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What is Arsenic Poisoning ?

Arsenic is a toxic metalloid that can cause poisoning in horses. The poison is typically not found occurring naturally in a stable or pasture where your horse roams. It will usually be present in items used by people; for example, car batteries can leak arsenic. Arsenic toxicity in horses will lead to a severely irritated gastrointestinal tract and can cause unsteadiness, paralysis and death. Arsenic poisoning in horses will usually be due to the horse having access to items that contain arsenic.

A toxic metalloid, arsenic is toxic to horses and can lead to severe symptoms, particularly in the gastrointestinal tract, and can be fatal.

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Symptoms of Arsenic Poisoning in Horses

Arsenic poisoning is typically acute and will significantly impact the gastrointestinal tract and cardiovascular system. Symptoms experienced as a result of arsenic poisoning will depend on the form of arsenic and how much has been consumed. In acute poisoning, general signs include:

  • Straining
  • Diarrhea (often with blood and mucus)
  • Dehydration
  • Abdominal pain
  • Weakness
  • Depression
  • Weak pulse
  • Unsteady gait
  • Convulsions
  • Cardiovascular collapse
  • Death

With the exception of when a large amount of the chemical has been consumed, it will usually take a day for symptoms to become severe and it may take a week or more for arsenic to be fatal. First symptoms may be noticed a few hours to one day after the poison has been ingested. In cases of chronic poisoning, you may see the following:

  • Loss of appetite
  • Weight loss
  • Joint swelling
  • Hair loss
  • Mucous membranes appearing a brick red color
  • Increased weakness as he nears death.

In peracute cases, the majority of animals are found dead. Often there will be some lesions found in the gastrointestinal tract.

Types 

Trivalent arsenicals, also called arsenites, are more soluble, making them more toxic than pentavalent or arsenate compounds. Whether the arsenic is in the organic or inorganic form will impact its toxicity. Inorganic forms tend to be more toxic. Other factors include valence state, solubility, physical state, how pure it is and its rates of absorption (the more rapidly absorbed the more toxic) and excretion. Arsenic compounds can be ranked in the following order as far as toxicity (the first being the highest):

  • Inorganic trivalent compounds
  • Organic trivalent compounds
  • Inorganic pentavalent compounds
  • Elemental arsenic

For most animals, the lethal amount of sodium arsenite is 1-25 mg/kg.

Causes of Arsenic Poisoning in Horses

Arsenic will impact the capillaries in your horse, damaging microvascular integrity, allowing plasma to pass through membranes, blood loss and hypovolemic shock.

Forms of arsenic compounds that are soluble are easily absorbed when ingested. Upon absorption, the majority of the arsenic becomes tied to the red blood cells, distributing to several tissues. The highest levels are found in the liver, kidneys, heart and lungs. In horses experiencing subchronic or chronic exposure, the arsenic will be found accumulated in the skin, hooves, sweat glands and hair. A good portion of the absorbed arsenic will be eliminated in the urine. 

How the toxicosis occurs will be dependent on the particular arsenical compound. In most cases, tissues that have a lot of oxidative enzymes (GI tract, liver, kidneys, lungs, endothelium and epidermis) are more likely to be damaged by arsenic.

Diagnosis of Arsenic Poisoning in Horses

Should you notice concerning symptoms in your horse it is best to contact your veterinarian. Take a good look around the area where your horse has been to note if there are any items that may contain arsenic that your horse may have gotten into. Should any items be of concern, bring them to the attention of your veterinarian as this might help with diagnosis.

After conducting a physical examination of your horse, your veterinarian will consider testing for arsenic in his stomach contents or the liver or kidney tissues. Testing stomach contents will only be helpful within 24-48 hours of ingestion of the substance. In a healthy liver and kidneys, arsenic levels should be greater than 0.1ppm arsenic (wet wt.). In cases of arsenic toxicity, concentrations will be greater than 3ppm (wet wt.). Urine of your horse can also be tested and arsenic levels can be high for a few days after it was consumed.

Treatment of Arsenic Poisoning in Horses

Treatment of arsenic toxicity will involve removing any arsenic from your horse’s stomach that has not yet been absorbed. This can be done through an oral purging with sodium thiosulphate. Activated charcoal may be an option and used in an attempt to bind the toxin. In addition, your veterinarian will seek to provide supportive fluid therapy (to restore blood volume and avoid dehydration), antibiotics and anti-inflammatory medication. Dimercaprol is recommended at 4-7 mg/kg, IM, for 2-3 days or until your horse has recovered. Thioctic acid can be used on its own or in conjunction with dimercaprol. If necessary, a blood transfusion can be conducted. Throughout treatment, kidney and liver function of your horse should be monitored.

Recovery of Arsenic Poisoning in Horses

Should your horse experience arsenic poisoning, you will want to work closely with your veterinarian to help him in his recovery. It is likely that follow up appointments will be necessary to ensure the best outcome for your horse. In addition, you will want to search the areas where your horse spends time to remove anything that can lead to further poisoning.