Frothy bloat is one of the most important diseases of cattle in southern Australia. It remains a serious impediment to grazing high quality, otherwise productive legume pastures and also occurs occasionally on nitrogen rich pastures and grazing cereals. Sheep generally graze high quality pastures with impunity. However, deaths due to frothy bloat occur occasionally. This report details two cases, the first of which was highly suggestive of primary bloat but the diagnosis was unable to be confirmed. The second case was confirmed by autopsy.
CASE 1: TUENA
On the 5th March 2010 a very experienced sheep producer from near Tuena on the central tablelands reported that a few days previously he discovered three dead lambs and 70-80 lambs with abdominal distension one hour and fifteen minutes after he the gave the mob of 300 Merino lambs access to a paddock of lucerne. The lucerne was growing actively after recent falls of rain totalling 150 mm. The owner commented that dead lambs had gas filled abdomens and a protruding purple anus. The surviving bloated lambs had obvious left sided abdominal distension that resolved after the lambs were removed from the lucerne.
CASE 2: TUMUT
On the 7th March 2010 a Gilmore Valley producer reported losses in a mob of 240 8-9 month old dorper cross lambs moved onto a paddock of lucerne the previous afternoon. The lucerne was about 30cms high in the pre bloom stage and 70mms of rain had been recorded over that weekend. Six lambs had died and another 10-12 lambs were distressed and bloated. All of these lambs recovered after the producer yarded them, fed them hay and kept them moving.
Autopsy findings on two of the dead lambs supported a diagnosis of frothy bloat. These findings included blood dripping from nostrils, purple gums and obvious abdominal distension with rupture of abdominal wall in the left inguinal area. On opening the carcases, the forequarters were noticeably more congested than the hindquarters and the thoracic contents appeared to be compressed cranially. The tracheal mucosa was haemorrhagic. There was marked distension of the rumens of both sheep with an explosive release of gas when opened, followed by a flood of frothy green rumen contents. The small intestines appeared reasonably normal with no indications of changes suggestive of 'red gut' or enterotoxaemia.
Sudden death in sheep grazing on lucerne can occur from a range of causes including the acute haemorrhagic enterocolitis known as 'redgut' and enterotoxaemia. However, neither of these diseases is likely to cause very rapid abdominal distension and deaths immediately post introduction to lucerne. Nor are affected sheep likely to recover once they are removed from the lucerne. Therefore, in the first case frothy bloat is suspected based on the observations of an experienced farmer. In the second case, the history is similar but the post-mortem findings are also consistent with frothy bloat.
In our experience, frothy bloat is rare in sheep in Australia despite the numbers of both sheep and lambs that graze on high quality legume pastures when conditions suit. However, the problem appears to be more common in North America. Extension literature available electronically includes frothy bloat in a list of common sheep diseases (Leffel et al., 2010) while Mike Neary, Extension Sheep Specialist from Purdue University writing in a 1997 edition of the 'Working Border Collie' gives detailed advice on the problem.
Jones, Hunt and King (1997) consider that frothy bloat in sheep is less common than in cattle because sheep have more 'conservative eating habits.' However as Colvin and Backus (1988) noted, sheep selectively grazed leaf versus stem and preferred clover. On this basis, they should therefore be more susceptible to bloat than cattle. They concluded that sheep suffered bloat less often than cattle because they ate more slowly.