Diagnostic and toxicology results indicate that nitrate toxicity is the most likely cause of death in July for 71 wild horses managed by the Bureau of Land Management on the Nevada Wild Horse Range.
High levels of nitrates were found in some water samples taken from a pond the horses used for drinking on a dry lake bed and also in the deceased horses' blood serum and ocular fluid. Water test results indicated nitrate levels of more than 3,000 parts per million. The United States Health Service standard for drinking water for human consumption is less than 45 ppm. Livestock can tolerate higher levels but problems are known to occur when levels exceed 400 ppm and acceptable levels should be below 100 ppm.
Acute nitrate toxicity in horses is not well understood and there haven't been many instances of it reported. There is no indication that the problem is attributable to a contagious or infectious disease. Tests conducted for salts, heavy metals and algae toxin have not identified other concerns. The pattern of the deaths, the test results and that the deaths stopped when the water hole was fenced off, all suggest that nitrates in the water is the cause of death.
Some soil test results are still pending, but the source of the nitrates has not been identified. BLM will coordinate with the Air Force to conduct additional environmental evaluations in the area. BLM will monitor the nitrate levels in the pond, which is now fenced, and conduct water testing on springs in the area.
The Nevada Wild Horse Range is completely within the U.S. Air Force Nevada Test and Training Range. The area is restricted from public access. The Air Force and Sandia Labs are assisting BLM with hauling water to the animals. The barrier around the pond will remain until future testing shows the water quality has returned to safe levels for he horses.
