Nitrate toxicity in a herd of cattle
By Will Sims, DVM, MS
A producer reported a sudden death loss of six head of cattle. Two of the animals were submitted to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) for necropsy to determine the cause of sudden death. Upon arrival at TVMDL, the owner indicated there had been a history of pneumonia and some of the animals had undergone treatment. He also indicated the animals had been grazing on irrigated wheat pasture for approximately one month. Additionally, the producer reported there was a system to add fertilizer into the irrigation for the wheat pasture, but the pasture had not been fertilized, even though it had been irrigated recently.
At necropsy, blood was noted as being dark brown, rather than red as is typically presented in recently deceased animals. This change is characteristic of methemoglobinemia. This coupled with the cattle’s history of being on an irrigated wheat pasture with a fertilizer system, elevated nitrate toxicity to the top of the differential list. Furthermore, nitrate toxicity would fit with the history of a sudden death loss rather than pneumonia, which would likely not cause a sudden death loss without any prior clinical signs in some of the animals.
TVMDL has a rapid test for nitratesthat can be performed on ocular fluid, urine, serum and water. In this case, ocular fluid and serum were sent to the toxicology lab for testing. Both were strongly positive. This coupled with gross necropsy findings provided a rapid diagnosis of nitrate toxicity. It was later learned that the check valve that prevents the fertilizer from entering into the irrigation system was faulty. This allowed the cattle to be unintentionally exposed to high levels of nitrates from the fertilizer.
Sources of nitrate include numerous plants known to concentrate nitrate, contaminated forages, contaminated water, and direct exposure to fertilizers. Acute nitrate poisoning is most likely to occur when forage nitrates exceed 10,000 ppm (dry weight bases) or when water contains 1500 ppm or more. Clinical signs include gastrointestinal irritation, dyspnea, ataxia, terminal convulsions, death 6 to 24 hours after exposure, and abortion. Diagnosis may be made with appropriate clinical signs and lesions such as “chocolate-brown” blood and muscle tissue. Testing of the ocular fluid, serum and water can confirm the diagnosis. TVMDL also offers quantitative nitratetesting on hay and fresh forages. Samples should be shipped to the lab within 24 hours of collection. Core hay samples should be collected from a bale using a probe and immediately placed in a glass jar and sealed tight. Do not send roots and or dirt with fresh forages as this can affect test results. Qualitative nitrate testing is also part of TVMDL’s bovine sudden death tissue panel.
For more information about this case, contact Dr. Will Sims, veterinary pathologist at TVMDL Amarillo. To learn more about TVMDL’s test catalog, visit tvmdl.tamu.edu or call 1.888.646.5624 for the Amarillo laboratory or 1.888.646.5623 for the College Station laboratory.