Equis ISSN 2398-2977

Toxicity: nitrate / nitrite

Synonym(s): Nitrate/nitrite poisoning, Brown blood disease

Contributor(s): Birgit Puschner, Karyn Bischoff

Introduction

  • Cause: nitrate (NO3) is reduced to Nitrite (NO2) by gastrointestinal microorganisms. Nitrite salts are used in food processing. Water can be contaminated with nitrate and nitrite. Some fertilizers contain nitrate.
  • Signs: methemoglobinemia: chocolate brown blood, cyanosis, abortion, restlessness, trembling, dyspnea, recumbence, death.
  • Diagnosis: access to nitrite or nitrate. Methemoglobinemia, elevated nitrate and nitrite in serum, plasma, or ocular fluid.
  • Treatment: methylene blue.
  • Prognosis: horses are resistant to nitrate toxicosis compared to ruminants because they do not convert nitrate to nitrite efficiently; horses are susceptible to nitrite toxicosis but no documented cases exist. Prognosis for a symptomatic horse is poor.

Pathogenesis

Etiology

  • Nitrate (NO3) is reduced to nitrite (NO2) by gastrointestinal microorganisms. This conversion is poor in horses.
  • Nitrite is an oxidant that leads to methemoglobin formation.

Predisposing factors

General

  • Feed or water related problem.
  • Monogastrics are less sensitive to nitrate than ruminants.
  • Some nitrate production in the cecum and colon can occur.

Specific

  • Feed containing 2% nitrate was not associated with toxicosis in experimental horses.
  • Access to nitrate salts can be associated with toxicosis.

Pathophysiology

  • Nitrate (NO3) is reduced to nitrite (NO2) by gastrointestinal microorganisms:
    • Nitrite causes toxicosis.
    • Microorganisms in wet hay can also reduce nitrate.
  • Nitrite causes oxidation of hemoglobin to methemoglobin.
  • Methemoglobin has reduced oxygen carrying capacity, leading to tissue hypoxia:
    • Clinical signs are present at 10-20% methemoglobin.
    • Death can occur at ≥ 60% methemoglobin.

Timecourse

  • Onset of clinical signs 1.5-20 h after ingestion of nitrate.

Epidemiology

  • High nitrate in feed has occasionally been suspected to cause deaths in horses.
  • Nitrite salts have occasionally been associated with human illness and death.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed Papers

Other sources of information

  • Burwash L, Ralston B & Olson M (2005) Effect of High Nitrate Feed on Mature Idle Horses. In: Proc 19th Symposium of the Equine Science Society. pp 174-179.
  • Bradley W B, Eppson H F & Beath O A (1940) Livestock poisoning by oat hay and other plants containing nitrate. In: Bulletin No. 241 of the University of Wyoming Agricultural Experiment Station. pp 3-20.
  • Thompson L J (online) Overview of Nitrate and Nitrite Poisoning. Merck Veterinary Manual. Website: http://www.merckvetmanual.com/toxicology/nitrate-and-nitrite-poisoning/overview-of-nitrate-and-nitrite-poisoning. Last accessed 20th January 2017.


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