| Scott Fritz, DVM, ABVT Toxicologist Beef Cattle Institute Kansas State University Scottfritz@vet.k-state.edu |
Sources
- Plants are typically fertilized, and drought-stressed, occasionally herbicide-treated plants can become more palatable and cause problems
- Plants (Johnson grass, corn stalks, Sudan grass), water (especially with fertilizer contamination)
Mechanism
Rumen reduces nitrate to nitrite. Nitrite reduces the iron in heme forming methemoglobin that can’t carry oxygen – the affected animal become anoxic. Clinical signs will occur at 30-40% methemoglobin, death can occur when methemoglobin is >70%.
Signs
Rapid onset (30 minutes to hours) of weakness, bloat, ataxia, recumbency, cyanosis, and death. The rapid progression from consumption to death precludes the formation of any reliable lesions. Pregnant animals that survive the acute disease may abort 3-7 days after exposure.
Treatment
Methylene blue is the traditional antidote administered at 10 mg/kg of a 1% solution. There are withdrawal concerns for the use of methylene blue due to it being a potential carcinogen, FARAD should be consulted. Remove the suspect source.
Diagnosis
- Brown discoloration of venous blood. Serum is a good antemortem sample, ocular fluid is the best post-mortem sample. Rumen content is not a good post-mortem sample as the rumen microbes will continue to degrade nitrate. Methemoglobin rapidly decays after collection so its diagnostic utility is limited.
- 1% nitrate in forage can result in acute deaths, 0.5% nitrate feed should not be fed to pregnant animals. 100 ppm in water can result in clinical signs. Laboratories that do feed and water analyses use different units to report the nitrate content, these units are different and it is imperative to recognize the differences.
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