Equine Science Update
Reports from the world of  equine research.
Hair analysis for plant toxins.
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Hair analysis could provide a way to investigate cases of suspected plant poisoning in horses, according to Dr Mark Dunnett formerly of the Royal Veterinary College, London.

Speaking at the Equine Nutrition Conference in Hannover, he explained that a big advantage of hair analysis is the ability to look at what the horse had been exposed to in the past. Many foreign materials are incorporated into the hair. They degrade very slowly and so can be detected for a long time. Hair provides a good subject for analysis because it is very stable. Samples do not need to be refrigerated.

Mane and tail hair grows at a fairly constant rate: 18 - 25mm/month. The rate varies with location and breed. Hair growth is fastest in the tail and at the top of the mane. Native pony breeds such as the Welsh Mountain and Shetland grow hair more rapidly than do Thoroughbreds. Season or time of year does not affect rate of growth of mane and tail hair.

By looking at segments of hair in relation to the hair root it is possible to estimate the time during which the animal was exposed to a certain substance. In many cases it may be possible to tell whether the horse was exposed to a single large dose, or if it has been exposed on several occasions or over a prolonged period.

For example salicylate ( natural "aspirin") is found in many pasture plants. Alfalfa is a particularly good source. Dunnett described how hair analysis can discriminate between horses that have been fed diets containing high or low levels of salicylate.

Hair analysis has the potential for identifying past exposure to plant toxins such as the pyrrolizidine alkaloids (PAs) found in ragwort. Ragwort is the most common cause of plant poisoning in horses in the UK. Common ragwort ( Senecio jacobaea) is the main culprit, but other related plants also contain the toxins - such as the groundsels ( Senecio plattensis and S. vulgaris ).

Fresh ragwort is unpalatable and so horses do not usually eat it. The main risk comes from the dried plant in hay. However, horses grazing ragwort infested pastures are not safe. Occasional leaves go un-noticed if taken in a mouthful of grass. If the horse does spit the ragwort out, the leaves will become more palatable after lying on the ground for a few days.

Comfrey also contains high levels of PAs, which have been shown to cause chronic liver disease and may cause cancer and abortion.

Results of a pilot study suggest that hair analysis may be used to investigate cases of suspected ragwort poisoning. Dunnett analysed hair samples from horses with no known exposure to PAs. He looked for the PAs seneciphylline, senecionine (from ragwort), and retorsine-N oxide (from groundsel). All samples gave negative results.

A second group of horses had raised liver enzymes, suggesting possible PA poisoning. In one of these horses he tentatively identified seneciphylline and senecionine at concentrations of up to 250pg/mg. By looking at sections of the hair he found that the peak levels had occurred about 20 months previously.

Hair colour seems to affect the concentration of toxin residues in the hair. The melanin pigment in coloured hairs binds certain substances - including organic bases. PAs are weak bases and so might be expected to be concentrated in pigmented hairs.

Dunnett emphasises that the work is not yet conclusive, and work is continuing.

Source: Equine nutrition conference, Hannover. Oct 2005.

Further reading:
Hair analysis for screening horses for exposure to dietary toxic residues. Mark Dunnett. Proceedings Equine Nutrition Conference: Pferdeheilkunde (2005) 21, 93 -95

Hair analysis for screening horses for exposure to dietary toxic residues. Mark Dunnett. Pferdeheilkunde (2005) 21, 457 - 467.
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