In the live export process, nitrate-nitrite poisoning may occur in assembly points when hungry cattle gorge on hay containing high concentrations of nitrate.
In ruminants it is the nitrite which is toxic. Rumen microbes convert nitrate to nitrite. If absorbed into the blood stream in sufficiently high amounts, nitrite can reduce the oxygen carrying capacity of blood by altering haemoglobin, causing hypoxia and death.
Dangerous concentrations of nitrate may be present in hay made from pasture or crop fertilised heavily with nitrogen. High levels may also occur in hay made from plants wilting from water, cold or heat stress or herbicide poisoning at time of cutting, or improperly cured at the time of baling. Mouldy hay may have high levels of preformed nitrite which can be absorbed directly into the blood stream after ingestion and cause respiratory distress and death.
Signs appear 6 to 24 hrs after toxic feed has been consumed. There will be weakness, ataxia, respiratory distress and brown cyanotic mucous membranes before collapse and death in asphyxial convulsions. At necropsy the blood is a dark chocolate brown but fades rapidly to near normal colour.
In the field, on fresh cases, urine test strips are adequate to test body fluids for nitrite. Laboratory confirmation requires serum or aqueous humor submitted chilled for toxicology. Test strips are available for field and laboratory testing for nitrate in plant samples. Plant samples submitted to the laboratory must be fresh and chilled. They must be frozen if there will be delays in reaching the laboratory.
The main differential diagnosis is cyanide poisoning which can be differentiated by the cherry coloured blood at necropsy and history of feeding sorghum hay.
Immediately prevent access to the suspect hay and offer an alternative low risk feed to dilute toxic feed already ingested. Handling must be minimal to reduce oxygen demand.
Intravenous administration methylene blue has been effective at saving seriously affected animals, but is no longer recommended for use. It is irritating to tissues and is a carcinogen. It is no longer approved by the Australian Pesticides and Veterinary Medicines Authority (APVMA) for use in food-producing animals. Other bodies such as the USA Food and Drug Administration have recommended a 6-month withhold in any food-producing species following administration of methylene blue.
Avoid feeds that may be high in nitrates. Seek advice on feed selection and management. Consider submitting feed samples for nitrate testing if concerned.