Urea is a valuable source of non-protein nitrogen but is potentially toxic if fed to ruminants in excess or if they are not adapted to it. This report describes a case of urea poisoning that occurred when sheep were inadvertently fed urea in a mineral mix.
Two hundred merino hoggets were being fed in a drought lot in the Central Tablelands of NSW. For the past three months they had been fed a ration of wheat and faba beans in a self-feeder and hay ad lib plus lime, magnesium oxide and salt in a trough. On one occasion urea was accidentally added to the salt/lime dry mix. Rain the previous day dissolved some of the urea in the trough.
On initial examination on 11 October 2018, ten hoggets were dead and another four lay in lateral recumbency. These animals were non responsive with tetanic convulsions, head tremors, jaw champing and profuse foamy salivation. Three of the affected lambs were blood sampled and treated with approximately 100 ml of vinegar. Despite treatment these lambs and an additional ten died.
Two hoggets were necropsied. The main abnormality was that the rumen was filled with fluid with little solid ingesta. There was white foamy material in the trachea. In case 1, the intestines were noticeably motile immediately post death and two intussusceptions could be seen continuing to develop (Figure 2).
|REFERENCE||0-200 umol/L||0-0.5 mmol/L||2.9-7.1 mmol/L||2.88-3.20 mmol/L||0.90-1.26 mmol/L|
In both cases the rumen pH was measured at 9.
The owner called when lambs commenced dying soon after being introduced to the pen. Salt intoxication (because a recent shower of rain was presumed to have dissolved some of the salt in the mineral trough) and urea poisoning were considered to be the main differentials given the rapidity of death. Urea was initially discounted as it was assumed that it was not included in the ration. However, a check of the mineral mix showed that urea had been accidently included in the mineral mix.
Signs of intoxication can occur within ten minutes of ingesting urea and include abdominal pain, frothing at the mouth and nose, muscle tremours, weakness and death (Constable et al., 2017).
Urease enzymes in the rumen break down urea to ammonia that is absorbed as ammonium ions when the rumen pH is low and as ammonia gas when the rumen pH is high. As ammonia gas is uncharged it passes through lipoprotein membranes more readily than ammonium ions. High blood ammonia levels can inhibit the citric acid cycle, produce a hyperkalaemic cardiac block, inhibit the respiratory centre and lead to rapid death (McBarron and McInnes, 1968; Seawright, 1982).
The laboratory findings of marked elevation in blood ammonia and urea levels and high rumen pH confirmed the diagnosis.