Nitrate/nitrite poisoning in animals is caused by ingestion of excessive amounts of nitrate or nitrite from forages or weeds, nitrate-containing fertilisers, or contaminated water.
Ingestion of large amounts of nitrate can cause gastrointestinal irritation, colic, and diarrhoea, but the most important consequence is the conversion of nitrate to the more toxic nitrite anion by gastrointestinal microorganisms.
Nitrite is absorbed from the gastrointestinal tract into the blood, causing injury to red blood cells and resulting in the inability of red cells to carry oxygen.
Clinical signs may include difficulty breathing, weakness, tremors, ataxia, rapid heartbeat, grey/blue or brown discolouration of blood and tissues, seizures, and rapid death.
Abortion can occur in animals that survive the initial clinical signs.
While nitrate/nitrite poisoning can occur in any species, ruminants are most susceptible, due to efficient conversion of nitrate to nitrite in the rumen.
Non-ruminant species such as horses are much less commonly affected because they do not readily convert nitrate to the more toxic nitrite.
Conversion of nitrate to nitrite occurs primarily in the large bowel in horses and is roughly one-fourth as efficient as ruminal conversion in cattle.
Hence, compared to ruminants, a much larger dosage of nitrate is required to cause clinical signs in horses. However, horses are very sensitive to nitrite.
Ingestion of nitrite can occur when nitrates in forages or water have been converted to nitrite by environmental microbes prior to ingestion.
Documented cases of nitrate poisoning in horses are rare. Most cases involve ingestion of nitrate/nitrite-contaminated water, nitrate fertilizer directly, or forage or hay grown in the area of a previous fertiliser spill.
A few cases in horses have occurred from ingestion of high-nitrate hay that was baled wet or became wet after baling. Nitrate was converted to nitrite by microorganisms in the hay, resulting in direct nitrite ingestion.
Only a few experimental studies have been published documenting effects of administration of high concentrations of nitrate to horses. No studies have been published that determine the amount of nitrate that horses can safely tolerate.
However, studies suggest that horses, including pregnant mares, can tolerate considerably more dietary nitrate than can cattle.
Chronic exposure to lower levels of nitrate has not been well researched in horses. Associations between chronic nitrate exposure and infertility, poor growth, hypothyroidism, and other disorders have been claimed, but none have been experimentally reproduced in horses, and much work remains to be done.
A small amount of nitrate is normally found in all animals, including horses, as nitrate is a normal component of the plants they eat. Many factors can increase the risk of excessive nitrate accumulation in plants, including species of plant, stage of growth, fertilisation practices, plant stress (drought, frost, hail, herbicide use), and many other factors.
Nitrate accumulates primarily in plant stalks, less in leaves, and not in grains or fruits.
Many important crop plants can accumulate nitrates, including oat plants, sorghum/sudan, and alfalfa (lucerne). Nitrate-accumulating weeds include ragweeds, pigweed, and Johnson grass, to name just a few.
Although high-nitrate forages and weeds pose significant risks to ruminants, horses are rarely poisoned by these plants unless they have been grown on sites of previous fertiliser spills or nitrates have been converted to nitrite by environmental microbes.
Treatment of affected animals is possible, but timing is critical, as animals can die very quickly. Prevention is key, and for horses includes the following: