Consumption of nitrate fertilizers or an excess of nitrate-concentrating plant species can cause severe or fatal blood disease. Nitrate toxicity can also take place through intake of contaminated water. It does this through eliminating the oxygen carrying capacity of the red blood cells.

Nitrate toxicity from plants 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.

Plants that can contain high levels of nitrate are common and include Ragweed, Pigweed, Wild Oats, Lamb's-Quarter, Bindweed, Jimsonweed, Barnyard Grass, Sunflowers, Kochia ( Fireweed), Cheeseweed ( Mallow ), Sweet Clover, Smart Weed, Russian Thistle, Nightshades, Goldenrods. Crop Plants include Oats, Sugar Beets, Rape, Soybeans, Flax, Alfalfa, Millet, Rye, Sudan Grass, Wheat and Corn.

For toxicity to take place, 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 usually occur within 6 hours of consumption, and include sudden death, respiratory distress, weakness, colic, diarrhea, muscle tremors, wobbly gait, high heart rate, and a brownish discoloration of the gums . The blood of animals with nitrate toxicity is brown instead of red.

DIAGNOSIS: Laboratories can test plants for nitrate levels, and body tissues and fluids for level of nitrite and methemoglobin. Blood and tissue samples must be frozen within 2 hours to preserve the methemoglobin.

TREATMENT requires methylene blue and supportive care. The goal is to revert methemoglobin back to hemoglobin. Methylene blue can itself cause toxicity in horses, so extreme care must be used.