I have written before about Dr David Marlin making questionable or unwarranted claims about ragwort and I am sad to say that he is at it again. So once again I am offering my honest opinion on what he has said based on the scientific evidence and legal facts. He has posted something on his website and on social media which is contains unsubstantiated assertions alongside factual errors and misleading assumptions.
I’m not claiming that Dr. Marlin is deliberately misleading people. That would not be rational as I cannot read his mind and gain insight into his private thoughts. What I can say, with confidence, is that his statements about ragwort are not supported by the best available evidence, and they mirror the kind of reasoning errors we often see in cases where strong emotion overrides sound evaluation of facts.
I am going to analyse the post he has made line by line, or perhaps more accurately piece by piece.
."Do Equestrians Want to Eliminate Ragwort from the Countryside?
Short answer: No. Just from the places horses graze!"
Well on the basis of the scientific literature there is little risk to horses in well managed pastures from the occurrence of ragwort in it. They eat around it.
"Most horse owners aren’t bothered about ragwort on verges, arable field margins, or in wild meadows, as the image above shows. We only want it removed from places where horses graze (or where hay and haylage for horses are being grown). That’s not scaremongering. That’s animal welfare."
Social media is full of people acting exactly as if they want ragwort eliminated and Prof Knottenbelt has said so in an on-line CV and is quoted in a Guardian article that it is his aim. Also to make the point again there is no proper evidence of risk just from it being in a pasture.
"Ragwort and Horses: What the Science Actually Says
Ragwort contains pyrrolizidine alkaloids (PAs) — toxic compounds that cause irreversible liver damage in horses if ingested in sufficient amounts.
This damage from PAs IS cumulative: While PAs are not stored, the liver damage caused by them is cumulative and irreversible."
NO not necessarily. The biochemistry shows that there are repair mechanisms for early damage and there are several published scientific papers showing no harm at low doses of the toxic alkaloids
"Ingestion of as little as 1% of bodyweight in ragwort, whether fresh or dried in hay, can prove fatal over time."
This is not supported by the evidence at all. No reference is given for this BUT he made the same claim in a Facebook post and then he gave a reference. It is probably best If I just repeat the points I made then.
As little ragwort as 1% of bodyweight can prove fatal over time - 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.
Just look at this reference! As soon as I saw it I was on the alert. I am very familiar with this paper. It DOES NOT SAY THAT! It is not about that kind of thing at all. What is going on here? Is this just carelessness or is it a deliberate attempt to mislead? In my honest opinion if someone wanted to bamboozle people with bad science this is exactly the sort of paper that would be used it is 55 pages long and full of degree level biochemistry. In fact it is worse than that. I took a random sample of text and put it through something that calculates a measure of readability called the Gunning Fog Index where Fog stands for Frequency Of Gobbledegook. An index of 17 requires you to have a university degree but this actually came out at 19.5! Most people wouldn’t have a clue about what it says. The word “ragwort” does not even appear in this scientific paper!
On the basis of what I have read in over 2 decades of detailed study of what is known from the scientific literature, claiming that 1% of body weight being fatal is I something I can honestly describe as scaremongering.
It’s estimated that up to 8% of equine liver samples submitted to labs show signs of ragwort-related pathology.
Vets across the UK continue to report suspected and confirmed cases of ragwort poisoning each year.
So no, this isn’t fake news or hysteria. It’s a fact, backed by published data, veterinary reports, and decades of pathology.
This is not what decades of pathology says at all. There are vets that don't know this, but it is an established scientific fact that the diagnosis is not this simple. To quote a paper from one of the worlds most highly prestigious scientific journals, Nature, that toxins produced by often invisible moulds that occur on preserved forage are, "Indistinguishable from ragwort.." This was decades ago in 1961, but it is still reflected in the text of some of the better veterinary textbooks and has never been superseded by more recent research.
One of the primary issues that emerges in this subject is what is known as moral panic where emotions such as fear and disgust override rational thinking and ill thought out postings such as these from Dr David Marlin only serve to fuel this. Science is more about nuance, abstract ideas, and evidence than these simple hyped up and exaggerated claims.
Then there are a set of false claims about the law
"A Reminder About Ragwort and UK Law!
Ragwort is one of five “injurious weeds” covered by the 1959 Weeds Act. The other weeds covered by this act are Spear Thistle, Field Thistle, Curled Dock and Broad-Leaved Dock."
Injurious in this context means harmful to agriculture.
"The law states that……
If you own horses, ponies or livestock, you must not allow them to graze on land where you know ragwort is present.”
