THE MERCK VETERINARY MANUAL
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Bloat in Ruminants

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Bloat is an overdistention of the rumenoreticulum with the gases of fermentation, either in the form of a persistent foam mixed with the ruminal contents, called primary or frothy bloat, or in the form of free gas separated from the ingesta, called secondary or free-gas bloat. It is predominantly a disorder of cattle but may also be seen in sheep. The susceptibility of individual cattle to bloat varies and is genetically determined.

Death rates as high as 20% are recorded in cattle grazing bloat-prone pasture, and in pastoral areas, the annual mortality rate from bloat in dairy cows may approach 1%. There is also economic loss from depressed milk production in nonfatal cases and from suboptimal use of bloat-prone pastures. Bloat can be a significant cause of mortality in feedlot cattle.

In primary ruminal tympany, or frothy bloat, the cause is entrapment of the normal gases of fermentation in a stable foam. Coalescence of the small gas bubbles is inhibited, and intraruminal pressure increases because eructation cannot occur. Several factors, both animal and plant, influence the formation of a stable foam. Soluble leaf proteins, saponins, and hemicelluloses are believed to be the primary foaming agents and to form a monomolecular layer around gas rumen bubbles that has its greatest stability at about pH 6. Salivary mucin is antifoaming, but saliva production is reduced with succulent forages. Bloat-producing pastures are more rapidly digested and may release a greater amount of small chloroplast particles that trap gas bubbles and prevent their coalescence. The immediate effect of feeding is probably to supply nutrients for a burst of microbial fermentation. However, the major factor that determines whether bloat will occur is the nature of the ruminal contents. Protein content and rates of digestion and ruminal passage reflect the forage's potential for causing bloat. Over a 24-hr period, the bloat-causing forage and unknown animal factors combine to maintain an increased concentration of small feed particles and enhance the susceptibility to bloat.

Bloat is most common in animals grazing legume or legume-dominant pastures, particularly alfalfa, ladino, and red and white clovers, but also is seen with grazing of young green cereal crops, rape, kale, turnips, and legume vegetable crops. Legume forages such as alfalfa and clover have a higher percentage of protein and are digested more quickly. Other legumes, such as sainfoin, crown vetch, milk vetch, fenugreek, and birdsfoot trefoil, are high in protein but do not cause bloat, probably because they contain condensed tannins, which precipitate protein and are digested more slowly than alfalfa or clover. Leguminous bloat is most common when cattle are placed on lush pastures, particularly those dominated by rapidly growing leguminous plants in the vegetative and early bud stages, but can also be seen when high-quality hay is fed.

Frothy bloat also is seen in feedlot cattle, and less commonly in dairy cattle, on high-grain diets. The cause of the foam in feedlot bloat is uncertain but is thought to be either the production of insoluble slime by certain species of rumen bacteria in cattle fed high-carbohydrate diets or the entrapment of the gases of fermentation by the fine particle size of ground feed. Fine particulate matter, such as in finely ground grain, can markedly affect foam stability, as can a low roughage intake. Feedlot bloat is most common in cattle that have been on a grain diet for 1–2 mo. This timing may be due to the increase in the level of grain feeding or to the time it takes for the slime-producing rumen bacteria to proliferate to large enough numbers.

In secondary ruminal tympany, or free-gas bloat, physical obstruction of eructation is caused by esophageal obstruction due to a foreign body (eg, potatoes, apples, turnips, kiwifruit), stenosis, or pressure from enlargement outside the esophagus (as from lymphadenopathy or sporadic juvenile thymic lymphoma). Interference with esophageal groove function in vagal indigestion and diaphragmatic hernia may cause chronic ruminal tympany. This also occurs in tetanus. Tumors and other lesions, such as those caused by infection with Actinomyces bovis, of the esophageal groove or the reticular wall are less common causes of obstructive bloat. There also may be interference with the nerve pathways involved in the eructation reflex. Lesions of the wall of the reticulum (which contains tension receptors and receptors that discriminate between gas, foam, and liquid) may interrupt the normal reflex essential for escape of gas from the rumen.

