Special Veterinary Pathology

NITRATES AND NITRITES

• Nitrates and Nitrites are closely linked as cause of poisoning in animals, however, it is difficult to differentiate between these poisonings as nitrates can be converted into nitrities and both the forms are toxic.
• Nitrates are relatively non-toxic as ruminants convert nitrate of the plants into nitrite in the rumen and then to ammonia by the action of ruminal and intestinal microbes.
• Ammonia so produced is utilized as a source of nitrogen by the rumen microflora for protein synthesis.
• However, if the rate of reduction of nitrate to nitrite exceeds  that of nitrite to ammonia, then excessive nitrite accumulates in the rumen and gets absorbed into the blood stream and produces toxicity as shown below.

Blood toxicity

• Reduction of nitrates depends on the supply of fermentable energy as carbohydrates supply hydrogen ions for reduction of nitrates and production of nitrite in the rumen.
• Poor feeding probably reduces the biotransforming capacity of ruminal microbes and thus increases the production of nitrite ions.
• However, the rate of reduction of nitrates in the digestive tract of horses is comparatively limited.
• Ruminants are most commonly affected, though human beings, horses, pigs and dogs are also poisoned.
• Pigs are most susceptible to nitrite poisoning.
• Non ruminants are susceptible to nitrite but not nitrate poisoning due to absence of rumen microflora.
• Toxicity of nitrates and nitrites not only depends on the dose but also the time e.g. single higher does is more toxic compared to the same or even higher does administered in divided does over a period of time say 24hrs.
• Toxic does are difficult to quantify precisely as toxicity varies depending on variation in susceptibility, production of nitrite from nitrate and food deprivation.
• Food deprivation increase the overall toxicity while concentrate rich or cereal based feeds reduce it.

Source of poisoning

• Main source of poisoning in animals, particularly ruminants is excessive ingestion of certain nitrates containing plants.
• More that 80 specific nitrate containing plants are known to cause nitrite poisoning.
• Roots and stems usually contain more nitrate that leaves.
• Nitrates are present in high concentrations in soil, ground after from deep wells, plants, animal excreta and silage etc. Plants / crops grown on nitrate rich soil and / or water accumulate higher concentrations of nitrates.
• Use of chemical fertilizers like ammonium and potassium nitrates, weedicide/herbicides (2,4 D), excessive rain, insufficient light, excessive and persistent frost and drought conditions, low soil pH, low soil temperature and aeration, deficiency of molybdenum, sulphur, phosphorus, iron etc. in soil and viral infection of plants etc. favour accumulation of higher concentration of nitrates in plants.
• In addition to plant sources, ingestion of nitrate fertilizers (ammonium or potassium nitrate), consumption of nitrate rich water from shallow surface well and ponds, animal wastes, sewage, preserved food left at room temperature for long time, silo juices, industrial effluents and lime stone deposits etc. may also result in nitrate/nitrite toxicity.
• Animals receiving roughages alone are more susceptible to toxicity compared to those on concentrate feed rich in carbohydrates.
• Young animals are more susceptible to poisoning compared to adults.
• Hypovitaminosis A, pre-existing anaemia, methaemoglobinemia and fasting aggravate the toxicity of nitrates and nitrites.
• Monensin facilitates the conversion of nitrate to nitrite in the rumen and may result in poisoning in cattle or sheep on high nitrate fodder.

Clinical signs

• Onset of acute nitrite poisoning symptoms is abrupt and characterized by dyspnoea with violent respiratory effects or gasping.
• Rapid respiration is the predominant sign.
• Salivation, voiding of colorless urine, vomition, diarrhoea, colic, rapid and weak pulse, accelerated heart rate, progressive development of cyanosis, muscular weakness, ataxia, recumbence, terminal anoxic convulsions and death within 2-24hrs. depending on the content of nitrite consumed.
• Bluish discoloration of the mucous membranes and unpigmented areas of the body and chocolate brown colour of blood are quite characteristic of nitrite poisoning, however, there is no change in the coagulation time.
• In pregnant animals it may induce abortion as nitrite ions cross the placental barrier and convert foetal-Hb to Met-Hb.
• Chronic nitrite poisoning is although rare, but sometimes observed in poorly nourished animals drinking high nitrate content water contaminated with chemicals and organic fertilizers, domestic waste waters, industrial effluents etc. or regular and frequent ingestion of plants or fodder rich in nitrates (e.g. rape seed, cabbage, brassica, rye grass) or of hay contaminated with wild plants (e.g. Amaranthus, Chenopodium or fat hen).
• Characteristic clinical features of chronic nitrite poisoning are abortion, poor development of mammary tissue, infertility in adults and lower birth weight in newly born lambs or calves if dam is exposed to nitrite during pregnancy.
• Ewes show hypothyroidism and hypovitaminosis A.
• Other common signs are ataxia, tremors, shaking, immunosuppression, increase in the incidence of mastitis, metritis and diarrhoea in calves and piglets etc.

Post mortem lesions

• Dark chocolate brown or coffee brown colour of blood, it clots poorly.
• Brown stained tissues.
• Congestion and inflammation of intra-abdominal organs.
• Petechial and large haemorrhages on the serous surfaces.
• Generalized cyanosis.
• Blood stained pericardial fluid.

Diagnosis

• History
• Clinical signs
• Chocolate brown colour of blood and tissue
• Post mortem findings if there is mortality.
• Analysis of the stomach/ruminal and intestinal contents for nitrate/nitrite detection
• Detection of nitrite in the urine
• Estimation of meth-Hb and serum nitrite levels (serum positive diphenylamine spot test; 20 µg/ml nitrate). In dead animals, positive diphenylamine spot test with ocular fluid indicating >50 µg/ml nitrate for confirmation of diagnosis.
• Response to methylene blue treatment.