Consumption of nitrate fertilizers or an excess of nitrate-concentrating plant species can cause severe or fatal blood disease. It does this through eliminating the oxygen carrying capacity of the red blood cells.
Nitrate toxicity is much more common in ruminants (cows, sheep and goats) than it is in horses. In ruminants, the rumen bacteria efficiently convert less toxic nitrate to very toxic nitrite. Unlike ruminants, horses are hindgut fermenters and convert less of the nitrate to nitrite.
Nitrite is absorbed into the blood, where it converts hemoglobin (the oxygen carrying molecule in red blood cells) to a form that cannot carry oxygen (methemoglobin). This starves the tissues of oxygen, resulting in cell death.
Signs of nitrate toxicity include respiratory distress, weakness, colic, diarrhea, muscle tremors, wobbly gait, high heart rate, brownish discoloration of the gums, seizures and/or sudden death. The blood of animals with nitrate toxicity is brown instead of red.
Laboratories can test plants for nitrate levels, and body tissues and fluids for level of nitrite and methemoglobin.
Treatment requires methylene blue and supportive care. The goal is to revert methemoglobin back to hemoglobin.
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Prognosis & Relevant Factors
The prognosis is guarded to poor, unless rapid emergency care can be delivered. Nitrate toxicity from ingestion of plants occurs rarely in horses. Fertilizer spills are more common causes of poisoning.
Water with high nitrates can be a source of poisoning.
High nitrate hay baled wet and stored allows bacterial conversion of nitrate to nitrite. This can be a source of poisoning in horses.
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