Ruminal tympany also can be secondary to the acute onset of ruminal atony that occurs in anaphylaxis and in grain overload; this causes a decrease in rumen pH and possibly an esophagitis and rumenitis that can interfere with eructation. Ruminal tympany also develops with hypocalcemia. Chronic ruminal tympany is relatively frequent in calves up to 6 mo old without apparent cause; this form usually resolves spontaneously.

Unusual postures, particularly lateral recumbency, are commonly associated with secondary tympany. Ruminants may die of bloat if they become accidentally cast in dorsal recumbency or other restrictive positions in handling facilities, crowded transportation vehicles, or irrigation ditches.

Bloat is a common cause of sudden death. Cattle not observed closely, such as pastured and feedlot cattle and dry dairy cattle, usually are found dead. In lactating dairy cattle, which are observed regularly, bloat commonly begins within 1 hr after being turned onto a bloat-producing pasture. Bloat may develop on the first day after being placed on the pasture but more commonly develops on the second or third day.

In primary pasture bloat, the rumen becomes obviously distended suddenly, and the left flank may be so distended that the contour of the paralumbar fossa protrudes above the vertebral column; the entire abdomen is enlarged. As the bloat progresses, the skin over the left flank becomes progressively more taut and, in severe cases, cannot be “tented.” Dyspnea and grunting are marked and are accompanied by mouth breathing, protrusion of the tongue, extension of the head, and frequent urination. Rumen motility does not decrease until bloat is severe. If the tympany continues to worsen, the animal will collapse and die. Death may occur within 1 hr after grazing began but is more common ~3–4 hr after onset of clinical signs. In a group of affected cattle, there are usually several with clinical bloat and some with mild to moderate abdominal distention.

In secondary bloat, the excess gas is usually free on top of the solid and fluid ruminal contents, although frothy bloat may be seen in vagal indigestion when there is increased ruminal activity. Secondary bloat is seen sporadically. There is tympanic resonance over the dorsal abdomen left of the midline. Free gas produces a higher pitched ping on percussion than frothy bloat. The distention of the rumen can be detected on rectal examination. In free-gas bloat, the passage of a stomach tube or trocarization releases large quantities of gas and alleviates distention.

Lesions:

Necropsy findings are characteristic. Congestion and hemorrhage of the lymph nodes of the head and neck, epicardium, and upper respiratory tract are marked. The lungs are compressed, and intrabronchial hemorrhage may be present. The cervical esophagus is congested and hemorrhagic, but the thoracic portion of the esophagus is pale and blanched—the demarcation known as the “bloat line” of the esophagus. The rumen is distended, but the contents usually are much less frothy than before death. The liver is pale because of expulsion of blood from the organ.

Usually, the clinical diagnosis of frothy bloat is obvious. The causes of secondary bloat must be ascertained by clinical examination to determine the cause of the failure of eructation.

In life-threatening cases, an emergency rumenotomy may be necessary; it is accompanied by an explosive release of ruminal contents and, thus, marked relief for the cow. Recovery is usually uneventful, with only occasional minor complications.

A trocar and cannula may be used for emergency relief, although the standard-sized instrument is not large enough to allow the viscous, stable foam in peracute cases to escape quickly enough. A larger bore instrument (2.5 cm in diameter) is necessary, but an incision through the skin must be made before it can be inserted through the muscle layers and into the rumen. If the cannula fails to reduce the bloat and the animal's life is threatened, an emergency rumenotomy should be performed. If the cannula provides some relief, an antifoaming agent can be administered through the cannula, which can remain in place until the animal has returned to normal, usually within several hours.

Fig. 1

When the animal's life is not immediately threatened, passing a stomach tube of the largest bore possible is recommended. A few attempts should be made to clear the tube by blowing and moving it back and forth in an attempt to find large pockets of rumen gas that can be released. In frothy bloat, it may be impossible to reduce the pressure with the tube, and an antifoaming agent should be administered while the tube is in place. If the bloat is not relieved quickly by the antifoaming agent, the animal must be observed carefully for the next hour to determine whether the treatment has been successful or whether an alternative therapy is necessary.