It says no such thing.
This is the only relevant piece of text in the law. "Where the Minister of Agriculture, Fisheries and Food (in this Act referred to as “the Minister”) is satisfied that there are injurious weeds to which this Act applies growing upon any land he may serve upon the occupier of the land a notice in writing requiring him, within the time specified in the notice, to take such action as may be necessary to prevent the weeds from spreading."
Sp to explain it the law says that you may be issued wth a control notice, these are rare. If you re issued with one of those notices, then you have to control ragwort. In the absence of one of these notices, there is no requirement placed on anyone to do anything. Horses are not even mentioned!
“You can be prosecuted if you allow animals to suffer by eating harmful weeds.”
Yes you might but as we've heard they don't eat the fresh plant unless starved. So we're back to hay again about which there is no dispute,
A link at the end is made to the government website as if this were the statute law. It is not! it is marked clearly as guidance. In a posting a few weeks ago I gave the key risk statistics to an independent evaluator the AI ChatGPT asking it in unbiased stages to evaluate the risk statistics used to justify ragwort control and it was scathing about it. The guidance issued by government is simply wrong!
Mythbusting the “Native Superflower” Narrative
“It’s not a cumulative poison.”
WRONG. While PAs are not stored, the liver damage caused by them is cumulative and irreversible.
This is based on a complete misunderstanding of the biochemistry and terminology. As I've said here are repair mechanisms so damage is not caused by small doses but he is not using the word cumulative correctly in the context. Alcohol can cause steady and increasing damage to the liver but we talk about chronic poisoning not cumulative poisoning because the alcohol doesn't stay n the body. Cumulative poisons are things like lead or cadmium that do not leave the body and build up in it.
It is a standard part of toxicology. Such a basic part that in most sources it is implied over a number of pages rather than stated as a definition as it is such a basic concept that it is assumed to be known.
Firstly the Oxford English Dictionary which is the definitive dictionary of the English language defines cumulative in this context as _
:-
Constituted by or arising from accumulation, or the accession of successive portions or particulars; acquiring or increasing in force or cogency by successive additions
:-
So the dictionary definition proves him wrong as the poison itself does not build up.
This definition from a scientific paper confirms it.
" Cumulative
poison is a slow acting poison, which has not been fully
discarded from the system. It retains in the tissues for a number
of years and therefore contaminates not only the tissues but also
at the level of cell too." ( Sud S. Diagn Ther Complement Tradit Med 2019(1): 01-02.)
"“It can’t penetrate human skin.”
Also WRONG. PA absorption through skin is low, but not zero, and handling ragwort regularly without gloves can cause liver damage."
This is just an extension of the false claim that it is a cumulative poison combined with the false idea that every dose has an effect which as we see above isn't the case. The idea that ragwort is poisonous to the touch like this was debunked by some Dutch scientists, one of whom has a PhD on the plant, some years ago.
“It’s not invasive.”
TRUE. It isn’t invasive by definition, BUT that doesn’t mean it can’t spread.
Each ragwort plant produces up to 200,000 wind-dispersed seeds, and these can travel many metres or even kilometres with secondary movement (e.g. water, animals, hay contamination).
An advert claiming just 150,000 seeds was banned some years ago by the Advertising Standards Authority and the seed calculations in the best scientific paper show the figures are much lower, which is why the ad was banned.
In fact the proper statistics show that typically a plant produces far fewer seeds than even that. There is no scientific basis for the claim that seeds are regularly dispersed over distances measured in kilometres by the wind, most in fact falling after a few metres and the occasional and rare outlier doesn't justify the exaggerated fear provoking claims Secondary dispersal is not at all significant in reality. This is another example of overstating things seemingly to create fear rather than to promote understanding
As Peter Medawar, the Nobel Prize winner, once remarked, “The spread of secondary and latterly of tertiary education has created a large population of people, often with well-developed literary and scholarly tastes, who have been educated far beyond their capacity to undertake analytical thought.”
This quote seems rather relevant today as there are people in the veterinary and in the equine communities who often speak very confidently and use scientific language. However, they repeatedly seem to show a cognitive failure to engage with the proper scientific concepts in a critical or careful manner. The ability to cite papers, use technical terms or make the firm pronouncements which we so often see does not necessarily reveal a proper analytical depth in understanding issues. In these scientific debates where we deal with such important issues as ecological and welfare concepts extreme care must be taken not to conflate verbal polish with proper scientific cognitive rigour.
