Animals most affected
Cattle, sheep, (all animals)
Ingestion, over a prolonged period, of water
which has been badly polluted by nitrates or nitrites (contaminated by chemicals
and organic fertilizers, domestic waste waters, industrial effluents (e.g. food processing
units, dairy factories), etc.).
Regular and frequent ingestion of plants
or fodder rich in nitrates (e.g. rape seed cabbage or other brassica, rye grass)
or of hay contaminated with wild plants (amaranthus, chenopodium or fat hen), which
all have a high nitrate content. Note: the manuring of soils significandy increases the nitrate
levels in plants.
Toxic doses depend upon the nature of the diet.
In general, oral intake of nitrates:
0.9-1.8% of the diet
However, if the diet is balanced with the levels of nitrates not exceeding 3% in
hay or 6% in feed concentrates,
toxicity does not develop.
9(l00 mg/l in drinking water)
recommended levels of nitrate in drinking
water: 20 mg/l;
maximum levels permissible in drinking water:
The following have been attributed to chronic nitrate poisoning.
Effects on the reproductive system
abortion, failure of the mammary tissue to prepare for lactation; where pregnancy
is maintained, no vaginal discharge and delivery is normal; later, fecundity
returns to normal; (the abortions caused by ingestion of vegetation rich in nitrates,
particularly in humid areas, are referred to as 'wetlands abortions');
infertility: levels of 70-3000 ppm of nitrates have been shown experimentally
to cause a reduction in fertility, low fecundity is exacerbated further when
the levels of nitrate in the diet are increased (shown by a reduced birth rate
without alteration to the sexual cycle).
Effects on the newborn
There is a lowering of birth weight (calves,
lambs) when the levels of nitrate in the maternal diet are increased.
Effects upon the endocrine system
thyroid: in ewes there is evidence of
'ivt)othyroidism, and a reduction in secretion of the thyroid hormones.
Effect on vitamin A
The oxidizing action of nitrates and
nitrites causes the destruction of vitamin A and carotenes in hay and silage
(especially where late level is high: 2%). In addition, nitrates and nitrites
inhibit the transformation of provitamin A to the active vitamin in the gut.
Other clinical effects have been described
following prolonged ingestion of nitrates:
neurological effects: ataxia, tremor,
immunodepression (reported by several
increase in the incidence of mastitis, metritis and diarrhoea in calves and in
transformation of nitrites into nitrosamines (however in domestic animals and
pets, this toxicological feature is not of great significance).