Overview of Nitrate and Nitrite Poisoning
Many species are susceptible to nitrate and nitrite poisoning, but cattle are affected most frequently. Ruminants are especially vulnerable because the ruminal flora reduces nitrate to ammonia, with nitrite (~10 times more toxic than nitrate) as an intermediate product. Nitrate reduction (and nitrite production) occurs in the cecum of equids but not to the same extent as in ruminants. Young pigs also have GI microflora capable of reducing nitrate to nitrite, but mature monogastric animals (except equids) are more resistant to nitrate toxicosis because this pathway is age-limited.
Acute intoxication is manifested primarily by methemoglobin formation (nitrite ion in contact with RBCs oxidizes ferrous iron in Hgb to the ferric state, forming stable methemoglobin incapable of oxygen transport) and resultant anoxia. Secondary effects due to vasodilatory action of the nitrite ion on vascular smooth muscle may occur. The nitrite ion may also alter metabolic protein enzymes. Ingested nitrates may directly irritate the GI mucosa and produce abdominal pain and diarrhea.
Although usually acute, the effects of nitrite or nitrate toxicity may be subacute or chronic and are reported to include retarded growth, lowered milk production, vitamin A deficiency, minor transitory goitrogenic effects, abortions and fetotoxicity, and increased susceptibility to infection. Chronic nitrate toxicosis remains a controversial issue and is not as yet well characterized, but most current evidence does not support assertions of lowered milk production in dairy cows due to excessive dietary nitrate exposure alone.
Nitrates and nitrites are used in pickling and curing brines to preserve meats, and in certain machine oils and antirust tablets, gunpowder and explosives, and fertilizers. They may also serve as therapeutic agents for certain noninfectious diseases, eg, cyanide poisoning. Toxicoses occur in unacclimated domestic animals, most commonly from ingestion of plants that contain excess nitrate, especially by hungry animals engorging themselves and taking in an enormous body burden of nitrate. Confounding interactions with nonprotein nitrogen, monensin, and other feed components may exacerbate effects of excessive nitrate content in livestock diets, especially when coupled with management errors.
Nitrate toxicosis can also result from accidental ingestion of fertilizer or other chemicals. Nitrate concentrations may be hazardous in ponds that receive extensive feedlot or fertilizer runoff; these types of nitrate sources may also contaminate shallow, poorly cased wells. Although nitrate concentrations are increasing in groundwater in the USA, well water is rarely the sole cause of excess nitrate exposure.
Water with both high nitrate content and significant coliform contamination has greater potential to adversely affect health and productivity than does either nitrate or bacteria alone. Livestock losses have occurred during cold weather due to the concentrating effect of freezing, which increases nitrate content of remaining water in stock tanks.
Crops that readily concentrate nitrate include cereal grasses (especially oats, millet, and rye), corn (maize), sunflower, and sorghums. Weeds that commonly have high nitrate concentrations are pigweed, lamb’s-quarter, thistle, Jimson weed, fireweed (Kochia), smartweed, dock, and Johnson grass. Anhydrous ammonia and nitrate fertilizers and soils naturally high in nitrogen tend to increase nitrate content in forage.
Excess nitrate in plants is generally associated with damp weather conditions and cool temperatures (55°F [13°C]), although high concentrations are also likely to develop when growth is rapid during hot, humid weather. Drought conditions, particularly if occurring when plants are immature, may leave the vegetation with high nitrate content. Decreased light, cloudy weather, and shading associated with crowding conditions can also cause increased concentrations of nitrates within plants. Well-aerated soil with a low pH, and low or deficient amounts of molybdenum, sulfur, or phosphorus in soil tend to enhance nitrate uptake, whereas soil deficiencies of copper, cobalt, or manganese tend to have opposing effects. Anything that stunts growth increases nitrate accumulation in the lower part of the plant. Phenoxy acid derivative herbicides (eg, 2,4-D and 2,4,5-T), applied to nitrate-accumulating plants during early stages, cause increased growth and a high nitrate residual (10%–30%) in surviving plants, which are lush and eaten with apparent relish even though previously avoided.
Nitrate, which does not selectively accumulate in fruits or grain, is found chiefly in the lower stalk with lesser amounts in the upper stalk and leaves. Nitrate in plants can be converted to nitrite under the proper conditions of moisture, heat, and microbial activity after harvesting.
