The Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) recently received several cases of suspected urea toxicity. Unfortunately, the diagnosis could not be confirmed as the samples were improperly preserved. To aid in proper sample submission and diagnosis, TVMDL compiled a brief review of urea poisoning and a list of suggestions for sample collection/storage.
To review, urea is a common source of non-protein nitrogen (NPN) used as a feed supplement in ruminants. The rumen microflora utilize the nitrogen in urea released as ammonia to make protein which then is absorbed and utilized by the animal for growth metabolism.
Because ammonia is toxic, balancing the ration with the appropriate amount of urea is essential. Slight excesses of urea can be handled in normal ruminants as the ammonia absorbed from the rumen will be converted back to urea in the liver and then secreted by the kidneys into the urine. The trouble arises when this safety mechanism is overwhelmed and ammonia accumulates in the blood to dangerous and often lethal levels.
Depending on the level of excess urea in the feed, illness may appear within minutes to several hours following ingestion. Onset and progression may be so rapid that animals may simply be found dead. Clinically ill animals may display twitching of facial muscles and ears, teeth grinding, excessive salivation, bloat with abdominal pain, and forced breathing that often progresses to weakness with a staggering gait and later, recumbence with terminal convulsive episodes.
A strong presumptive diagnosis of urea toxicity can be made during necropsy soon after death by the distinctive odor of ammonia when opening the rumen and an elevated rumen pH of greater than 7.5-8. Other post mortem lesions may include obvious bloat, pulmonary edema, and epicardial/endocardial hemorrhages.
If you are called out to investigate animal losses where urea poisoning is suspected, it is critical to follow through with necropsy, sample collection and preservation as soon as possible after death. Testing involves distillation and titration of rumen content for ammonia. Ammonia is a gas and will dissipate rapidly post mortem if the rumen content is not frozen immediately after collection. Preferably, submit at least 500 grams of frozen rumen content stored in a leak-proof plastic container or sealed plastic bag.
Urea can also be measured in feed. Feed samples for urea should be submitted to Servi-Tech Laboratories (www.servitechlabs.com).This will be important in those situations where animal feed may have been mis-formulated and contain unwanted urea.
Questions about sample submission or test information should be directed to the laboratory: Amarillo, 1.888.646.5624; College Station, 1.888.646.5623. Visit tvmdl.tamu.edu for the full test catalog, submission forms or specimen information.