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Nitrates and Nitrites

(588) If these substances are absorbed from the small intestine in sufficient quantities, nitrite reacts with haemoglobin in the blood to form met-haemoglobin which reduces its oxygen carrying capacity. Poisoning occurs when nitrates and ammonia in slurry and straw are converted to nitrites by bacteria, or if the pig drinks drainage water that as become heavily contaminated. Nitrate itself is only slightly toxic, until it is converted to nitrite. Silage effluent is a particularly heavy source of nitrites.

Clinical signs

These are related to a shortage of oxygen in the blood stream. There is a marked increase in the respiratory rate with animals showing a staggering gait and general weakness. The mucous membranes become dark red, almost blue. There may be sufficient irritation of the digestive tract to cause a gastro-enteritis. Clinical signs of poisoning will develop when 10 to 20mg of nitrites per kg liveweight are eaten. Mortality becomes very high above the latter level.


Sudden prostration of groups of pigs or a pen of pigs, that have had access to potential sources of nitrates and nitrites, including whey and milk bye products, must always raise a suspicion. Post-mortem examinations show that the blood and musculature are a dark brown colour due to the formation of met-haemoglobin. This is a diagnostic feature.


  • Intravenous injections of 10mg/kg methylene blue. Consult your veterinarian.

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