Bloat

Also known as: Ruminal tympany

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 most 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 the rumenal pH and optimise conditions for the formation of froth. Gas present as foam is insufficient to trigger eructation, and so the rumen becomes distended.

Secondary bloat

Secondary bloat or free gas bloat results from any condition that interferes with eructation and includes physical or functional disroders of the oesophagus (e.g.oesophageal obstruction)or the forestomachs. Secondary bloat usually affects a single animal in a group and generally has a more sporadic occurrence.

It may be acute in the case of sudden oesophageal obstruction, caused by a foreign body such as a potato or turnip, or chronic resulting from partial oesophageal obstruction, compression, or from a forestomach motility disorder that interferes with normal eructation.

Causes of secondary bloat:

Physical oesophageal obstruction
Lesions of the oesophagus or oesophageal groove such as tumours, abcessation or infection with Actinobacillus lignieresii.
Prolonged lateral recumbency
Excessive cereal ingestion
Rumenal atony
Inability to eructate due to Tetanus 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 prevalence in New Zealand due to the high level of clover. In the UK occurrence 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 potatoes 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 obvious 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 trochar will release large quantities of gas in the case of secondary bloat. A full clinical exam should then be carried out in order to determine the underlying cause.

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

Post mortem

Post mortem change resulting in rumen distention following other causes of death can complicate the diagnosis of bloat. If the post mortem exam is carried out shortly after death then bloat will be obvious due to distension at the left paralumbar fossa. Other post mortem changes consistent with bloat are:

Pale liver due to compression by the rumen
Rupture of the diaphragm
Inguinal and ventral perineal oedema
Oesophageal "bloat line" at the thoracic inlet.

Treatment

Primary, or frothy bloat is relieved by drenching with an antifoaming agent containing silicone or poloxalene, such as Bloat Guard or Birp, or a vegetable oil mixed with detergent. Clinical signs should improve within one hour, if there is still no resolution after a second dose then alternative diagnoses should be considered.

In peracute cases where the bloat is life threatening an emergency rumenotomy should be performed.

Secondary bloat can be relieved by passage of a stomach tube in most cases. There will be instantaneous relief as the gas is released. In cases where the oesophagus is completely obstructed so that a stomach tube cannot be passed a 5mm diameter trochar and cannula should be passed into the rumen via the left paralumbar fossa.

Prognosis

If treatment is prompt prognosis is good, however the condition is rapidly fatal if left untreated.

Literature Search

Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).

Bloat/ruminal tympany in cattle and/or sheep publications

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