for making hay or haylage and if not making your own check that your hay or haylage is free from any signs of dried Ragwort.
Ingested regularly Pyrrolizidine alkaloids cause irreversible liver damage, mostly fatal and expressed after a silent clinical phase. The cases of intoxication are more numerous and no treatment currently exists. Moreover, the diagnosis remains very complicated because of the delay of expression of the symptoms, the lack of specificity of the clinical signs and the biochemical examinations. Ragwort proliferates very effectively so it is very important to inform the owners of horses and put in place effective means of control quickly.