A variety of antifoaming agents are effective, including vegetable oils (eg, peanut, corn, soybean) and mineral oils (paraffins), at doses of 250–500 mL. Dioctyl sodium sulfosuccinate, a surfactant, is commonly incorporated into one of the above oils and sold as a proprietary antibloat remedy, which is effective if administered early. Poloxalene (25–50 g, PO) is effective in treating legume bloat but not feedlot bloat. Placement of a rumen fistula provides short-term relief for cases of free-gas bloat associated with external obstruction of the esophagus.

Prevention of pasture bloat can be difficult. Management practices used to reduce the risk of bloat include feeding hay, particularly orchard grass, before turning cattle on pasture, maintaining grass dominance in the sward, or using strip grazing to restrict intake, with movement of animals to a new strip in the afternoon, not the early morning. Hay must constitute at least one-third of the diet to effectively reduce risk of bloat. Feeding hay or strip grazing may be reliable when the pasture is only moderately dangerous, but these methods are less reliable when the pasture is in the pre-bloom stage and the bloat potential is high. Mature pastures are less likely to cause bloat than immature or rapidly growing pastures.

The only satisfactory method available to prevent pasture bloating is continual administration of an antifoaming agent during the risk period. This is widely practiced in grassland countries such as Australia and New Zealand. The most reliable method is drenching twice daily (eg, at milking times) with an antifoaming agent. Spraying the agent onto the pasture is equally effective, provided the animals have access only to treated pasture. This method is ideal for strip grazing but not when grazing is uncontrolled. The antifoaming agent can be added to the feed or water or incorporated into feed blocks, but success with this method depends on adequate individual intake. The agent can be “painted” on the flanks of the animals, from which it is licked during the day, but animals that do not lick will be unprotected.

Available antifoaming agents include oils and fats and synthetic nonionic surfactants. Oils and fats are given at 60–120 mL/head/day; doses up to 240 mL are indicated during dangerous periods. Poloxalene, a synthetic polymer, is a highly effective nonionic surfactant that can be given at 10–20 g/head/day and up to 40 g/head/day in high-risk situations. It is safe and economical to use and is administered daily through the susceptible period by adding to water, feed grain mixtures, or molasses. Pluronic agents facilitate the solubilization of water-insoluble factors that contribute to formation of a stable foam. A pluronic detergent (Alfasure®) and a water-soluble mixture of alcohol ethoxylate and pluronic detergents (Blocare 4511) also are effective but are not approved by the FDA. Ionophores effectively prevent bloat, and a sustained-release capsule administered into the rumen and releasing 300 mg of monensin daily for a 100-day period protects against pasture bloat and improves milk production on bloat-prone pastures.

The ultimate aim in control is development of a pasture that permits high production, while keeping incidence of bloat low. The use of pastures of clover and grasses in equal amounts comes closest to achieving this goal. Bloat potential varies between cultivars of alfalfa, and low-risk LIRD (low initial rate of digestion) cultivars are available commercially. The addition of legumes with high condensed tannins to the pasture seeding mix (10% sainfoin) can reduce the risk of bloat where there is strip grazing, as can the feeding of sainfoin pellets.

To prevent feedlot bloat, rations should contain ≥10–15% cut or chopped roughage mixed into the complete feed. Preferably, the roughage should be a cereal, grain straw, grass hay, or equivalent. Grains should be rolled or cracked, not finely ground. Pelleted rations made from finely ground grain should be avoided. The addition of tallow (3%–5% of the total ration) may be successful occasionally, but it was not effective in controlled trials. The nonionic surfactants, such as poloxalene, have been ineffective in preventing feedlot bloat, but the ionophore lasalocid is effective in control.

Last full review/revision May 2015 by Peter D. Constable, BVSc (Hons), MS, PhD, DACVIM

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