Bloat

Over-distension of the rumen with gases of fermentation is termed bloat. If foam is present it is called frothy bloat, otherwise it is called free-gas bloat. Bloat causes compression of the thorax which in turn causes respiratory distress and death from asphyxiation or heart failure if severe.

Frothy bloat occurs after rapid engorgement of high protein, highly digestible feed that results in formation of excess amounts of stable foam in the rumen. Free gas bloat occurs when any physical (obstruction) or functional problem prevents normal eructation of rumen gas (burping).

When cattle gorge on feed, either because they are hungry or they are competing for feed, the rumen receives an oversupply of fermentable carbohydrates in a short space of time. Rapid consumption of pellets without roughage of an adequate fibre length means minimal production of salivary buffers when chewing feed and increased viscosity of the rumen contents. The influx of fermentable carbohydrates leads to the proliferation of certain bacterial species that produce slimy bacterial mucopolysaccharides (especially Streptococcus bovis). Bacterial slime can prevent bubbles of gas created during the fermentation process from coalescing resulting in frothy rumen contents. Gas bubbles held within frothy ingesta are unable to rise to the gas layer in the dorsal rumen. Frothy gas cannot be freely expelled by eructation and the presence of frothy ingesta means that the normal eructation sequence is not initiated by the presence of free gas in the dorsal sac of the rumen. In some cases, highly viscous rumen ingesta may block the cardia and completely prevent the animal’s ability to eructate.

Some observers have attributed frothy bloat at sea to ingestion of large quantities of rapidly fermented fine feed particles from pellets that have disintegrated during storage and handling. Lucerne hay may also cause frothy bloat. Free gas bloat at sea has been attributed to cattle eating plastic or string found in pens. On a ship, cattle may chew and swallow any foreign object that they can reach.

Frothy bloat usually affects groups of animals within an hour of feeding. Free gas bloat occurs sporadically in individuals. Distension of the left flank is detected during pen inspection. The right flank will also be distended in severe cases, and there may also be respiratory distress (mouth breathing, tongue protrusion, outstretched neck).

Cattle with tetanus, rabies, oesophageal obstruction and ruminal acidosis may also develop bloat. With tetanus the animal is also stiff, with rabies there is aberrant behaviour, with oesophageal obstruction there is profuse salivation and bilateral nasal discharge, and with acidosis there is also dehydration and liquid, rich-smelling, brown faeces.

Cattle often develop bloat when lying down. This is because it is difficult for cattle to burp unless positioned almost exactly upright. When they become too uncomfortable they will reposition themselves or stand, belch and be okay. If they are unable to reposition themselves, for example if they get stuck under a pen rail, sat on by another animal, or if weakened by hypocalcaemia, they are in danger of bloating and asphyxiating.

Bloat is confirmed by the history and gross pathology. Cattle affected by bloat may die suddenly and are unexpectedly found dead during pen inspection. At necropsy there is intense congestion of the head and neck (especially the tongue, eyes and tonsils), relative pallor of the chest and abdomen, and moderate congestion of the hindquarters. The abdomen is hugely distended by the rumen which may still contain some froth in cases of frothy bloat. The sudden death and rapid decomposition can make differentiation from clostridial diseases difficult.

Note: well-conditioned animals dying of other causes can bloat quickly after death and develop signs resembling true bloat such as congestion of the head and neck, and an oesophageal bloat line (if post-mortem bloating occurs before blood coagulates).

Cases of frothy bloat, if mild, can be treated by adding chaff or hay to the feed, or painting vegetable oil on the flank which is licked off by the patient. If severe, vegetable oils, mineral oils (paraffins) or antifoaming agents may be administered by oral drenching or via large bore needle directly into the rumen. The resulting free gas layer is then more readily eructated. Life threatening distension is treated by venting the rumen through a 10 cm stab incision with a knife targeting the middle of the left paralumbar fossa. Use a sharp pointy knife with a non-slip handle. Prepare the site so it is aseptic and use local anaesthesia and possibly sedation. Extend the stab incision to a minimum 5 cm and maximum 10 cm length, maintaining and rotating the knife in the hole during decompression to help the foam escape to outside the abdominal cavity. A trochar and cannula will usually have too small a bore to allow escape of the stable foam. The stab wound should not be sutured. Clean the wound area, administer antibiotics (procaine penicillin, oxytetracycline) for three days, keep flies away and hope for the best. If the stab incision and venting have been done properly, the wound will heal over in two to three weeks. If not, there will be peritonitis, a chronic festering wound and some animals will die.

In cases of free gas bloat, antibiotics (procaine penicillin, oxytetracycline) should be administered in case infection in or around the oesophageal groove is the cause. A stomach tube passed down the oesophagus into the rumen may dislodge an obstruction in which case it will be diagnostic and curative. It should at least provide temporarily relief. Severely distended cattle have been known to drop dead with this procedure due to further circulatory embarrassment as the tube passes the aortic arch. In severely affected animals, partial release of pressure via flank cannulation before passing a stomach tube may avoid risk of collapse while passing the tube.

Cattle areas must be kept free of rubbish, in particular plastic, string, rope and wire. This will help prevent free gas bloat and other cases of indigestion.

Prevention of frothy bloat is achieved by slowing the eating of high protein, highly digestible feeds and increasing salivary secretion, or by incorporating substances (alcohol ethoxylate teric) into feed to prevent rapid fermentation and foaming.

It is important to ensure the continuity of feed availability and feed composition by careful planning and close management of feeding regimens and fodder reserves. Avoid erratic feeding or creating undue competition at feed times. Roughage of adequate fibre length stimulates chewing and salivary production. A consistent supply of roughage will help to stabilise rumen fermentation, prevent ruminal acidosis, reduce the viscosity of the rumen contents and optimise rumen motility.

During routine inspection, animals that are lying in lateral recumbency or that have left flank distension should be forced to stand. Crew members should be trained to take such action and immediately report to the veterinarian or senior stockperson if any animal appears unable to stand.