Research shows that Ragwort is toxic to animals. Many reports in the press in the UK do not, however, derive their information from proper scientific sources. What is overwhelmingly clear from the scientific journals is that actual poisoning is a rare event. Detailed questions on the number of horse deaths are dealt with here Ragwort horse deaths
There is also the claim from the British Horse Society that exaggerates the toxicity ten thousand fold. See See Ragwort is not extremely toxic
The rest of this article deals with general toxicity.Common ragwort contains compounds that are poisonous to most vertebrates .These are pyrrolizidine alkaloids . These substances occur in other plants as well. In fact they occur in 3% of the world's flora (1). Inside the plants, they occur in a non-toxic form, but after the plant has been eaten it is first changed by the intestines and then broken down by the liver. These processes are necessary for toxicity. The breakdown products formed in the liver are toxic. (2, 3). Contrary to what is often thought by the general public, the alkaloids do not accumulate inside the body of an animal. The fact is that they are excreted in about 24 to 48 hours (3). It is the damage that is caused to liver cells that can, if sufficient ragwort is consumed at each dose, be cumulative to the point of death occurring.
The question then is how much needs to be consumed for an animal to be poisoned. Again research provides the answer. It has been found that is lies between 5 % and 25% of body weight for horses and cattle. For goats the figure is much higher, between 125% and 404% (4). In another study several horses were deliberately poisoned with continous doses of ragwort and it showed a slightly higher figure than the minimum of around 6% of body weight for each one.(5) It is important to realise that not every dose is harmful. The toxins formed have certain properties in common with paracetamol being detoxified in the same manner. Even then if the toxic effect is exerted there are repair mechanisms that can stop problems occuring. This matter is covered here in the article. Ragwort is not strictly a cumulative poison.
References
(1) Fu, P. P., Q. Xia, G. Lin & M. W. Chou. 2004. Pyrrolizidine alkaloids - Genotoxicity, metabolism enzymes, metabolic activation, and mechanisms. Drug Metabolism Reviews 36: 1-55.
(2) Stewart, M. J. & V. Steenkamp. 2001. Pyrrolizidine poisoning: a neglected area in human toxicology. Therapeutic Drug Monitoring 23: 698-708.
(3) Chojkier, M. 2003. Hepatic sinusoidal-obstruction syndrome: toxicity of pyrrolizidine alkaloids. Journal of Hepatology 39: 437-446.
(4) Goeger, DE, PR Cheeke, JA Schmitz & DR Buhler (1982). Toxicity of tansy ragwort (Senecio jacobaea) to goats. Am. J. Vet. Res. 43(2): 252-254
(5) Clinicopathologic studies of tansy ragwort toxicosis in ponies: sequential serum and histopathological changes AM Craig, EG Pearson, C Meyer, JA Schmitz - Journal of Equine Veterinary Science Volume 11, Issue 5, September–October 1991, Pages 261271 1991