Methemoglobinemia Occurs in Certain Toxic States, Notably Nitrite Poisoning

When the normal ferrous iron of hemoglobin is oxidized by nitrites and other toxins to ferric iron, brown-colored methemoglobin is formed. Methemoglobin does not bind oxygen; thus the oxygen capacity of the blood is reduced. Nitrite can be ingested directly from spoiled feeds, but rumi­nants more often form nitrite in the rumen after ingestion of nit rate-rich feeds such as Sudan grass and mangel tops. An enzyme, methemoglobin reductase, which is present in erythrocytes to reduce the normally small amounts of met­hemoglobin back to hemoglobin, is inadequate in the presence ofexcessive oxidation, so therapy must be provided.