Over-distension of the rumen with gases of fermentation is termed bloat. If from foam 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).
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 entering 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 behavior, 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 oil 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 escape of foam to the 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 be 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 pressue via flank cannulation before passing a stomach tube may avoid risk of collapse while passing the tube.
If the bloat keeps recurring, consider inserting a commercially available plastic, self-retaining cannula into the rumen. The accompanying trochar is not usually robust enough to penetrate the skin so it is best to make an incision with a scalpel at the intended place of cannula insertion. Make a vertical incision through skin and underlying fascia of a length equal to twice the diameter of the cannula body. With the trochar in place in the lumen of the cannula, sharply thrust through the abdominal and rumen walls. With the trochar still firmly in place, screw the cannula in so the spiral flange holds the rumen firmly against the abdominal wall. The trochar is then removed and the gas allowed to escape. Gas escape is best controlled in order to avoid a sudden fall in blood pressure accompanying the sudden fall in intra-abdominal pressure. The cannula can remain in place for as long as necessary. This may be as long as three to four weeks.
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 into feed substances preventing rapid fermentation and foaming (alcohol ethoxylate teric). Options include provision of hard durable pellets, supplemental roughage and incorporation of ionophores such as monensin or lasalocid into pellets.
During routine inspection recumbent animals that are lying lateral 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 stockman any animal that appears unable to stand.