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Scientists believe that members of the Equisetum genus, composed of scouring rushes and horsetails, have been around for over 540 million years, and may have populated much of the world back then. Today, scouring rushes can be found growing in moist areas, such as in marshes or ditches, near ponds and lakes, or in pastures along a water source. Found in dense colonies consisting of dark green, jointed, hollow stems, scouring rushes can grow waist high. They produce no leaves or flowers, and boast an extensive rhizomatic root system that can reach as deep as 6 feet. Reproduction occurs between March and July, when spores are produced that burrow into the soil, creating new scouring rushes.
Scouring rushes, or Equisetum arvense, has been called many names, such as field horsetail or common horsetail. The toxins in this silica based plant disrupt metabolism and can compromise brain and nerve functions, leading to weight loss, incoordination, and eventually seizures and heart failure.
A poisoning from scouring rushes, called equisetosis, is most often seen in young horses. Consumption over several days or weeks results in a slow progression of symptoms. At first, you may see your horse appear scruffy and show some weight loss. Uncoordinated movements can turn into a loss of muscle control and balance problems, until your horse refuses to get up. Symptoms can include:
Scouring rushes contains many different compounds, including silicates, nicotine, and aconitic acid, but it is the presence of thiaminase that is believed to cause the toxicity when ingested. Thiaminase breaks down thiamine, or Vitamin B1, making it inactive. Thiamine is an essential vitamin that helps the body metabolize carbohydrates, proteins and fats, strengthens the immune system, helps to sustain normal brain function, and is needed to form adenosine triphosphate, which is used by the entire body for energy. Once thiamine has been broken down and cannot be utilized by the body, metabolism and energy extraction is affected, as well as brain and central nervous system functionality.
While horses generally do not eat scouring rushes due to the high silica content, they can ingest them when there isn’t better vegetation to graze, or if it has been mixed into their hay. Scouring rushes can be toxic both fresh and dried. If your horse has been eating hay containing at least 20% of scouring rushes, symptoms are often seen within 1 to 5 weeks.
If you have evidence of your horse eating scouring rushes, diagnosis is based on a correct plant identification, symptoms, and the results of testing. Bring in a sample of the plant to be correctly identified by your veterinarian. If you do not have knowledge of your horse ingesting this plant, your veterinarian will often begin with a physical exam, and then run a series of tests to narrow down a cause.
Bloodwork will reveal a decreased level of thiamine or abnormal erythrocyte transketolase activity, both clear signs of a thiamine deficiency. Other tests that could be run are a urinalysis, serum analysis, and perhaps imaging techniques to analyze the state of the heart, as this condition can lead to enlargement of this essential organ.
Once it is established that your horse is suffering from a thiamine deficiency, you and your veterinarian will seek to identify and remove any plants containing thiaminase from your horse’s reach, including scouring rushes. Be sure to check your horse’s hay for accidental contamination of this toxic plant.
The main method of treatment is the intravenous or intra-muscular administration of thiamine over several days. If the condition was caught early, thiamine injections and the removal of the source of thiaminase generally results in a full recovery.
Recovery is generally good with quick treatment and removal of scouring rushes from your horse’s reach.
To prevent your horse from ingesting scouring rushes, be sure to inspect all hay for any potentially toxic plants that have accidentally been rolled into the bale.
Due to its rooting system and reproduction through spores, scouring rushes is difficult to manage once it is on your property. Drain excessive water to any infested areas, low areas, or ditches, and be sure to wash any equipment used for removal to prevent scouring rushes from spreading to other areas. Herbicides are often ineffective due to the high silica concentrations which prevent absorption. Products containing glyphosate can suppress scouring rushes, but they will usually grow back.
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