Horse owners warned to scout pastures for toxic Alsike clover

Crusts on the light-coloured skin on the muzzle of a horse show the effects of Alsike clover toxicity. |  Jamie Rothenburger photo

Unlike their counterparts in the southern United States, Canadian horse owners have relatively few toxic pasture plants to contend with.

One is Alsike clover.

The plant is thought to have originated in Sweden and was introduced to Canada as a forage plant in the 1800s. Since it thrives in cold climates, it is particularly common in northern Canadian pastures.

The plant ranges from 38-75 centimetres in height and features a small, light-pink flower. The toxin produced by the plant remains unknown, although some scientists have speculated that a mycotoxin produced from fungi on the clover may be responsible for causing disease in horses.

There are two main conditions associated with alsike clover in horses. The first is photosensitization.

Light-coloured skin, particularly on the face and muzzle becomes red and inflamed. Next, the outer layers die, forming unsightly and painful crusts. Skin can also swell or ooze yellow, crusting serum. Dark skin, black, brown or red areas, are unaffected. Horses may also have sores in their mouths and experience digestive upset in the form of colic and diarrhea. Horses can recover from this form and require supportive care.

This includes providing feed that is free of the clover and removing them from direct sunlight. Infected skin sores may require additional attention should they develop.

The second condition associated with Alsike clover arises from chronic exposure to the clover over the long term and is generally fatal. In these cases, the exposure to the clover over at least one year repeatedly damages the liver enough to cause signs of liver disease. Over time, the liver becomes enlarged or shrinks. There is scar tissue formation, creating nodules, compromising critical functions and ultimately culminating in liver cirrhosis. This is similar to the chronic liver damage seen in alcoholic people.

Affected horses lose weight and their appetite, followed by neurological conditions including excitement, depression, head-pressing and lack of co-ordination. Some horses may also become jaundiced, run a fever and produce dark urine.

The condition worsens and leads to coma and death in most cases. There is no specific treatment of the chronic condition so preventing consumption of the clover is important.

Diagnosis of Alsike clover toxicity is based on consistent signs of disease and identifying the clover in pasture or hay. There are other toxic plants and some medications that can lead to similar-looking photosensitization, so these need to be ruled out.

Dr. Frank Schofield of the Ontario Veterinary College has conducted the only experimental studies linking Alsike clover poisoning to chronic liver disease in the 1920s and 1930s. During these years, he fed horses hay with Alsike clover. However, his results did not definitively prove that it causes liver disease and the connection remains unproven.

Alsike clover poisoning appears to be more of an issue in northern areas. This is supported by a study of cases in Alberta between 1973 and 1988 by Dr. Nick Nation.

Of the 28,752 horses that were submitted for autopsy examination to the Alberta Agriculture laboratories, 49 were diagnosed with the condition and 40 originated from the Peace River area of northwestern Alberta. This is an area where Alsike clover is particularly common.

One of Dr. Schofield’s key observations was that horses would prefer to eat other plants if possible and that many horses avoid the clover all together. This may explain why only some horses on a given pasture with Alsike clover develop disease.

It also supports the recommendation that Alsike clover should make up five percent or less of pastures so that horses have plenty of other forage plants to eat.

Dr. Jamie Rothenburger is a veterinarian who practices pathology and a PhD student at the Ontario Veterinary College. Twitter: @JRothenburger

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