Ruminal tympany

What it is

Ruminal fermentation continuously produces large volumes of gas (carbon dioxide and methane), which are removed by eructation. To eructate, the gas must collect in the dorsal sac of the rumen and the cardia must be clear of fluid and foam. Bloat appears when this mechanism fails:

• Frothy (primary) bloat: the gas is trapped in small bubbles within a stable foam that fills the rumen and covers the cardia, so the eructation reflex is not triggered. On pasture it is caused by legumes in the vegetative stage, such as alfalfa (lucerne) and clovers, which are rich in soluble proteins that stabilise the foam. In the feedlot, it is caused by diets rich in rapidly fermented grain, in which bacteria produce a slime that increases the viscosity of the rumen fluid.
• Free-gas (secondary) bloat: the gas separates from the contents, but the animal cannot expel it because of a physical or functional failure of eructation. Causes include oesophageal obstruction by foreign bodies (tubers, fruit), external compression of the oesophagus (abscesses, enlarged mediastinal lymph nodes), vagal indigestion from reticular adhesions or abscesses, tetanus, hypocalcaemia and lateral recumbency.

The distended rumen pushes the diaphragm into the thorax and compresses the large abdominal vessels: death is due to respiratory failure and reduced venous return.

Who it affects

• Pasture frothy bloat: cattle grazing alfalfa- or clover-dominant pastures, especially when they go onto pasture hungry or when the forage is young and wet. Some animals are more susceptible than others, and susceptibility has a heritable component.
• Feedlot frothy bloat: feeder calves and finishing cattle on concentrate-rich, low-fibre rations.
• Free-gas bloat: animals of any age and system with a cause that prevents eructation; it can be an isolated, recurrent case.

Anatomy involved

• Rumen: the dorsal sac, in contact with the left paralumbar fossa, is where gas collects; this is why distension is seen first in the left flank. In severe cases the whole abdomen is distended.
• Oesophagus: the exit route for the gas. Obstruction in the neck or thorax prevents eructation and is a common cause of free-gas bloat; it is also the route for passing the stomach tube for diagnosis and treatment.
• Reticulum: the cardia opens at the junction of the reticulum and the atrium of the rumen. Reticuloruminal contractions controlled by the vagus nerve move the contents and clear the cardia for eructation; adhesions and abscesses from traumatic reticuloperitonitis can damage the vagus and cause recurrent bloat.

Clinical signs

Distension of the left flank, which can spread to the whole abdomen; discomfort, kicking at the belly, lying down and getting up frequently, frequent urination and defecation. As it progresses, dyspnoea, open-mouth breathing, tongue protrusion and an extended head appear. Percussion of the left flank gives a tympanic sound. In pasture frothy bloat animals may be found dead without signs having been seen. In free-gas bloat distension is often moderate and recurrent.

Diagnosis

• Clinical diagnosis from left flank distension and the history (change of pasture, diet, several animals affected).
• Stomach tube: if gas escapes easily and the flank deflates, the bloat is free-gas; if little gas escapes and there is foam, it is frothy. The tube also locates an oesophageal obstruction.
• Underlying causes of free-gas bloat: examination of the oesophagus, the reticulum (foreign body tests, ultrasonography) and the thorax.
• Necropsy: in an animal that died of bloat the so-called "bloat line" is seen in the oesophagus (congested cervical portion and pale thoracic portion), which helps distinguish it from distension occurring after death.

Treatment and prevention

• Frothy bloat: remove the animals from the causative pasture or feed and give an oral antifoaming agent (poloxalene, vegetable or mineral oils, or detergents). When death is imminent an emergency rumenotomy is performed; a trocar is of little use because the foam blocks it.
• Free-gas bloat: remove the gas with a stomach tube or, if that is not possible, with a trocar in the left paralumbar fossa, and treat the cause (for example, removing the foreign body from the oesophagus). In recurrent cases a temporary rumen fistula (rumenostomy) can be created.
• Prevention on pasture: introduce animals to legumes gradually and not hungry, offer hay before turnout, use mixed grass-legume pastures or non-bloating legumes (sainfoin, birdsfoot trefoil), and give antifoaming agents daily during the risk season or ionophores such as monensin, where authorised.
• Prevention in the feedlot: enough roughage in the ration, appropriate grain processing and gradual adaptation to concentrate-rich diets.
• Drug use in food animals is subject to each country's regulations and to withdrawal periods for milk and meat.

Key studies

The evidence on bloat is old and limited: it comes mainly from feeding trials with small groups of rumen-fistulated cattle. No controlled trials on the treatment of clinical bloat (stomach tube, trocar, antifoaming agents) were found, so those recommendations rest on clinical experience and reference texts.

• Majak et al. (1995), synthesis of two decades (1973–1993) of trials at a Canadian research station, with rumen-fistulated cattle grazing or fed freshly cut alfalfa. Every alfalfa cultivar tested caused bloat, while sainfoin, birdsfoot trefoil and cicer milkvetch (Astragalus cicer) did not. Bloat was associated with the soluble protein content of alfalfa, which falls as the plant matures, and not with saponins. Cation supplementation did not prevent it and, of all the additives tested, only poloxalene prevented it completely. It is a review of the centre's own trials.
• Gadberry et al. (2022), meta-analysis of the effects of monensin in growing cattle on forage. The bloat analysis included 12 experiments with 23 comparisons: on bloat-prone pastures, monensin reduced incidence by 20% and severity by 0.7 points on the bloat score. It reduces the problem but does not eliminate it.
• Cheng et al. (1998), narrative review of feedlot bloat; it is the standard reference and there are no higher-level studies. It explains that excess rapidly fermented carbohydrate favours acid-tolerant bacteria (Streptococcus bovis, Lactobacillus spp.), that low pH reduces rumen motility and that bacterial slime stabilises the foam. The amount of roughage, grain processing and cereal type, dietary adaptation and additives such as ionophores influence its occurrence.