Over the last 5 years there has been an explosion of ragwort in the UK. One can hardly avoid seeing it along road verges and in central reservations of motorways. However, animals seldom graze these areas and so many lay people driving along comment on how pretty the flowers are in high summer and how the fluffy seeds can be seen to blow away in the wind - giving the impression of a dandelion clock! The yellow peril is lurking and expanding its grip on the UK - it presents a serious hazard for horses and other grazing animals.

Ragwort in full bloom

The potential danger of ragwort has been known for many years - indeed the plant was classified as a noxious weed in terms of the Act and as such every owner of land was required to control ragwort either by direct spraying or by lifting and burning it. Up to the last 5 years the value of this policy was widely appreciated by farmers and horse owners alike. It was rare to see any ragwort growing anywhere and prosecutions were rarely needed. There has been a clear change in policy over the weed. Now a landowner on whose land ragwort is growing can only be prosecuted if it can be proven that by allowing the plant to proliferate on his land someone else's animals are proven to have suffered. This is nonsense of course because it is quite clearly impossible for an owner of a ragwort-poisoned horse to be sure within the law that the problem arose from one source! Imagine trying to sue the Ministry of Transport because they have allowed ragwort to grow on the central reservation of a motorway.

A field covered in Ragwort

Interestingly some areas of the UK have very progressive and caring local authorities and they are clearly taking action against the scourge. Every now and again you will find swathes of ragwort free roads and fields! By contrast you will certainly see whole areas of the country where nothing is done to control it at all. Meadows of lovely yellow flowers and deep green florets can be attractive to the uninformed passer-by! We all know it is a time bomb waiting to explode (in our horses!).

The florets of the Ragwort Plant

Do horses actually eat it?

For many years it has been assumed that horses will not eat green growing ragwort plants but this is far from proven. Indeed it is only based on supposition that when horses are grazing in a field in mid summer the ragwort appears to be untouched.

Grazing near Ragwort

A horse grazing a Ragwort infested field

This is a very simplistic approach and it is impossible to know categorically that the horses are never even tempted to take a mouthful. The biggest difficulty lies in the palatability of the plant. In its green growing state it is probably fairly unpalatable but the wilted or cut and dried plant is not detected at all. Thus hay that has been cut from a field with ragwort will be potentially lethal. The repeated ingestion of a small amount is probably the worst possible scenario!

Ragwort in Hay

Ragwort in Haylage

As the plant is most palatable in dried or preserved forage the greatest risk appears to come from the winter feeding! How many of us know if the hay or haylage we use is certainly ragwort free? It is unlikely that any meadow hay is totally free of ragwort in the UK. This means that damage can be occurring in the horse over the winter without any sign! It is a time bomb quietly ticking away! This probably explains why we see cases of ragwort poisoning in the late spring and early summer.

What does it do to the horse?

Ingestion of the ragwort plant (in any state in any amount) will result in the absorption of the pyrrolizidine alkaloid that passes to the liver in the portal blood vessels. On arrival the toxin damages the liver cells to an extent that is proportional to the concentration of the chemical. As the blood is delivered to the liver in the portal vessels the changes are very characteristic. Pathologists can easily detect the changes and can be reasonably certain that they are caused by ragwort. However, the diagnosis would relay on biopsy and this is not an easy procedure and is certainly invasive. Repeated and sustained ingestion (or the ingestion of a large amount) of ragwort causes progressive fibrosis and shrinking of the liver. Once more than 75% of the organ is damaged irreparably the horse will show the characteristic clinical signs. These include lethargy, weight loss, diarrhoea or at least pasty sloppy faeces, and in the summer months photosensitisation.

Photosensitisation occurs because the liver is badly damaged and therefore fails to remove plant pigments that then are allowed to circulate in the blood. These are deposited in the white skinned areas and when the sun shines on this skin it causes massive destruction and release of bioactive inflammatory mediators that are associated with serious skin damage. The pigmented skin is not usually affected.

The effects of Photosensitisation

The effects of Hepatoencephalopathy

Perhaps the most dramatic sign of hepatic failure is hepatoencephalopathy. Here chemicals (and in particular ammonia) that should be removed from the blood by the liver poison the central nervous system. Progressive accumulation of these chemicals results in serious brain disease, which shows as depression, abnormal behaviour (either mania or convulsions and seizures) and sometimes even blindness.

