Ragwort poisoning is frequently mentioned in the press, but it is actually not a simple diagnosis. It is caused by the breakdown products of Pyrrolizidine Alkaloids (PAs) which are not just present in ragwort but in many many other plants as well. Also the symptoms that present themselves are not unique to ragwort poisoning but are just the symptoms of liver failure. In order to progress further with the diagnosis you have to establish the presence of a enlarged liver cells in a form known as "Hepatic Megalocytosis".
Professor Robert B. Moeller, Jr Professor of Clinical Veterinary Pathology at University of California, Davis has written about this(1) and he says.
"Hepatic megalocytosis is observed with certain toxic insults, particularly pyrrolizidine alkaloid toxicosis and aflatoxin toxicosis. This lesion is not observed grossly and must be reviewed histologically."
Reviewing something histologically and not observing is grossly means that it cannot be determined just by looking at the liver but a sample must be taken and examined under a microscope.
Also, the reader will note, that it is not only PAs that can cause the problem. Aflatoxins, which are very common, are produced by mould species that may grow on stored hay, silage or other feed and are an indistinguishable poison.
We also have this item from the scientific literature from the book.Pathology of Domestic Animals by K.V.F Jubb , Peter C Kennedy and Nigel Palmer(2)
"Megalocytosis is not a change specific for pyrrolizidine alkaloidosis; it is seen in intoxication by other alkylating agents such as nitrosamines and aflatoxins."
So we see that there is yet another set of substances that cannot be distinguished from PAs. Seeing these texts, which are absolutely consistent with others on the subject, would surely appear clearly obvious that the only rational conclusion we can draw is that there is no test that can positively identify ragwort poisoning and distinguish it with 100% confidence from other causes.
We also know that when a study was made where a large number of liver samples from horses was examined it was found that only around 8% of them were discovered to display megalocytosis, which cannot of course be attributed to any particular cause with certainty.(3)
Although it has been common to attribute any cause of liver damage in a horse to the consumption of ragwort, we now know it is not a particularly likely cause and that it cannot be identified as a cause with certainty due to the existance of other causes with identical effects.
References
1. Toxic response of the hepatobilliary system Robert B. Moeller, Jr In Clinical Veterinary Toxicology 2004 Ed Konnie H Plumlee.
2. Pathology of Domestic Animals 3rd edition 1985 K.V.F Jubb , Peter C Kennedy and Nigel Palmer
3. Surveillance focus: ragwort toxicity in horses in the UK Andy E. Durham Veterinary Record 2015 176: 620-622