Nitrate & Nitrite Poisoning in Horses (Brown Blood Disease): Causes, Risk Factors & Prevention

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

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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
• Colic
• Head pressing
• Blue gums
• Seizures
• Sudden death

Brown Blood Disease

Healthy blood appears red. The more saturated blood is with oxygen, the brighter red it appears. ^[9] In cases of nitrate poisoning, as the blood’s capacity for oxygen decreases, the red color dulls and may appear brown.

This is where the common term “brown blood disease” comes from.

Risk Factors

Horses are most at risk of nitrate exposure from excessive use and/or inappropriate storage of fertilizers. However, several environmental factors can significantly increase the levels of nitrate uptake by plants, which can add to the total quantity ingested by horses feeding on affected pastures.

Understanding these factors can help mitigate the risks associated with nitrate accumulation in plants that could be consumed by horses.

Plants more likely accumulate larger amounts of nitrate when growing under stressful conditions. Stress factors include: ^[10]

• Drought: inadequate precipitation can disrupt normal metabolic processes in plants. This stress often results in higher nitrate accumulation in the plant tissues, as reduced water availability limits the plant’s ability to convert nitrate into amino acids and proteins.
• Extreme temperatures: extreme temperatures, whether high or low, can impair the enzymatic activities involved in nitrate reduction within plants. High temperatures can cause heat stress, while low temperatures can slow down metabolic rates, both leading to increased nitrate levels in plants.
• Insufficient sunlight: adequate sunlight is fundamental for photosynthesis, which provides the energy needed for nitrate reduction. Plants growing in shaded areas or during overcast conditions may not receive sufficient light, leading to a buildup of nitrates as the plants cannot metabolize them efficiently.

Additional risk factors for nitrate poisoning in horses include: ^[10][11]

• Part of the plant ingested: stalks and stems tend to accumulate the most nitrate compared to other parts of the plant. Leaves and seeds generally contain lower levels of nitrate.
• Management and lifestyle: horses that suffer from malnutrition and those living in very cold environments are more likely to consume larger amounts of feed in a shorter period, increasing their nitrate intake
• Health status: foals, pregnant mares, and horses with compromised immune systems or underlying conditions may have a reduced ability to metabolize nitrate efficiently, leading to a higher risk of toxicosis
• Body weight: as with all forms of poisoning, the severity of nitrate toxicity depends on dosage relative to body size. Foals, ponies and miniature horses are at higher risk of ingesting a toxic dose of any poison than larger horses.

By recognizing these risk factors, horse owners and caretakers can take proactive steps to manage the grazing environment and diet of their horses, reducing the risk of nitrate toxicosis.

Nitrates and Blue-Green Algae

Although not directly related to brown blood disease, the overuse and incorrect disposal of fertilizers can lead to toxic algae blooms in bodies of water. If horses drink water contaminated with these algae, it can quickly lead to sudden death. ^[6]

These toxic algae blooms, commonly known as blue-green algae and scientifically as cyanobacteria, occur worldwide in both fresh and brackish bodies of water. In recent years, the prevalence of algae blooms and the number of related animal deaths are increasing due to climate change. ^[6]

Horses and dogs are at particular risk of blue-green algae poisoning as many have outdoor lifestyles where they can interact freely with natural bodies of standing water. Never let horses swim or drink from water that looks stagnant, has an obvious blue-green algae bloom on the surface, or if the water appears turgid (swollen).

Diagnosis

Diagnosis of nitrate poisoning is complex, as blood tests and serum analyses do not reveal any indicative changes other than low oxygen levels. Initial diagnosis is based on clinical presentation and discussion of known or suspected history of exposure. ^[6]

Specific tests can provide a definitive diagnosis, including: ^[6]

• Analysis of ocular fluid
• Feed analysis

Differential Diagnosis

In cases of unknown exposure, medical professionals may be required to rule out other potential conditions that can cause similar symptoms. These include: ^[6]

• Yew poisoning
• Cyanobacteria toxicosis
• Ionophore poisoning
• Fumonisin toxicosis
• Hepatic encephalopathy

Treatment and Prognosis

Brown blood disease in horses usually has a rapid onset and often leads to sudden death. If treatment is possible before life-limiting symptoms set in, the first step is immediate removal of the source of nitrate exposure.

Once the horse is safe from further exposure and stabilized, IV administration of methylene blue is the antidote. Supportive care and hospitalization may be required during recovery. ^[3][6]

Prognosis depends on the severity of poisoning. Many equine cases of nitrate poisoning are catastrophic due to the rapid onset and progression of symptoms followed by sudden death. ^[6]

Prevention

Fortunately, horses have a higher resistance to nitrates compared to other animals and can consume fair amounts of nitrate in forage without developing toxic symptoms.

Most cases of toxicosis in horses are caused by direct ingestion of fertilizers or consumption of recently fertilized crops, both of which are preventable by the following strategies:

• Proper storage: ensuring fertilizers and all other toxic farm chemicals are stored securely and in accordance with safety guidelines is essential. This prevents accidental ingestion by horses and other animals.
• Soil sampling: applying fertilizers at recommended quantities and times and subsequently monitoring the fields can help mitigate the risk of horses accessing recently fertilized areas where nitrate concentrations are higher
• Pasture management: implementing rotational grazing and robust pasture management techniques can help reduce nitrate accumulation in forage plants, lowering the risk of toxicosis
• Water monitoring: regularly testing water sources for nitrate levels helps mitigate the risk of poisoning for free-roaming animals; visually monitoring for signs of blue-green algae and restricting equine access to water hazards is also important

By following these strategies, horse owners and all farm personnel can significantly reduce the risk of nitrate toxicosis in equines, promoting the health and well-being of their animals. On farms that house multiple species of animals, owners and personnel need to pay additional attention, as ruminants are highly susceptible to nitrates.

If you are unsure about the toxicity of your pasture forage, consider submitting a hay sample for a forage analysis and working with an equine nutritionist to ensure your herd is getting everything they need.

Summary

Nitrate and nitrite poisoning in horses results from ingestion of large quantities of nitrates in the environment, usually due to direct consumption of synthetic fertilizer.

• Nitrate poisoning, also called brown blood disease, results from impairment to the blood’s oxygen carrying capacity.
• The main symptoms of nitrate toxicosis in horses include diarrhea, colic, respiratory distress, and sudden death.
• Treatment is based on eliminating further exposure and the administration of methylene blue.
• Prevention involves proper fertilizer storage, monitoring and pasture management.
• Prognosis is dose-dependent and varies from good to poor.