The book, Handbook of Poisoning in Dogs and Cats Alexander Campbell and Michael Chapman, is compiled from more than 15,000 cases referred to the UK's Veterinary Poisons Information Service The facts are however extremely clear on this subject. The risk just does not exist to any significant degree at all.
The reality is that a search of the scientific literature shows not one single case of ragwort poisoning reported in dogs or cats. There are plenty of scare stories in the press from time to time but there is no evidence that ragwort poses any serious risk at all to dogs.
Ragwort is of course poisonous to mammals. So if a dog (or a cat) were to be force fed this intensely distasteful plant it would be poisoned. However, the poisonous dose for the grazing animals where it is a risk if it is in hay is measured in percentages of body weight so it would be safe to assume that the same sort of dosage risk would apply to dogs. There would be no risk to them from just encountering the plant.
If dogs or cats were found to have megalocytosis which is what has to turn up under the microscope to even consider ragwort poisoning Then it is far more likely that it comes from mouldy food which contins a different set of toxins which cause the same microscopic changes. THere is even a scientific paper in the journal Nature, the world's best scientific journal, which says thsy poisoning by these moulds is "indistinguishable from ragwort. " For more details on the absence of a definitive test click here
One of the issues is that there are vets who are so concerned to warn people about ragwort that this overrules telling the truth, often due to a surprising lack of critical thinking skills in examining the facts. The author of this site is in the process of compiling a good list of examples of this which will prove this beyind any doubt and explain the psychological science behind all of this.
The message here is just don't panic about this scare story. Look at the facts of which you will find plenty on this website, and make your decisions on the basis of the evidence.