Signs of nitrite poisoning usually appear suddenly because of tissue hypoxia and low blood pressure as a consequence of vasodilation. Rapid, weak heartbeat with subnormal body temperature, muscular tremors, weakness, and ataxia are early signs of toxicosis when methemoglobinemia reaches 30%–40%. Brown, cyanotic mucous membranes develop rapidly as methemoglobinemia exceeds 50%. Dyspnea, tachypnea, anxiety, and frequent urination are common. Some monogastric animals, usually because of excess nitrate exposure from nonplant sources, exhibit salivation, vomiting, diarrhea, abdominal pain, and gastric hemorrhage. Affected animals may die suddenly without appearing ill, in terminal anoxic convulsions within 1 hr, or after a clinical course of 12–24 hr or longer. Acute lethal toxicoses almost always are due to development of ≥80% methemoglobinemia.
Under certain conditions, adverse effects may not be apparent until animals have been eating nitrate-containing forages for days to weeks. Some animals that develop marked dyspnea recover but then develop interstitial pulmonary emphysema and have continued respiratory distress; most of these recover fully within 10–14 days. Abortion and stillbirths may be seen in some cattle 5–14 days after excessive nitrate/nitrite exposure but likely only in cows that have survived a ≥50% methemoglobinemia for 6–12 hr or longer. Prolonged exposure to excess nitrate coupled with cold stress and inadequate nutrition may lead to the alert downer cow syndrome (see Bovine Secondary Recumbency) in pregnant beef cattle; sudden collapse and death can result.
Blood that contains methemoglobin usually has a chocolate-brown color, although dark red hues may also be seen. There may be pinpoint or larger hemorrhages on serosal surfaces. Hydroperitoneum and ascites have been reported in stillborn calves, as well as edema and hemorrhage in the lungs and digestive system of perinatal calves with excessive maternal nitrate exposure. However, dark brown discoloration evident in moribund or recently dead animals is not pathognomonic, and other methemoglobin inducers must be considered. If necropsy is postponed too long, the brown discoloration may disappear with conversion of methemoglobin back to Hgb.
Excess nitrate exposure can be assessed by laboratory analysis for nitrate in both pre- and postmortem specimens. High nitrate and nitrite values in postmortem specimens may be an incidental finding, indicative only of exposure and not toxicity. Plasma is the preferred premortem specimen, because some plasma protein–bound nitrite could be lost in the clot if serum was collected. Nitrite present in whole blood also continues to react with Hgb in vitro, so these specimens must be centrifuged immediately and plasma separated to prevent erroneous values of both. Additional postmortem specimens from either toxicoses or abortions include ocular fluids, fetal pleural or thoracic fluids, fetal stomach contents, and maternal uterine fluid. All specimens should be frozen in clean plastic or glass containers before submission, except when whole blood is collected for methemoglobin analysis. Because the amount of nitrate in rumen contents is not representative of concentrations in the diet, evaluation of rumen contents is not indicated.
Bacterial contamination of postmortem specimens, especially ocular fluid, is likely to cause conversion of nitrate to nitrite at room temperature or higher; such specimens may have abnormally high nitrite concentrations with reduced to absent nitrate concentrations. Endogenous biosynthesis of nitrate and nitrite by macrophages stimulated by lipopolysaccharide or other bacterial products may also complicate interpretation of analytical findings; this should be considered as a possible maternal or fetal response to an infection.
Methemoglobin analysis alone is not a reliable indicator of excess nitrate or nitrite exposure except in acute toxicosis, because 50% of methemoglobin present will be converted back to Hgb in ~2 hr, and alternative forms of nonoxygenated Hgb that may be formed by reaction with nitrite are not detected by methemoglobin analysis. Nitrate and nitrite concentrations >20 mcg NO3/mL and >0.5 mcg NO2/mL, respectively, in maternal and perinatal serum, plasma, ocular fluid, and other similar biologic fluids are usually indicative of excessive nitrate or nitrite exposure in most domestic animal species; nitrate concentrations of up to 40 mcg NO3/mL have been present in the plasma of healthy calves at birth but are reduced rapidly as normal neonatal renal function eliminates nitrate in the urine. In acutely poisoned ruminant livestock, nitrate and nitrite concentrations as high as 300 mcg NO3/mL and 25–50 mcg NO2/mL, respectively, can be found in plasma or serum, with ~⅓ less in postmortem ocular fluid because of diffusion equilibrium delay. However, postmortem ocular fluid nitrate concentrations are relatively stable and remain diagnostically significant for up to 60 hr after death. Once collected, plasma, serum, and ocular fluid specimens have stable nitrate concentration for at least 1 mo at –20°C.
