Poisoning by ingestion of excess urea (a nitrogen compound) or other sources of nonprotein nitrogen is usually sudden, rapidly progressive, and highly deadly. After ingestion, nonprotein nitrogen undergoes a chemical reaction and releases excess ammonia into the gastrointestinal tract, which is absorbed and leads to excess ammonia in the blood.
The most common sources of nonprotein nitrogen in feeds are urea, urea phosphate, anhydrous ammonia, and salts such as monoammonium and diammonium phosphate. Because feed-grade urea is unstable, it is formulated (usually pelleted) to prevent degradation to ammonia. Horses are sometimes fed nonprotein nitrogen as a feed additive. Horses are more sensitive to urea than some other animals. Ammonium salts can be toxic and deadly to all species and ages of farm animals.
Diets low in energy and high in fiber are more commonly associated with nonprotein nitrogen toxicosis. Highly tasty supplements (such as liquid molasses) or improperly maintained lick tanks may lead to consumption of lethal amounts of nonprotein nitrogen. Early signs include muscle tremors (especially of face and ears), protrusion of eyeballs, abdominal pain, drooling, passing large amounts of urine, and grinding the teeth. Tremors progress to lack of coordination and weakness. Fluid in the lungs leads to difficulty breathing and gasping. Horses may exhibit head pressing. Eventually, there is a bluish tinge to the skin and mucous membranes, difficulty breathing, an absence of urine output, fever, and metabolic abnormalities. Death usually occurs within 3 to 12 hours in horses. Survivors recover in 12 to 24 hours with no lasting effects.
Ammonia or nitrogen poisoning is suggested by the history, signs, and dietary exposure. Blood and urine can be tested for ammonia nitrogen, and feed can be analyzed for nonprotein nitrogen. Definitive test results will probably not be possible in animals that have been dead more than a few hours in hot temperatures or 12 hours in moderate climates.
Examination and treatment may be difficult because of uncooperative behavior. Supportive treatment includes controlling fever, correcting dehydration with fluids, and calcium gluconate and magnesium solutions for muscle tremors.