Bloat

Description

Bloat is the ruminal distension caused by excessive accumulation of gas produced by the fermentation process. Bloat can be categorised according to cause into Primary bloat (frothy bloat) and secondary bloat (free gas bloat). The condition progresses rapidly and can be fatal.

Primary bloat

Primary bloat, or frothy bloat is the common type of bloat, and usually affects several animals in a group at once. The condition can occur in feedlot cattle but is most frequently associated with cattle grazing lush, leguminous pastures. it occurs following the ingestion of large amounts of succulent green plant material which is highly fermentable and produces large amounts of gas. The material also contains high levels of soluble proteins, these act as foaming agents by reducing surface tension but increasing surface viscosity of rumenal liquids and a stable rumenal foam forms. The volatile fatty acids of the plant material consumed lower pH, optimising conditions for formation of froth. Gas present as foam is insufficient to trigger eructation, and so the rumen becomes distended.

Seconday bloat

Secondary bloat or free gas bloat results from any condition that causes oesophageal obstruction or interferes with eructation. It usually affects a single animal in a group and generally has a more sporadic occurance.

It may be acute in the case of sudden oesophageal obstruction, caused by a foreign body such as a potatoe or turnip, or chronic resulting from partial oesophageal obstruction, compression, or from interference with the normal eructation mechanism.

Causes of secondary bloat:

• Physical oesophageal obstruction
• Leisions of the oesophagus or oesophageal groove such as tumours, abcessation or infection with Actinobacillus lignierii.
• Prolonged lateral recumbency
• Excessive cereal ingestion
• Rumenal atony
• Inability to eructate due to Tentanus or Milk fever
• Chronic inflammatory or adhesive disease of the reticulum

Signalment

Primary bloat can occur in feedlot cattle but is generally associated with pastures rich in leguminous plants. There is a particularly high prevelence in New Zealand due to the high level of clover. In the UK occurance of the disease coincides with turnout to lush pasture, especially in spring and autumn.

Secondary bloat due to oesophageal foreign bodies can occur in any animal fed feeds such as potatoe or turnips which may lodge in the oesophagus. Cattle with underlying conditions such as tetanus and milk fever are predisposed.

Diagnosis

The diagnosis of bloat can usually be made based on the obvios distension of the left paralumbar fossa. When several animals are affected on lush pasture the cause is invariably primary bloat. If only one animal is affected, or there is doubt over the cause, the passage of a stomach tube or trocar will release large quantities of gas in the case of secondary bloat. A clinical exam will aid in finding the underlying cause of secondary bloat.

Clinical signs

• Visible distension of the left paralumbar fossa
• Kicking ventral abdomen and bellowing in pain
• Sudden death

Pathology

In severe cases, the rumen almost fills the abdomen, the diaphragm is displaced forwards, compressing the lungs. The posterior vena cava is also compressed, reducing the venous return to the heart. Respiration is impaired and animals die from hypoxia.

Oesophageal "bloat line" (Courtesy of Elspeth Milne)

Post mortem

• • • Blood is dark (hypoxic) and clots poorly.
• The anterior parts of the body are often characteristically acutely congested.
  • Oedema and subcutaneous haemorrhage of head and neck tissues.
  • Oesophageal "bloat line" at thoracic inlet.
    • Congested and haemorrhagic cranially, pale caudally.

References

• Andrews, A.H, Blowey, R.W, Boyd, H and Eddy, R.G. (2004) Bovine Medicine (Second edition), Blackwell Publishing
• Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial

Also known as:

Ruminal tympany