Normally expected nitrate and nitrite concentrations in similar diagnostic specimens are usually <10 mcg NO3/mL and <0.2 mcg NO2/mL, respectively. Nitrate and nitrite concentrations >10 but <20 mcg NO3/mL and >0.2 but <0.5 mcg NO2/mL, respectively, are suspect and indicate nitrate or nitrite exposure of unknown duration, extent, or origin. The possible contribution of endogenous nitrate or nitrite synthesis by activated macrophages must also be considered. The biologic half-life of nitrate in beef cattle, sheep, and ponies was determined to be 7.7, 4.2, and 4.8 hr, respectively, so it will be at least five biologic half-lives (24–36 hr) before increased nitrate concentrations from excessive nitrate exposure diminish to normally expected values, allowing additional time for valid premortem specimen collection.
A latent period may exist between excessive maternal dietary nitrate exposure and equilibrium in perinatal ocular fluids. Aqueous humor is actively secreted into the anterior chamber at a rate of ~0.1/mL/hr, and nitrate and nitrite are thought to enter the globe of the eye by this mechanism. Equilibrium between aqueous and vitreous humor is by passive diffusion rather than by active secretion, so nitrate or nitrite may be present in comparatively lesser concentrations in vitreous humor after acute exposure.
Field tests for nitrate are presumptive and should be confirmed by standard analytical methods at a qualified laboratory. The diphenylamine blue test (1% in concentrated sulfuric acid) is more suitable to determine the presence or absence of nitrate in suspected forages. Nitrate test strips (dipsticks) are effective in determining nitrate values in water supplies and can be used to evaluate nitrate and nitrite content in serum, plasma, ocular fluid, and urine.
Differential diagnoses include poisonings by cyanide, urea, pesticides, toxic gases (eg, carbon monoxide, hydrogen sulfide), chlorates, aniline dyes, aminophenols, or drugs (eg, sulfonamides, phenacetin, and acetaminophen), as well as infectious or noninfectious diseases (eg, grain overload, hypocalcemia, hypomagnesemia, pulmonary adenomatosis, or emphysema) and any other cause of sudden unexplained deaths.
Slow IV injection of 1% methylene blue in distilled water or isotonic saline should be given at 4–22 mg/kg or more, depending on severity of exposure. Lower dosages may be repeated in 20–30 min if the initial response is not satisfactory. Lower dosages of methylene blue can be used in all species, but only ruminants can safely tolerate higher dosages. If additional exposure or absorption occurs during therapy, re-treating with methylene blue every 6–8 hr should be considered. Rumen lavage with cold water and antibiotics may stop the continuing microbial production of nitrite.
Animals may adapt to higher nitrate content in feeds, especially when grazing summer annuals such as sorghum-Sudan hybrids. Multiple, small feedings help animals adapt. Trace mineral supplements and a balanced diet may help prevent nutritional or metabolic disorders associated with longterm excess dietary nitrate consumption. Feeding grain with high-nitrate forages may reduce nitrite production. However, caution is advised when combining other feed additives/components, including nonprotein nitrogen, ionophores (such as monensin) and other growth and performance enhancers, with high-nitrate diets in ruminants. Proper management, especially regarding acclimation, is critical. Forage nitrate concentrations >1% nitrate dry-weight basis (10,000 ppm NO3) may cause acute toxicoses in unacclimated animals, and forage nitrate concentrations ≤5,000 ppm NO3 (dry-weight basis) are recommended for pregnant beef cows. However, even forage concentrations of 1,000 ppm NO3 dry-weight basis have been lethal to hungry cows engorging themselves in a single feeding within an hour, so the total dose of nitrate ingested is a deciding factor.
High-nitrate forages may also be harvested and stored as ensilage rather than dried hay or green chop; this may reduce the nitrate content in forages by up to 50%. Raising cutter heads of machinery during harvesting operations selectively leaves the more hazardous stalk bases in the field.
Hay appears to be more hazardous than fresh green chop or pasture with similar nitrate content. Heating may assist bacterial conversion of nitrate to nitrite; feeding high-nitrate hay, straw, or fodder that has been damp or wet for several days, or stockpiled, green-chopped forage should be avoided. Large round bales with excess nitrate are especially dangerous if stored uncovered outside; rain or snow can leach and subsequently concentrate most of the total nitrate present into the lower third of these bales.
Water transported in improperly cleaned liquid fertilizer tanks may be extremely high in nitrate. Young, unweaned livestock, especially neonatal pigs, can be more sensitive to nitrate in water.