Lucerne (Medicago sativa) is a common fodder crop in Australian mixed farming enterprises, valued for its high quality and high protein. An important limitation of lucerne is that is causes bloat in cattle. Fortunately bloat in sheep is rare. However, this paper describes a case of bloat causing mortalities of 1.6% in lactating merino ewes, confirmed by necropsy findings including anterior congestion, a 'bloat line' and frothy rumen contents.
In April 2014 a producer from Bimbi, near Grenfell in the southern Central West Local Land Services region reported losing lactating ewes on a fresh lucerne paddock. He had lost six ewes out of 450 over the past few days, with two dying the previous night. The flock had just finished lambing. Prior to lambing a worm test showed a negligible worm burden, and a booster 5in1 vaccination was administered. The ewes had lambed on lucerne, were being fed oats approximately twice a week and were supplemented with a loose lick of lime and salt. Deaths occurred in a lucerne paddock that the sheep had been grazing for a week. The paddock was fresh following good rain a couple of weeks prior. At the initial visit the sheep had already been moved to a second lucerne paddock and offered a bale of hay. Lucerne in this second paddock, although fresh, was more mature with plants at the flowering stage.
Both carcases were distended, and the first had a loop of ruptured large bowel protruding from the anus. Necropsy findings and changes consistent with mild to moderate autolysis were similar in both carcases. In both animals anterior congestion was evident with generalised reddening or darkening of tissues and prominence of superficial blood vessels cranially from the level of the thorax (Figure 1). The lungs were heavy and dark red in colour with a shiny surface and a visible rib imprint (Figure 2). Blood oozed freely from the cut surface of the lung. There was severe generalised reddening of the tracheal mucosa particularly in the first animal, with diffusely scattered petechial haemorrhages concentrated in the cranial 10 cm (Figure 3). In both sheep there was approximately 200mls of bloody fluid in the pericardial sac and the liver was pale and soft (Figure 4). The oesophagus of the second animal was pale in the thoracic portion compared to the cervical end, the demarcation between indicating a 'bloat line' (Figure 5). The rumen contained large amounts of fresh green lucerne but no froth.
Following this initial visit a diagnosis of frothy bloat was made. Use of anti-bloat preparations was discussed but as other lucerne paddocks to be grazed were not as fresh, and the sheep were readily consuming hay provided, the producer opted not to use a preventative product.
One week later in the early morning the ewes were moved to another lucerne paddock. A trail of grain was fed out. The producer returned one and a half hours later to find another dead ewe.
This ewe had been dead a maximum of three hours prior to necropsy. She lay in right lateral recumbency with a grossly distended abdomen (Figure 6). Fresh green ingesta was present around the mouth and the tongue slightly protruded. Both eyeballs also appeared prominent (Figure 7). Anterior congestion was again evident and there was abdominal wall muscle separation on the lower left side (Figure 11). The lungs were slightly darker than normal with a mild mottled appearance and clear rib imprints (Figure 8). Blood vessels were slightly prominent in the tracheal mucosa. No bloat line was evident on the oesophagus but a firm bolus of green ingesta was found in the cranial oesophagus (Figure 9). The distended rumen was full of fresh green lucerne and frothy bubbles with a 'soapy' feel were clearly present (Figure 10).
The abdominal distension, protrusion of the anus and tongue and prominence of the eyeball in these carcases suggest bloat as the cause of death. The classic necropsy findings of anterior congestion, a bloat line and frothy rumen contents confirmed the diagnosis.
While bloat is regarded as the disease with the greatest impact to southern beef producers (Sackett et al 2006), it is rarely reported in sheep in Australia. Belschner (1965) commented that under some circumstances a large proportion of a flock can be affected by bloat and that sporadic cases also occur. Hungerford (1990) also alluded to a 1965 report from the New England reporting significant losses in sheep on clover dominant pastures. A survey in southern Queensland and northern NSW found losses in cattle grazing lucerne ranged from 0-6% whereas losses in sheep were reported as ranging from 0-1.25% (Queensland DPI report 2010). However, this survey relied on farmer diagnosis. Losses from bloat would be difficult to differentiate from other causes particularly enterotoxaemia and redgut without a necropsy. More recently Masters and Watt (2010) reported losses from bloat in young sheep.
The difference in susceptibility of cattle and sheep has been explained by the grazing characteristics of sheep - despite selectively choosing to eat leaves over stems and chewing more frequently than cattle, sheep eat more slowly and presumably ruminal gas takes longer to build up. In addition, it is thought sheep are more resistant than cattle to the effects of an increase in intra-rumen pressure (Colvin and Backus 1988).
Management techniques suggested which minimise the risk of bloat to sheep grazing lucerne include not introducing hungry animals into a fresh lucerne paddock; not grazing young, fresh lucerne; supplementing with suitable roughage; under-sowing with a grass or other non-bloating forage; or providing antifoaming agents as a preventative. In this case the first round of deaths were in a paddock where the lucerne was very young and fresh and not grazing the pasture at this stage may have avoided those deaths. The subsequent death occurred in a paddock where the lucerne was more mature and where hay and grain were available, however it did occur shortly after the sheep were introduced to the paddock. Introducing the sheep to the new paddock later in the day after allowing them to fill on less risky pastures may have avoided this death.