Toxicology for the Equine Practitioner

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Key points

  • The identification of gross lesions produced by toxins by equine practitioners is often the first step in formulating a diagnostic plan.

  • Two structural toxicoses of the nervous system in horse which produce gross lesions are equine nigropallidal encephalomalacia (ENPE) and equine leukoencephalomalacia (ELM).

  • The most common toxin responsible for liver failure in horses is pyrrolizidine alkaloid (PA).

  • Red maple toxicosis and chronic sorghum poisoning produce detectable gross lesions within the

Toxins affecting the nervous system

Toxins affecting the nervous system can be categorized into two main groups based on mechanism of toxicoses: structural toxicoses and functional toxicoses. Toxins affecting the structure of the nervous system may harm one or more of three components (neurons, axons, or myelin) resulting in a neuropathy, axonopathy, or myelinopathy, respectively. Structural toxicoses commonly result in detectable lesions at the gross and/or microscopic level, which facilitate their diagnosis. However, functional

Toxins affecting the liver and biliary system

The liver is highly susceptible to toxins because of its role in excretion of xenobiotics. Toxins taken orally reach the liver through the portal system and are metabolized by a group of enzymes known as mixed-function oxidases. These enzymes usually increase the water solubility of metabolized compounds but occasionally produce more toxic metabolites. Depending on the dose and the duration of exposure the toxic injury results in hepatic necrosis, fibrosis, bile duct proliferation,

Toxins affecting the urinary system

The kidney is a primary target for toxicity because of its high perfusion. Toxins affecting the kidney can be categorized into two groups: exogenous toxins and endogenous toxins. Exogenous toxins include a long list of chemicals, drugs, plants, and metals, whereas endogenous toxins include hemoglobin myoglobin and bilirubin. Both endogenous and exogenous toxins cause tubular necrosis with proximal convoluted tubules being most susceptible. With the exception of hemoglobinuric nephrosis, acute

Toxins affecting the cardiovascular system

Drugs, chemicals, and plants can produce toxic effects on the heart. Common equine myocardial toxicoses in North America include ionophore antimicrobials (eg, monensin, lasalocid), blister beetles (discussed later), many plants, and rarely mycotoxins. Most plant toxicoses affecting the equine myocardium result in sudden onset of clinical signs and fall into the alkaloid and glycoside groups. An alkaloid is a nitrogen base containing organic compound that has a pharmacologic effect in animals

Bister Beetle (Cantharidin) Toxicosis

The Meloidae family of blister beetles contains more than 200 species of beetles within North America, which contain the toxin cantharidin at different levels. Beetles are often killed and trapped within hay during harvesting.40 Because beetles swarm for mating purposes, large numbers can be trapped with a single flake of hay. Cantharidin is produced by mature males and is passed to females during copulation, which later incorporates it into eggs to deter feeding by other insects. The minimum

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