Etiology and Pathogenesis:
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.
In primary ruminal tympanum, 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 6.0. 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 if 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, 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). 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 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 that is 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 reduction 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.
Clinical Findings
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. Occasionally, vomiting occurs. 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. 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 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 distension 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 due to expulsion of blood from the organ.
