Urea is commonly used as a feed supplement in ruminant diets, as rumen microbes are able to synthesise protein from this cheaper, non-protein nitrogen source. Urea may be provided by inclusion in solid blocks, liquid mixes, or added to the concentrate or roughage component of a ration. However, care is required to prevent overconsumption, which can lead to urea poisoning. The effects of urea are influenced by a number of factors, and so establishing a toxic dose for individuals is difficult (McDonald et al., 2011). Reportedly, doses as little as 0.2 to 0.5 g urea/kg body weight (BW) can cause toxic signs in unadapted ruminants, whereas adapted animals can tolerate up to 1 g urea/kg BW. Method of delivery also needs to be considered. For example, there is a risk of overconsumption of liquid mixes of urea and molasses if access is not restricted (Lieske & Volmer, 2003). Urea is also used as a nitrogen fertiliser, which can cause toxicosis if ingested (Niles, 2017). Therefore, when considering exposure, it is important to assess not only the presence of urea in feed, but also stock access to urea fertiliser.
The following report describes a case of sudden death in Merino ewes due to urea poisoning, likely through ingestion of water contaminated with urea fertiliser.
In March 2018, the District Veterinarian was contacted to investigate the sudden death of 26 merino ewes in a flock of 400 mixed-age ewes. The property was located in the Central Tablelands region of New South Wales. It was primarily a wool enterprise, with a self-replacing flock.
The affected flock had been grazing a stubble pasture and receiving wheat until the previous day. Rain had been forecast, and to prevent soil erosion the ewes had been moved off the stubble into a laneway. They had appeared normal that afternoon. The following morning, 26 ewes were found dead, with no clinical signs observed prior.
The ewes were homebred, and there had been no stock movements on or off the farm in the previous six weeks. The ram had been with the flock for the past four weeks. The ewes had been vaccinated with 6-in-1 and drenched with Jurox Q-drench prior to joining. The wheat that had been fed was from the same batch and there was no mould present. The property was located in the anthrax belt, but there was no history of anthrax on the property.
A trough was located at the far end of the laneway and was supplied from the same water source as the paddock they had previously been in. There was some green pick in the laneway, which consisted of mixed native pasture and a minor portion of mature phalaris. In the laneway was one uncovered pile of superphosphate and two piles of urea fertiliser covered with black plastic. Overnight four millimetres of rainfall was recorded and there were pools of water on the plastic covering the urea fertiliser.
The dead animals were found within 50 m of the urea/superphosphate piles, with evidence of struggling/paddling before death. The remainder of the flock had been shifted, with the producer reporting that they appeared normal, and there were no further deaths.
An anthrax immunochromatographic (ICT) test was conducted using blood from the nostrils of the dead animals, and returned a negative result.
Most of the dead ewes had bloody froth from the nostrils and blood in the anterior chamber of the eye, and all were bloated to some degree.
Three animals were necropsied, with all in an advanced state of decomposition. Necropsy findings were purple discolouration and congestion of the lungs, pulmonary oedema with presence of rib impressions, serosanguinous fluid in the pericardial sac, and ulceration of the oesophagus. All tissues were autolysed.
Rumen contents were collected from the three necropsied animals, and aqueous humour samples were collected from six animals. A range of fixed tissue samples were submitted for histology. Two formalin-fixed brains, and two fresh segments of spinal cord were collected for transmissible spongiform encephalopathy (TSE) analysis. The neural tissue samples were taken from animals that were approximately three years of age. Three water samples from the pools on the plastic covering the urea fertiliser were also collected.
The negative anthrax ICT test was confirmed at Elizabeth Macarthur Agricultural Institute by polychrome methylene blue stains on multiple aqueous humour samples. All aqueous humour samples were negative for Bacillus anthracis.
On histology, there were no lesions suggestive of a transmissible spongiform encephalopathy identified at the brain sites specified in the Australian and New Zealand Standard Diagnostic Protocols for Animal Diseases (Cook, Richards, & Middleton, 2010).
Histology of additional tissue samples showed no significant findings besides advanced autolysis.