More effects of hepatoencephalopathy

At this stage the horse is probably beyond treatment but just occasionally we can save them at least temporarily. The clinical entity is a very distressing condition for both horse and owner and indeed many horses hurt themselves and / or hurt the owner.

How can we diagnose the problem?

The biggest problem with ragwort is that it is extremely poisonous to horses in particular and somewhat less so to ruminants. Even small amounts cause significant liver damage. It is easy to ask then why we do not see plenty of cases every year! The reason is that the liver has an enormous functional reserve (over 75%). Clinical signs of liver disease only appear when there is less than 25% of the functional tissue left! This can be some years later and although the liver can repair itself to some extent it is also known that the pyrrolizidine alkaloid poison inhibits the repair mechanisms. Therefore repeated ingestion of even small amounts is potentially very dangerous. The signs may only appear some years later when the liver is beyond repair and beyond treatment.

The effects of Ragwort on the Horses' liver

While the mildest signs of liver damage are not usually detectable from the outside, damage to liver cells can be detected by blood samples. The acute (fresh) damage to liver cells causes leakage of enzymes from the cells into the blood stream. These enzymes should not normally be present in abnormal amounts and so detection of elevated enzyme concentrations in the blood is highly suggestive of liver cell damage. Also as the liver becomes more and more damaged the functional reserve is gradually eroded until it can no longer cope with all the tasks it is required to do. Then some signs of liver disease become apparent.

Recently it has become possible to detect the alkaloid in the blood of horses at a very minute concentration! Of course any of the chemical indicates that the horse is eating ragwort and the food can be changed to a better type! Unfortunately this test is not yet available but we hope this year to get it running on a preliminary trial. We would then expect that a proportion of the horses in a yard would be blood tested every month or two and if no alkaloid was present then the feeding could reasonably be expected to be safe (with respect of course only to ragwort!) If you are interested in helping us to develop the test please contact me. We need financial support for this project and we need owners and vets who will take part in the trial phase at first!

Can it be treated?

Liver failure cannot be treated - it can only be assisted to repair itself. Therefore once the clinical signs appear it means that there is no reserve of liver and that recovery is virtually impossible. Over the last few years there have been cases that have been suspected as being Ragwort-related liver failure that have recovered from a serious state. Of course a few cases that have limited damage do recover but if Ragwort toxins extensively damage the liver then it will not repair effectively. Most of the cases that are diagnosed at an early stage can recover sufficiently to have a useful life but of course many cases with other forms of liver disease are diagnosed as being Ragwort poisoning. There are other types of liver damage including acute and chronic hepatitis that can be treated and that hold out a much improved prognosis.

Signs of Liver damage

For the most part terminal Ragwort damage results in a wide variety of signs and each of the deficient functions needs to be supported by medications and management. For example because the liver makes clotting factors for the blood, these need to be replaced in horses with serious terminal liver failure. Similarly the protein metabolism is impaired and so plasma transfusions may be helpful. It is important that the attending vet knows the full range of signs that are being shown and then suitable measures can be used to treat each of these directly. Cases showing neurological signs (blindness, aimless wandering, seizures and collapse often need sedation to prevent themselves from self-injury. Throughout the management of the Ragwort poisoned horse the over-riding sentiment must be for the welfare of the animal. Most cases are destroyed on humane grounds as soon as the diagnosis is confirmed.

The untreatable nature of terminal Ragwort poisoning merely serves to illustrate the importance of an accurate diagnosis even at this stage. It would be far better of course to detect the poison long before the extent of the damage is irretrievable. This is why we need to eliminate Ragwort from the UK - the reality is that it is here to stay - we have largely lost the battle and we can expect many more cases of fatal hepatic disease in our horses. If we have to accept that it is going to be around for some years yet then we need to identify whether individual horses are eating any of it - even one molecule of the toxin causes damage. A blood test would identify if the food contains any Ragwort and then at least there would be a chance to change the feed and avoid progression of the disease. Finding Ragwort in hay or haylage is very difficult task and not many of us has time to carefully sift through every hay net of food to ensure it is safe.

The danger of Ragwort

I am not sure that any pasture in UK can now be guaranteed to be free of the plant and this is a terrible indictment of our care for the countryside over the last 5 - 10 years.

Derek C Knottenbelt BVM&S, DVM&S, MRCVS

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