English
Nitrate poisoning in horses occurs when they ingest excessive amounts of nitrates, found in certain plants and synthetic fertilizers. Nitrates are converted into nitrites in the horse’s digestive system, which then interfere with the blood’s ability to carry oxygen throughout the body.
Nitrate poisoning can lead to brown blood disease with symptoms including diarrhea, colic, rapid pulse, and respiratory distress. This condition is rapidly progressive, leading to sudden death in most cases.
Many forages contain small amounts of nitrates and are safe for horses to graze on. However, horses may be at risk of toxic exposure if there are elevated nitrate levels in the environment. The most common cause of nitrate toxicosis in horses is direct consumption of fertilizers by consuming recently treated crops.
Responsible agricultural practices are key to preventing nitrate poisoning in horses. Measures such as secure storage of fertilizers, careful application practices, rotational grazing, and monitoring water sources are important to keep horses safe.
Nitrate Exposure in Horses
Nitrate is a common form of nitrogen, an essential nutrient that is required by all living organisms. Nitrogen is needed to synthesize various molecules that are fundamental to biological function, particularly amino acids and protein.
Plants need nitrogen to survive because it is a key component of chlorophyll, essential for photosynthesis, in addition to its role in other biological processes.
While all plants require nitrogen, the amount of nitrates in plants differs between species, and the concentration in individual plants varies based on: [1][2]
- Maturity
- Environmental conditions
- Soil composition
- External stress factors
In agriculture, nitrate is frequently used in fertilizers to improve crop health and yield. While synthetic fertilizers are important for ensuring abundant food supply, when used improperly they pose environmental and health hazards to horses, other animals and humans. [3][4][5]
Excessive use, accidental spills, and improper disposal of nitrate containing fertilizers can lead to: [5]
- Water pollution
- Algae blooms
- Aquatic dead zones
These forms of pollution, as well as treated crops themselves, are all possible routes of exposure to high nitrate levels for horses.
Plant Uptake of Nitrogen
The most common form of nitrogen absorbed by plants is nitrate, as it is highly soluble in water and moves easily through the soil to the plant roots. Once absorbed, the nitrogen is metabolized and transported throughout the plant cells, where it is the most abundant mineral, making up 1 – 5% of the plants’ total dry matter. [3][4][6]
Plants adapted to different environments may exhibit preferences for different nitrogen forms based on availability. Several plant species have evolved to be naturally high in nitrates. In the United States, these plants include: [7]
- Oats
- Millet
- Sorghum grass
- Sudan grass
- Corn
- Pigweed
- Lamb’s quarters
In addition to evolutionary differences between plant species, several factors can cause elevated concentrations of nitrates in a variety of plants. These include: [1][2]
- Maturity: young and rapidly growing plants tend to contain higher amounts of nitrates
- Environmental factors: plants accumulate more nitrates when normal growing conditions are disrupted; stressors include drought, hail, frost, and unusually long periods of reduced sunlight
- Soil fertilization: application of synthetic nitrate-containing fertilizers can cause excessive uptake
Toxicology
Horses are less susceptible to nitrate poisoning than ruminants (e.g. cows) due to differences in their digestive system. However, horses can still develop life-threatening nitrate poisoning and brown blood disease if they ingest large enough quantities of nitrates.
Horses can tolerate consuming forages that contain up to 2% nitrates on a dry matter basis. This means that horses are more likely to be develop brown blood disease from other routes of exposure than by consuming plants with naturally high levels of nitrates.
Horses affected by nitrate poisoning are usually exposed due to: [5][6]
- Ingesting fertilizers
- Consuming recently fertilized crops
- Drinking contaminated water
Effects on Horses
Once ingested, nitrate is converted to nitrite in the horse’s body. This minor chemical conversion that precedes toxic damage is why brown blood disease is often medically referred to as “Nitrate and Nitrite Poisoning.”
Nitrite is rapidly absorbed by the horse’s digestive tract where it interacts with red blood cells, converting hemoglobin into methemoglobin. Hemoglobin is a protein that is abundant in the blood and responsible for transporting oxygen to the tissues. [5][8]
Methemoglobin is unable to carry oxygen. As hemoglobin throughout the blood is converted into the non-functional form, oxygen transport throughout the body decreases. Without sufficient oxygen delivery to cells, tissues and organs are unable to function properly.
Symptoms of oxygen deficiency set in once 30 to 40% of the hemoglobin in the blood is converted into methemoglobin, with death occurring when methemoglobin levels exceed 80%. In some cases, mortality can occur when conversion is as low as 50-60%. [5][8]
Additionally, both nitrate and nitrite are vasoactive compounds, meaning they influence the dilation and constriction of blood vessels.
While the mechanism of action is not well understood, current evidence suggests that as the hemoglobin is converting to methemoglobin, the blood vessels relax (vasodilate), lowering blood pressure. [5][8]
Low blood pressure exacerbates the effects of oxygen deprivation, as the rate of blood circulation slows at the same time as the quantity of available oxygen is rapidly declining. [5][8]
Symptoms
Following the ingestion of excessive amounts of nitrates, symptoms of toxicosis tend to occur within six hours of exposure and affect multiple organ systems. Clinical signs in horses can include: [6][7]
- Diarrhea
- Difficulty breathing
- Elevated heart rate
- Weakness
- Muscle tremors
- Incoordination (ataxia)
- Shaking