Progress towards a blood test for ragwort.
The fresh plant is unpalatable, so horses are most likely to eat it when it is present in hay. It contains several pyrrolizidine alkaloids (PAs) - including jacobine, erucifoline and senecionine. After being eaten, these PAs are metabolised in the liver to become pyrroles, which are toxic.
The pyrroles react with DNA in the liver cells, leading to cell damage. They can also escape into the circulation where they react with the sulphur-containing thiol groups in blood proteins.
The toxic effects are cumulative. Poisoning occurs slowly - signs of disease may appear only long after the ragwort is eaten. However, once clinical signs appear, the damage to the liver is usually so severe that treatment is ineffective. Estimates suggest that, in the UK alone, ragwort may be responsible for 500 or more deaths in horses and ponies each year.
The researchers at Liverpool University have been trying to identify changes in blood proteins caused by the toxic pyrroles. Such altered proteins could act as markers to indicate recent ragwort consumption. Their aim is to develop a test to identify the by-products of the ragwort toxins in a blood sample.
In this laboratory study, the researchers used monocrotaline, a pyrrolizidine alkaloid closely related to those found in ragwort, which is commercially available. The monocrotaline was chemically treated to mimic the process of metabolism that the PAs undergo in the liver. The resulting toxic pyrrole (called dehydromonocrotalin or DHM) was mixed with samples of horse plasma, and the researchers studied the changes in the blood proteins that occurred.
The first possibility they considered was haemoglobin, the main oxygen-carrying molecule, found in the red blood cells. It is a large molecule, and contains thiol groups, and so might have been expected to show characteristic toxic damage.
They incubated normal equine haemoglobin, with different concentrations of DHM. Then they analysed the haemoglobin for signs of changes, but could not find any.
So they looked at other potential targets for damage with pyrroles - such as plasma proteins.
They found that when DHM was added to the plasma protein an aggregate in the high molecular weight region was formed. This aggregate was only found when DHM was added to the protein. It was not found if either monocrotaline (the PA itself before being metabolised to the toxic metabolite DHM) or the solvent were included in the reaction mixture
They identified the proteins in the aggregate - serum albumin, fibrinogen, transferrin and IgG.
The next step was to check the reaction of DHM with albumin, fibrinogen and transferrin individually.
Fibrinogen, in particular, formed high molecular weight aggregates when mixed with DHM.
These toxic changes in fibrinogen could be detected by techniques regularly used in diagnostic laboratories. So this test could be easily performed in commercial laboratories.
Ragwort is recognised as a threat to horses. Without a specific way of identifying exposure to the toxin it is impossible to assess the true level of threat. The discovery that fibrinogen is modified by the toxic pyrrole is the first step towards a specific test for ragwort poisoning.
Perhaps the greatest benefit of such a test would be to allow owners to check that the hay they were feeding was not contaminated with ragwort.
For more details see:
Biomarkers for ragwort poisoning in horses: identification of protein targetsRE Moore, D Knottenbelt JB Matthews RJ Beynon and PD Whitfield
BMC Veterinary Research 2008, 4:30 (doi:10.1186/1746-6148-4-30)
Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews - Equine Science Update 2008
VFH - Futher Information
Identification of Injurious Weeds
Indentification of Injurious Weeds - DEFRA
Guidance on the disposal options for common ragwort
Preventing the spread of harmfull weeds
Guidance note on the methods that can be used to control harmful weeds
Natural England is the body responsible for enforcing the Weeds Act 1959 and the Ragwort Control Act 2003 and each Local Highways Agency (for action on motorways) is also supposed to take anti-ragwort action.
For those who would like to prompt these organizations into action call Natural England 0845 600 3078 and ask for the Weeds Department. For the Highways Agency call 08457 504030
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Added on: 07/10/08.
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