Brown Blood Disease (Nitrate and Nitrite Toxicosis) in Horses

Nitrate or nitrite poisoning, also referred to as brown blood disease, is a rare and potentially life-threatening condition in horses caused by excessive consumption of nitrate-containing forages, contaminated waters, and some kinds of fertilizers.

Once ingested, nitrate (NO3) is converted into nitrite (NO2)
Excessive levels of nitrite cause a reduction in oxygen transportation, preventing the tissues from receiving adequate oxygen
Inadequate oxygen in the body leads to weakness, tremors, recumbency, blue membranes, and sudden death
Diagnosis is based on the symptoms, blood tests, and a potential history of exposure
Treatment options include IV administration of methylene blue salts, removal of nitrate sources, and supportive care
Symptoms of brown blood disease rapidly progress; horses often die before treatment attempts are able to take effect
Due to the rapid onset of life-threatening symptoms, prevention is paramount

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A closer look: Brown Blood Disease (Nitrate and Nitrite Toxicosis) in Horses

Nitrate is a naturally occurring compound present in some types of plants as well as fertilizers. Nitrate is converted into nitrite when it is ingested. Excessive levels of nitrate can lead to insufficient oxygen uptake in horses. Oxygen is required continuously by all body systems to maintain life. Sustained low oxygen levels leads to loss of tissue function. In severe cases, nitrate toxicosis causes complete oxygen deprivation (anoxia).

Crops that can contain high levels of nitrate include:

Millet
Oats
Corn
Wild sunflower
Smartweed
Pigweed
Lambsquarter
Sorghum-sudangrass
Russian thistle

Other potential sources of excessive nitrate include:

Fertilizers ingested accidentally
Contaminated water sources

Nitrate/Nitrite toxicosis can present as acute or chronic in horses.

Acute presentation is the most common form of nitrate/nitrite poisoning in horses.

Symptoms include

Decrease in body temperature

Tying Up (Muscle Twitching and Cramping) in Horses

Weakness

Lack of Coordination (Ataxia) in Horses

Brown mucus

Frequent urination

Difficulty Breathing (Dyspnea) in Horses

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Chronic presentation : The symptoms of chronic nitrate toxicosis are not well-defined in horses, however several symptoms are thought to be related to this toxicosis:

Delayed growth
Decreased lactation
Abortion

Risk factors

Nitrate toxicosis is a rare and life-threatening condition in horses.

Nitrate poisoning must be treated as an emergency, as any delay in treatment can lead to death.

Due to the rapid onset of severe symptoms, prevention is of the utmost importance.

Horses living close to fertilized land are at greater risk of developing nitrate poisoning.

Possible causes

Nitrate/nitrite toxicosis is caused by the ingestion of elevated amounts of nitrate (NO3).

NO3 can be found in forage, weeds, fertilizers, animal waste, and contaminated waters.

Once ingested, the nitrate is converted into nitrite (NO2). NO2 is rapidly absorbed into the body and causes formation of methemoglobin, a form of hemoglobin which cannot bind oxygen appropriately. In turn, elevated methemoglobin levels cause a decrease in oxygen levels in the blood.

Main symptoms

Symptoms of nitrate toxicosis generally develop within a few hours after ingestion.

These include

Blue gums

Muddy brown gums

Weakness

Decreased body temperature

Tying Up (Muscle Twitching and Cramping) in Horses Difficulty Breathing (Dyspnea) in Horses

Rapid breathing

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Recumbency in Horses

Rare symptoms due to non-plant sources of nitrate include

Colic in Horses

Gastric hemorrhage

Diarrhea in Horses

Testing and diagnosis

Initial diagnosis of nitrate toxicosis is based on symptoms and a probable history of exposure. Further diagnostic tools include:

Physical examination
Bloodwork
Identification of brown blood during sampling
Methemoglobin analysis
Forage tests: diphenylamine blue test is used to identify the presence of nitrate in feed
Water source tests: nitrate test strips are helpful in estimating the concentration of nitrate in water sources

Due to the rapid progression of life-threatening symptoms, specific diagnostic tests are not always possible.

Steps to Recovery

Once diagnosed, treatment options include:

IV methylene blue: Methylene blue is the definitive treatment for nitrate toxicosis in other species, however may not be effective in horses
Removal of nitrate source
Supportive and symptomatic care

Most cases of nitrate toxicosis carry a poor prognosis. After the onset of symptoms of acute nitrate toxicosis, most animals deteriorate rapidly and die as the result of oxygen deprivation (anoxia).

If treatment is started prior to the onset of severe symptoms, the prognosis is guarded.