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.

It is worth looking at the history of the research on this subject as it confirms as fact that toxins from moulds cause the same symptoms as ragwort. First let's look at a 1961 paper in the highly respected journal Nature. (This is the Journal where only a few years earlier where the discovery of the structure of DNA had been published.) This paper states that poisoning by aflotoxins was, "indistinguishable from Ragwort."(4)

This used as is source a scientific letter which appeared in the journal called the Veterinary Record on August 19th 1961. (5) It is worth quoting a section from this learned letter as it shows that the authors had the evidence even then to state cagorically that something else was causing identical symptoms to ragwort.

For clarity the technical term pathognomonic needs explaining. It means a sign or symptom which is specifically characteristic or indicative of a particular disease or condition. It is also used in the form pathognomic, both forms are in the Oxford English Dictionary.

It was believed that megalocytosis was specifically charcteristic (pathognomonic) of PA poisonng or "seneciosis" but this research shows that it was not.

The research says:-

Allcroft, Camaghan, Sargeant and O'Kelly (1961) noted the close similarity of Brazilian groundnut meal poisoning in ducklings and turkey poults to experimental seneciosis in chickens as described by Campbell (1956). In cattle the disease is indistinguishable from ragwort (Senecio jacobaea) poisoning, the commonest British example of seneciosis.

Not only are the symptoms and the gross morbid anatomy the same, the tetrad of hepatic lesions (Markson, 1960) is essentially identical. Hill (1960) considered the veno-occlusive lesion to be a characteristic feature of seneciosis, while Bull (1954) has stated categorically that the hepatocellular abnormality, which he called "megalocytosis," is pathognomonic of poisoning by pyrrolizidine alkaloids. (By "seneciosis" we mean intoxication by pyrrolizidine alkaloids, which occur naturally in species of Senecio, Crotalaria and Heliotropium.)

There was no evidence of ragwort having been eaten in any of the outbreaks and the evidence indicted groundnut meal as the cause of the disease. The similarity of the hepatic lesions in the calves to those produced experimentally in poultry (Allcroft, et al.) and pigs (Loosmore and Harding) supported this conclusion, while calf nuts from some of the outbreaks and Brazilian groundnut meal were shown to be similarly toxic for guinea-pigs.

Experimental Brazilian groundnut meal poisoning in calves has been studied by colleagues in the Biochemistry Department at Weybridge (Allcroft and Lewis) and we have not duplicated this work. There is now no doubt of the source of the poison; but Allcroft and her colleagues (lac. cit.) have demonstrated by chemical analysis that it is "neither a pyrrolizidine alkaloid nor the N-oxide of such an alkaloid."

The letter concludes with the following paragraph.

"The main purpose of this letter is to describe a new disease of cattle which general practitioners may expect to encounter. In the absence of information about diet and pasture, it is indistinguishable from ragwort poisoning."

The issue here is that the problem of aflatoxins has largely been ignored by the veterinary profession. It is not mentioned as a possible alternative to ragwort poisoning in all textbooks. but as quoted above it does appear in some, and therefore equine vets are quite often unaware of it. The author of this website has had to correct an equine vet who insisted on social media that there was a definitive test for ragwort on this basis when there wasn't.(6)

There is a further example of a vet getting it wrong in a scientific article. It is important here not to fall into the trap of illogically assuming that a more modern reference must be right on account simply of its newness. That would be an example of a well-known logical fallacy. Only the evidence behind the claim maes something correct not when it was written. An article written for Vet Times in September 2013 (7) a vet called Vicky Rowlands made the following statement which has just been demonstrated on this page as false, "Megalocytosis is pathognomic for pyrrolizidine alkaloid toxicity."

Indeed a proper detailed and large book published in 2016 The Pathologic basis of veterinary disease, showed that this vet was wrong. It (8) stated.

Megalocytes are the result of the antimitotic effects of pyrrolizidine alkaloids, which prevent cell division but not DNA synthesis because the hepatocytes attempt to divide to replace those that have undergone necrosis. This change, although indicative of pyrrolizidine alkaloid intoxication, is not pathognomonic because it can also be observed with other toxins such as aflatoxins and nitrosamines.

It is important to note that while these very early scientific papers dealt with goundnuts, also known as peanuts, it has been established that aflatoxin producing moulds do most definitely occur in preserved food like hay. (9) It would also seem that vets are quite commonly idenifying ragwort poisoning as a cause with just the vague signs of blood tests which can only identify liver damage and not the cause. This site's author has had to deal with four letter insults hurled at him by an equine charity called Canterbury Horse Rescue who had evidently been convinced, falsely, by their vet that ragwort poisoning could be defninitively identified in this way, which is of course most definitely not the case. (10)

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
4. SARGEANT, K., SHERIDAN, A., O'KELLY, J. et al. Toxicity associated with Certain Samples of Groundnuts. Nature 192, 1096-1097 (1961).
5. Loosmore, R. M., and Markson, L. M., Vet. Rec., 73, 813 (1961).
6. Jones N.(2022) Ragwort hysteria:Equine vet gets it wrong on ragwort. Available at: http://ragwort-hysteria.blogspot
7. Rowlands v. 2013 More than just blood work – first opinion approach to hepatopathy Vet Times Available at : https://www.vettimes.co.uk/app/uploads/wp-post-to-pdf-enhanced-cache/1/more-than-just-blood-work-first-opinion-approach-to-hepatopathy.pdf 8. Zachary, J.F. et al. (2016) ‘Hepatobiliary System and Exocrine Pancreas1’, in Pathologic basis of veterinary disease. 6th edn. St. Louis, MO: Elsevier. 9. C. W. Hesseltine, O. L. Shotwell, M. L. Smith, G. M. Shannon, E. E. Vandegraft & M. L. Goulden (1968) Laboratory Studies on the Formation of Aflatoxin In Forages, Mycologia, 60:2, 304-312, DOI: 10.1080/00275514.1968.12018571
10. Jones N. (2023) Canterbury Horse Rescue Lousy Charity's Ragwort Nonsense! Available at : https://ragwort-hysteria.blogspot.com/2023/10/canterbury-horse-rescue-lousy-charitys.html