Nitrate toxicity in sheep is an uncommon diagnosis in the more northern areas of central western NSW. This finding likely reflects the epidemiology of the disease, with animals commonly run on unimproved native pastures, little or no nitrogenous fertiliser application and infrequent overcast days. This case describes an event of nitrate poisoning in sheep in the Nyngan district. Several risk factors, including hungry animals, recent drought and highly nitrogenous soils in sheep yards were likely contributors.
On the 27 August 2020, 20 two-year-old ewes out of a mob of 400 were found dead having been yarded without feed overnight following classing. The ewes were homebred and had been run on native pasture immediately prior to yarding. They were up-to-date with vaccinations and drenches and had been mustered without incident. The surviving animals were released from the yard and did not appear to be clinically affected.
The yard was largely bare of foliage with a light cover of capeweed (Arctotheca calendula) and marshmallow (Malva parviflora) that had been grazed down to the base of the stems.
The dead ewes were scattered across the yard with varying amounts of dark-brown froth exuding from their nose and mouth. One ewe was found to be alive and paddling in lateral recumbency. She was hypothermic (37.3C) with tachycardia, muscle tremors and dark, muddy mucous membranes. Dark red-brown blood was obtained by jugular venipuncture and the ewe exhibited generalised ataxia and apparent central blindness when placed on her feet. The mucous membranes of dead ewes also appeared muddy.
A field immunochromatographic (ICT) kit for anthrax was negative and a necropsy was performed on one of the dead ewes. The ewe was in reasonable body condition and had digested green feed in the rumen. Rumen wall and liver samples were taken as the farmer was concerned about poor growth rates over the preceding months despite access to excellent nutrition. Nitrate/nitrite levels in the aqueous humour was assessed semi-qualitatively with Quantofix Nitrate Test strips, which revealed a strong positive result of approximately 50-100mg/L nitrate and 10-20mg/L nitrite (Reference Range: aqueous humour <10mg/L nitrate and <0.2mg/L nitrite up to 24 hours post-mortem at room temperature). A strong positive was also observed for nitrite on a Multistix dipstick analysis of urine collected from the bladder. No other significant findings were observed in the mildly autolysed carcass.
The combination of clinical and post-mortem signs (dark-brown blood, central nervous system dysfunction, dark foamy exudate in the upper respiratory tract) and high nitrate/nitrite levels in the aqueous humour allowed a field diagnosis to be rendered. Given the location of the property and the sudden death of a number of animals, anthrax was an important exclusion. The absence of hyperthermia in the surviving affected ewe, lack of ongoing deaths and the negative ICT test result meant that anthrax was deemed unlikely. No treatment beyond removing surviving stock from the yard was recommended as administration of methylene blue is no longer supported by the APVMA in food-producing animals.
The epidemiology of nitrate toxicity involves a complex interaction between environmental, plant and animal factors. Nitrate toxicity can occur when plant nitrate uptake exceeds its reduction to ammonia. Plant protein synthesis is dependent on photosynthesis and conditions of low light or metabolic stress can therefore lead to nitrate accumulation (Hall, 2018). Ruminants are particularly susceptible to nitrate toxicity as rumen microbes convert nitrate to nitrite that can then be absorbed across the gut. Nitrite interacts with haemoglobin to form methaemoglobin, which is unable to carry oxygen, resulting in hypoxia. Muscle tremors and tachycardia become apparent at approximately 30-40% methaemoglobinaemia, which progresses to convulsions and deaths at 80-90% methaemoglobinaemia (Bolan & Kemp, 2003).
Capeweed and marshmallow are both known nitrate accumulators with higher levels recorded in the stems and connective tissues compared to the leaves, where more rapid reduction to ammonia occurs (McKenzie, 2012). In this case, heavy grazing by hungry animals likely increased their exposure to nitrates, which is consistent with previous cases of livestock intoxication in NSW (Watt, Staples & Bunker, 2017). While it was not a particularly overcast morning, there had been both recent frosts and stormy weather, which may have increased the metabolic stress placed on the plants, further increasing nitrate levels.
The preceding three years of drought in the region and poor ground cover in the yards likely led to increased concentrations of nitrates in the soil, making them readily available for plant uptake post-rain (Bolan & Kemp, 2003). Additionally, there is evidence that the deposition of urine and faeces in stockyards can lead to 10-30 fold localised increases in the nitrate concentrations of plants within yards compared with similar plants in other areas of a property (McKenzie et al., 2004). As the yard in question had been used as a drought feedlot from autumn 2017 to autumn 2020, heavy faecal and urine contamination over that period probably led to very high soil nitrogen concentrations.
It is speculated that the exposure to drought conditions as young animals could also have predisposed the sheep to nitrate toxicity. Histopathology was performed on the rumen of the necropsied animal revealing papilla atrophy and hyperkeratosis. While the animals had been raised on grain during the drought, they had not been fed concentrates for several months preceding the toxic event. The impacts of long-term grain feeding on ruminal architecture and microbiome following resumption of a pasture-based diet are unknown. However, a recent longitudinal study of long-term grain diets in Japanese black cattle indicates that highly complex microbe-host interactions (extending to transcriptomic changes in ruminal epithelium) vary over time and raises the possibility of more persistent changes following long-term exposures (Ogata et al., 2020). The ill-thrift and tissue changes found in this case could indicate that an ongoing lack of effective dietary fibre (eg. lush pasture conditions following drought-breaking rain) potentiated papilla-blunting and hyperkeratosis initially induced by a high concentrate diet. As approximately 70% of ruminal microbes are solid-associated (adhered to fibre particles), these animals may have been experiencing an ongoing dysbiosis despite being run on pasture (Forsberg & Lam, 1977). Any dysbiosis could be significant as defaunation (extinction of ruminal protozoa) in sheep can result in nitrite accumulation in the rumen following exposure to dietary nitrates (Villar et al., 2020). Therefore, it is hypothesised that dysbiosis may increase an animal's risk of nitrate poisoning. However, further research on ruminal recovery following long-term grain-based diets and the role of the microbiome in nitrate poisoning are necessary.
I would like to thank Jillian Kelly (District Veterinarian, Coonamble) for her assistance in working up the case and report.