Brown Blood Disease (Nitrate and Nitrite Toxicosis) in Horses

Published on
Last updated on
5 min read

Key takeaways

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
Are you concerned?

Connect with a vet to get more information about your pet’s health.

Book an online vet

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.

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.

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.


Nitrate toxicosis is not contagious.

Prevention is the best way to ensure that horses do not die of nitrate poisoning. Strategies include:

  • Proper use and storage of fertilizer
  • Not using fertilizer tanks to transport or contain drinking water
  • Testing potential sources of nitrates
  • Not feeding moldy or wet hay
  • Waiting until hay is dried appropriately before baling

Is Brown Blood Disease (Nitrate and Nitrite Toxicosis) in Horses common?

Nitrate toxicosis is a rare condition in horses.

Horses living in close proximity to fertilized land are at greater risk of developing nitrate toxicosis.

Typical Treatment

  • IV methylene blue
  • Removal of nitrate source
  • Supportive and symptomatic care


Bob Judd, DVM, DABVP (Equine Medicine), DABVP (Canine and Feline Practice) - Writing for Veterinary Partner
Larry J. Thompson , DVM, PhD, DABVT - Writing for MSD Veterinary Manual
Equine Disease Quarterly - Writing for The Horse
Karyn Bischoff BS DVM MS DipABVT; Birgit Puschner DipABVT PhD DVM - Writing for Vetlexicon

Our editorial committee

Our medical review team is responsible for validating and maintaining the quality of our medical information.