Two aqueous humour samples were used for ruminant biochemistry profiles including urea, glucose, β-hydroxybutyrate, calcium, magnesium, nitrate/nitrite, phosphorus, D-lactate, and ammonia. The remaining were analysed for ammonia and urea. The findings are presented in Table 1 below.
|Analyte (reference range)||Sheep 1||Sheep 2||Sheep 3||Sheep 4||Sheep 5||Sheep 6|
Rumen contents and water samples were analysed for urea, with the results presented (Table 2 and Table 3). Note that no reference ranges were provided by the laboratory.
|Rumen sample 1||17||285|
|Rumen sample 2||8||139|
|Rumen sample 3||5||76|
|Water sample 1||347||5776|
|Water sample 2||195||3246|
|Water sample 3||158||2637|
Ruminants have the ability to utilise non-protein nitrogen sources, such as urea, to synthesise protein. In the rumen, urea undergoes hydrolysis via bacterial urease to form ammonia (NH3), which is used for the production of microbial protein. When microbes are carried through to the abomasum and intestine, this microbial protein is digested and absorbed by the animal. If ammonia is produced at a greater rate than can be utilised for protein synthesis, excess ammonia passes through the rumen wall into the blood. Once in the blood it is carried to the liver where it is converted to urea, which may be recycled back to the rumen via saliva or by passing directly through the gut wall, or excreted in the urine (McDonald et al., 2011). When used as a feed supplement, care must be taken to introduce urea slowly, to allow rumen microbial adaptation (Abbott, 2018). Urea should also be fed with a readily fermentable carbohydrate source to provide energy for protein synthesis (McDonald et al., 2011).
Urea poisoning can occur when adapted animals are fed larger quantities than appropriate, un-adapted animals gain access to urea, or urea is not adequately mixed through a ration. It may also occur when contaminated water or urea fertiliser is ingested (Constable, Hinchcliff, & Grünberg, 2017; Niles, 2017). In cases of urea poisoning, the ammonia produced from hydrolysis is the toxic agent (McDonald et al., 2011), with poisoning occurring when it is produced at a rate greater than can be utilised by microbes (Niles, 2017). When the rumen pH is 7 or below, a majority of the excess ammonia is in the form of ammonium (NH4+). Ammonium does not easily pass through the rumen wall, and is therefore not significantly absorbed into the circulation. As ammonia continues to be converted to ammonium, the pH of the rumen increases, causing the reaction to be shifted in the direction of ammonia production. At approximately pH 8, the ammonia to ammonium ratio is in favour of ammonia (Lieske & Volmer, 2003). Ammonia readily passes through the rumen wall and enters the circulation (Lieske & Volmer, 2003; Ensley & Rumneiha, 2012). Signs of toxicosis occur when the mechanisms of detoxification in the liver are overwhelmed, and ammonia accumulates in the blood (Lieske & Volmer, 2003). Ammonia causes inhibition of the citric acid cycle, lactic acidosis, decreased ATP production, disruption of the sodium-potassium ATPase pump, and interferes with cerebral energy metabolism (Lieske & Volmer, 2003; Cope, 2012).
Clinical and laboratory findings
In this particular case, no clinical signs were observed. However, the presence of froth from the nostrils and evidence of struggle prior to death are consistent with urea poisoning (Constable et al., 2017). Clinical signs of urea toxicity in ruminants described include muscle and skin tremors, hypersensitivity to sound, frequent urination and defecation, dyspnoea and tachypnoea, stiffening of the forelimbs, incoordination, convulsions, bloat, rumen atony, and regurgitation (Lieske & Volmer, 2003; Antonelli, Mori, Soares, Kitamura, & Ortolani, 2004; Constable et al., 2017; Niles, 2017). Clinical signs, when observed, can appear as soon as 10 minutes post-ingestion (Constable et al., 2017). Generally, signs are seen within 30 to 90 minutes in sheep, with terminal convulsions and death often within four hours (Lieske & Volmer, 2003).
There are no pathognomonic post-mortem lesions (Lieske & Volmer, 2003). However, bloat, pulmonary congestion and oedema, and fluid in the pericardial sac have been reported (McBarron & McInnes, 1968; Lieske & Volmer, 2003; Antonelli et al., 2004) and were observed in this case.
Reports of oesophageal ulceration and blood in the anterior chamber of the eye were not found in the literature. However, terminal regurgitation of rumen contents has been reported (McBarron & McInnes, 1968). Oesophageal ulceration may have been a result of regurgitation of rumen contents containing high concentrations of ammonia (S. Raidal, personal communication, September 7, 2020). Ingurgitated episcleral veins have been reported in cases of experimental urea poisoning (Antonelli et al., 2004), however haemorrhage was not observed, indicating that this finding may be uncommon in cases of urea poisoning.
