Nitrate Poisoning Case Studies

Aetiology

Nitrate poisoning is caused by the intake of excess nitrates (NO3¯) through feed, water, fertiliser or other chemicals. The excess nitrates accumulate in the animal and is broken down to nitrite which is 10 times more toxic than nitrate.

Epidemiology

Nitrate poisoning occurs around the world. Spring is the most common season for nitrate poisoning. Ruminants can tolerate a wide range of nitrates, but are more susceptible to nitrate poisoning because rumen microbes reduces nitrate to ammonia with nitrite as an intermediate product. Hungry animals may engorge feed or plants with excess nitrates and take in an enormous amount of nitrates. Unhealthy animals for example anaemic cattle, due to lice or stomach worm infestations, are more

Under normal circumstances (where the nitrate intake is spread over a whole feeding day and the diet is high in readily available carbohydrates) nitrite is further converted to ammonia which is used by rumen microbes to produce protein. When excess nitrate are consumed over a short period, the nitrite accumulates in the rumen and is absorbed into the bloodstream. The nitrite then oxidises the iron in haemoglobin from the ferrous to the ferric state and thereby changes oxyhaemoglobin to methaemoglobin. Methaemoglobin is unable to carry sufficient amounts of oxygen and this leads to tissue anoxia. Nitrite also causes dilatation of blood vessels and a drop in blood pressure. Nitrate and nitrite may also directly irritate the mucosa of the gastrointestinal tract and may sometimes lead to abdominal pain and diarrhoea. Whether the disease will follow an acute, subacute or chronic course, depends on a number of factors: the amount of nitrate in the feed or water, the amount and speed of ingestion of nitrate and the rate of conversion of nitrate to nitrite in the rumen. Acute nitrate poisoning occurs frequently in hungry animals consuming large quantities of feed or water containing nitrate in a relatively short time. Subclinical or chronic nitrate poisoning occurs in animals taking in sublethal amounts of nitrate over a prolonged period and results in poor growth rate, infertility, vitamin A

If these feeds have a high level of nitrate, add non-nitrate sources of feed (which is also analysed) to the ration. A chemical analysis of the water supplies is also recommended and any adjustments to the ration must take into account the nitrate contribution of the water. By ensiling or allowing high nitrate pastures to set seed, the nitrate levels will be reduced by approximately 40 to 60%. The cutter heads of machinery can be raised during harvesting to leave the more hazardous stalk bases on the field. A high grain diet fed with a high nitrate forage has a protective effect in ruminants because the carbohydrates enhance the conversion of nitrate to ammonia, which is used by ruminal bacteria to produce microbial protein, and nitrite production is thus reduced. Feed additives such as non-protein nitrogen, ionophores and growth and performance enhancers must be combined cautiously with diets of ruminants containing high amounts of