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Equine Factors in Plant Poisoning


    There are physiological and behavioral factors which are very specific to equines that may instigate, or even mitigate, toxic events.  The tolerance for toxins may vary between horses, depending upon factors such as size or weight, age, nutritional condition, and pre-existing health issues or disorders.

  • Physical condition

Poor physical condition is one element that can have a direct effect on an animal’s response to toxins and recovery from poisoning. It is perhaps the single attribute of a horse over which its owner has control, and also the factor that is easiest to modify. An animal’s weight is significant because it affects the amount of toxin the animal can ingest before it becomes a critical amount, since lethal dose is related to percentage of body weight.  Improving an animal’s condition through diet and nutrition often leads to more rapid detoxification, lessening of toxic effects, and damage from toxic compounds.

  • Metabolism and vitamins/minerals

Individual animals vary in their ability to handle inadequate or excessive minerals and vitamins. However, it is accepted that either excessive or deficient levels of these can impact livestock health and production, which could subsequently affect susceptibility to toxic plants.  Vitamins are often catalysts for metabolism, which means that proper amounts are beneficial for the animal’s entire system.  Deficiencies may result in severe illness, while an excess may lead to serious side effects.  Selenium is one mineral that can have deleterious effects if present in disproportionate amounts in equine diets; this will be covered later in the section on toxins.  Logic and common sense dictate that any equine’s health and ability to cope with illness depends on proper diet and nutrition.

  • Age or prior diseases

Other factors that may affect a horse’s individual ability to handle toxins are age and existing or prior health issues.  Age may be an issue with certain toxins.  Ingestion of sensitive fern may be lethal if the animal is quite old, while young animals are far more susceptible to the toxins in locoweeds.  Existing health issues may make the horse more susceptible to toxins and may alter the course of treatment.  The owner and consulting veterinarian must consider these on an individual basis. 

  • Breed

The breed of horse has been named as a risk factor in some studies for colic, which may mean that it also affects other gastric problems including plant poisonings, though this has not yet been thoroughly demonstrated. 

  • Color

The color of a horse has a recognized correlation to susceptibility to some toxins.  In particular, toxin-induced photosensitization is far more likely in animals with light-colored skin or skin patches.

  • Grazing habits

The individual horse’s tolerance of or susceptibility to poisons found in plants may be due in part to its grazing efficiency, which varies between animals.  This factor includes the amount of unwanted plant matter ingested, and may possibly be used to predict which horses are likely to be vulnerable to toxins.

  • Past grazing history and tolerances

There may exist individual immunities within animals that are a result of long-term exposure to certain poisonous plants.  Horses raised on particular forage will continue to accept that forage if it is offered.  As a group, equines are less immune and therefore more highly susceptible to some toxins, including those found in tansy ragwort or stinking willie (Senecio jacobaea), locoweeds, and the nightshade family (Solanum spp.). 

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