Analysis of ammonium nitrate (NH3N) is used for diagnosis of urea poisoning, and can be performed on serum, whole blood, ocular fluid, cerebrospinal fluid (CSF), and rumen contents. After collection, these samples should be frozen until analysis. It should be noted that diagnosis can be complicated by post-mortem changes including decreases in rumen pH and production of ammonia due to autolysis. Animals dead for longer than 12 hours in a moderate climate are considered to be of no diagnostic value due to autolysis (Lieske & Volmer, 2003). In this case, it was determined that samples would still be of diagnostic value. Collected samples were put on ice, and sent to the laboratory the same day.
For post-mortem diagnosis to be made, elevated ammonia should be seen in rumen fluid, whole blood or serum, and ocular fluid or CSF (Lieske & Volmer, 2003). In this case, all samples of aqueous humour showed marked elevations in ammonia. The large degree of elevation was considered supportive of urea poisoning, and not as a result of post-mortem changes. In fresh serum, plasma and ocular fluids, ammonia concentrations of greater than 1000 µM are expected with urea poisoning (D. Paynter, laboratory report, March 29, 2018). This value was well exceeded for the aqueous humour samples analysed in this case. Analysis of rumen fluid for ammonia was not performed due to the instability of ammonia levels in the rumen post-mortem. Therefore, elevated ammonia concentrations in aqueous humour, combined with history and other clinical findings, were enough to diagnose urea poisoning without ammonia measurement in other samples.
All aqueous humour samples demonstrated elevated urea. This finding may be as a result of increased hepatic production of urea in an attempt to detoxify excess ammonia in the blood (Latimer, 2011). The clinical course was likely too acute for the elevated urea to be a result of azotaemia. The elevations in calcium, magnesium and phosphorus shown in aqueous humour sample 2 were attributed to post-mortem changes.
In this case, rumen urea content was measured, and was used as a qualitative test to determine the presence of urea in the ingesta, given the sheep were not receiving urea supplements. It is hypothesised that urea poisoning occurred as a result of ingesting water contaminated with urea fertiliser. Analysis of water pooled on the plastic covering of the urea fertiliser supports this hypothesis, with urea contamination detected.
Toxicosis can be observed in unadapted ruminants at doses of 0.2 to 0.5 g urea/kg BW (Lieske & Volmer, 2003). The toxic dose is 10 to 25 g of urea in a 50 kg ruminant. Using an average of the water analysis results, a 50 kg sheep would need to ingest approximately 40 to 100 mL of urea-contaminated water to consume a toxic dose (D. Paynter, personal communication, September 14, 2020). It is likely that the volume of contaminated water the ewes consumed was in in excess of this amount. The fact that the affected sheep were found dead approximately 50 m from the urea pile suggests rapid clinical progression due to ingestion of a large dose of urea.
Treatment and Prevention
Treatment was not undertaken in this case and the remainder of the flock had been shifted out of the laneway with no further abnormalities reported. Generally, due to the rapid progression of urea poisoning, treatment is not possible. Oral administration of 5% acetic acid can lower the pH in the rumen. Cold water can also be given to dilute the rumen contents and reduce activity of ureases by lowering rumen temperature. Repeated treatment may be required if clinical signs reappear (Lieske & Volmer, 2003). Evacuation of the rumen via stomach tube or rumenotomy has also been suggested, but success may vary (Constable et al., 2017).
Prevention involves ensuring that supplementation is appropriately managed and animals do not have access to urea fertiliser or water sources contaminated with urea. Prevention is particularly important as treatment of large numbers of animals may be ineffective due to rapid clinical progression (Constable et al., 2017).
This case demonstrates the use of history and a systematic approach to excluding differential diagnoses in cases of sudden death, in particular where clinical signs are not observed and pathognomonic lesions are not found on necropsy. It also highlights the need to consider stock access to urea even if not provided as a feed supplement. While it is a commonly used source of non-protein nitrogen, urea is also used as a fertiliser and so may be present on farms for this purpose. The likely cause of poisoning in this case was ingestion of water contaminated by urea fertiliser. With the exception of oesophageal ulceration and haemorrhage in the anterior chamber of the eye, the findings in this case were consistent with what has been previously reported.
The authors would like to thank Professor Shane Raidal and David Paynter for their time in discussion of the case.