A horse is required to eat quite large quantities of acorns for poisoning to occur; this can occur on poor pasture in early autumn after a high wind. Certain animals develop a craving for green acorns, the main sign being colic. An impaction will be felt on rectal examination and the faeces will be very dry. The colic should be controlled with NSAIDs and liquid paraffin, and isotonic saline given by mouth.

Caused by high concentrations of blue-green algae in the drinking water, the acute form will kill a horse within 1 h. The presenting sign is cyanosis, but in the more chronic case there will be photosensitization and raised liver enzymes. Diagnosis is made by laboratory identification of algae. The treatment is supportive, including the obvious removal of the animal from contaminated water and providing clean water.

Poisoning from arsenic can arise through either an inorganic or an organic source. The source for inorganic poisoning is either rat bait or acaricide, the principal sign being severe dysentery. Diagnosis can be confirmed by analysis of the ingesta, liver or kidney. There is a specific treatment other than obvious supportive therapy, i.e. sodium thiosulfate at the rate of 3 mg/kg daily. However, the mode of administration is contentious: in other species it can be given i/m, but this causes massive swelling in the horse, which may even be life threatening; very slow i/v injection is the method of choice in the equine, as oral administration is not effective. The organic source is pig growth promoters; organic poisoning has a slower onset compared with acute inorganic poisoning, and shows neurological signs. There is no specific treatment.

Horses may become addicted to this fruit, which will then cause diarrhoea. Another feature that will be observed is oedema of the lips and tongue. Treatment is symptomatic only.

Not normally eaten by the horse unless starving, this common plant causes chronic poisoning, evident as muscle weakness and other neurological signs. The specific antidote is vitamin B₁ (thiamine), which is given i/m daily at 1 mg/10 kg.

Blister beetles (Epicauta spp.) swarm on to lucerne during harvesting; these secrete the terpenoid cantharidin, which is very toxic to the horse. Cantharidin causes acute colic with diarrhoea, with shreds of mucosa being seen in the faeces; it also causes haematuria. The horse will die unless stabilized through an isotonic saline drip.

Carbamates are commonly in use as both herbicides and insecticides, and can cause poisoning in horses. The signs and treatment are as for organo-phosphorus poisoning, described below.

Although sometimes included in horse feed, this will cause diarrhoea in the horse.

These insecticides and acaricides are very potent poisons in the horse; they cause nervous signs from overstimulation of the CNS. There are no specific antidotes and supportive treatment is rarely effective.

Copper poisoning in the horse is extremely rare, but may be seen when fed copper-supplemented pig feed. The main sign is jaundice, and if this is evident the outcome is fatal. There is no antidote.

Although this poison could potentially be ingested by the horse when used in the inorganic form as a rodenticide, this is not the normal type of poisoning seen. Many plants such as linseed, flax, wild black cherry, sorghum and Sudan grass contain cyanogenetic glycosides, which can be released by damage to the plants through herbicides or wilting. The main signs are bright red mucous membranes and severe asphyxia convulsions, followed by death; the blood will appear bright red on post-mortem. Tests for hydrocyanic acid can be performed on the stomach contents to confirm the diagnosis. Treatment is specific: 660 mg/kg sodium thiosulfate should be given by mouth hourly until the horse improves.

The Latin name for this plant, which is found in hedgerows in the UK, is Belladonna atropina. The signs of poisoning are of that of atropine overdose, i.e. enlarged pupils and ileus. The specific antidote is neostigmine at 0.01 mg/kg given s/c; ileus should be treated with flunixin i/v and isotonic saline by stomach tube.