by: Jenny Pope, DVM,
Dipl. ACVP

Both nitrate/nitrite and ammonia/urea toxicity in cattle can cause multiple deaths in a herd with few clinical signs and few to no gross or microscopic lesions of animals who die. These deaths can be frustrating for farmers and veterinarians and can present a diagnostic challenge to pathologists.

Nitrate/nitrite toxicity can oc-cur from ingestion plants/hay that have accumulated nitrates. Some plants are more likely to concentrate nitrate and include weeds like pigweed, dock, lamb's quarter, and Johnson grass or crops like cereal grasses (oats, wheat, barley, rye), com, or sorghum. Plant stress from drought or lack of sunlight, age of plants (younger), and excess use of fertilizer can all increase nitrate accumulation within the plant. Toxicity can also occur due to ingestion of water contaminated with fertilizer or ingestion of fertilizer. Most cases we have seen at the lab have been during winter due to high nitrate concentrations in hay.

Nitrate is converted to nitrite in the rumen and absorbed in the blood causing methemoglobinemia, which re-duces the blood's ability to supply enough oxygen to the body tis-sues. Clinical signs/symptoms include weakness, depression, muscle tremors, and blue or brown discoloration of mucus membranes. Calf loss through abortions and/or stillbirths can also occur. Chocolate- colored blood is characteristic, but not always present at necropsy. Hemorrhages can be observed in dead animals. Testing on live animals can be done on blood. Testing on dead animals can be done on ocular fluid (aqueous humor). Screening for high nitrate levels can be done on hay through the state diagnostic lab and quantification can be done through Auburn University's Soil, Forage, & Water Testing Lab.

Ammonia/urea toxicity occurs most commonly due to improper feeding of non-protein nitrogen (NPN). Improper feeding can be due to inadequate mixing, feeding too much NPN, lack of acclimation by the animals to the feed supplement, free choice feeding to hungry cows, intermittent feeding of NPN, or even inaccurate levels on the supplement block label. Last year at the lab, we saw multiple cases of ammonia toxicity due to improperly labeled NPN containing feed/protein blocks, with up to twice the labeled con-centration of NPN. At the lab, we have also seen cases of ammonia toxicity related to feeding chicken litter in combination with an inadequate diet/ low carbohydrate containing diet. This is an extremely uncommon cause and is not expected when chicken litter is included in a properly balanced diet.

Bacteria in the cow's rumen convert NPN to ammonia, which would normally be used by these bacteria to build microbial/ bacterial protein. In the case of toxicity, excessive ammonia is absorbed into the blood. Normally, the liver can detox absorbed ammonia, but when overwhelmed, too much ammonia in the blood can result in toxicity. Clinical signs/symptoms include tremors, excessive salivation/urination, teeth grinding, rumen atony, un-coordinated gait, convulsions, and downer cows. Bovine Bonker's Syndrome, or cows with clinical signs of hyper-excitability and aggressiveness when stimulated, can also occur with improper feeding of NPN because of the formation of the toxin 4-methylirnidazole. Findings at necropsy can include pulmonary edema, bloat, and a high rumen pH (>7.5). Testing on live animals can be done on blood for ammonia levels. Testing on dead animals can be done on ocular fluid (aqueous or vitreous humor, preferably frozen or rapidly tested after collection). Ammonia levels in blood become elevated after death or with improper storage so testing on blood is not recommended in dead animals and rapid testing after collection in live animals is recommended. Testing for levels of NPN in blocks/supplements can be done through the Alabama Department of Agriculture and Industries Agriculture Compliance Division.

Death due to toxicity from any cause not just nitrate/nitrite or ammonia/urea should be suspected in cases with multiple, closely timed, deaths in the herd. Clinical history is important in toxicity cases and may be associated with introduction of new feed, hay, or minerals (either never previously fed or a new batch), other diet changes, or pasture changes (either new pasture or change in season, etc.). The most important thing to prevent additional losses or additional sick cattle in the herd is to remove any newly in-troduced feeds or to remove cattle from the current pasture until a source can be identified. Samples to submit to the lab from dead cattle for toxicology testing are liver, kidney, rumen contents, ocular fluid (aqueous and vitreous humor), and hay/feed.

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