Drugs for Specific Purposes - The Merck Veterinary Manual

Drugs for Specific Purposes

Esophageal Obstruction

Ruminotorics

Ruminal Fluid Transfer

Antifoaming Agents

Ruminoreticular Antacids

Ruminoreticular Acidifying Agents

Modulators of Ruminoreticular Motility

Esophageal Obstruction:

Esophageal obstruction due to a foreign body ( Esophageal Obstruction (choke): Overview) leads to severe discomfort and acute free-gas bloat. Physical removal of the object may be hampered by marked spasm of the surrounding muscle. Specific spasmolytic drugs such as acepromazine may be used. Alternatively, the moderate sedative and muscle relaxant effects of a low dose of xylazine (0.05 mg/kg, IM in cattle) aid the removal of obstructions.

Ruminotorics:

Agents and mixtures that promote forestomach function (fermentation and motility) are known as ruminotorics. Formulations that contain glucogenic substrates, minerals, cofactors, and bitters (eg, nux vomica) have limited application in current therapy of ruminoreticular indigestion. Generally, restoration of the normal ruminoreticular environment using a physiologic approach is much more satisfactory. The mild laxative and antacid effects of magnesium hydroxide may be beneficial in those animals that do not already have alkaline ruminoreticular fluid.

Mineral oil (1-2 L) or dioctyl sodium sulfosuccinate (DSS, 90-120 mL in 1-2 L of water) administered PO or via nasogastric tube followed by gentle ruminal massage can be helpful in promoting the dissolution and passage of impacted fibrous ruminal contents. DSS can markedly depress rumen protozoa; thus, ruminal transfaunation should follow the use of this agent if ruminal hypomotility continues.

Ruminal Fluid Transfer:

Fresh ruminal fluid is considered to be the best available “ruminotoric” because it contains viable ruminal bacteria (1 × 10⁸-10¹¹ /mL) and protozoa (1 × 10⁵ -10⁶/mL) as well as many useful fermentation factors (volatile fatty acids, microbial protein, minerals, vitamins, buffers). Strained fresh ruminal juice (at least 3 L; 8-16 L is ideal in cattle; sheep require ~1 L) given PO or by tube is indicated in cases of ruminoreticular stasis. Ruminal fluid can be aspirated through a stomach tube from the ruminoreticulum of healthy animals using an extractor pump or by siphoning, or it can be collected at slaughterhouses. A rumen-cannulated donor animal is particularly convenient. It is best for the donor to be on a ration similar to that of the recipient because the ruminal microflora will then be more appropriately adapted. Provided the initiating condition or lesion is responding favorably, improvement almost invariably follows the reestablishment of normal ruminal microflora, with consequent normalization of the fermentation process and ruminoreticular motility. When the ruminoreticular contents are putrified, ingesta must first be removed prior to transfer of fresh ruminal fluid. This can be accomplished using a large-bore stomach tube or by performing a ruminotomy. (See also Digestive System Introduction: Introduction .)

Antifoaming Agents:

The therapeutic approaches to the control of acute frothy bloat involve the administration of antifoaming agents to reduce foam stability and to promote release of free gas, which is then promptly eructated. (See also bloat, Bloat .)

Acute frothy bloat in cattle should be treated with poloxalene, which may be administered as a drench or by stomach tube (25-50 g). Frothy bloat can be prevented by administering poloxalene as a top dressing to feed (1 g/45 kg body wt/day) or in a molasses block (1.5 g/45 kg body wt/day). The direct intraruminal injection of poloxalene often does not give satisfactory results. Polymerized methyl silicone (3.3% emulsion [cattle: 30-60 mL; sheep: 7-15 mL]) may be used in a similar manner as poloxalene, although direct intraruminal injection via a needle or cannula may be more satisfactory in this case. Administration of docusate sodium in emulsified soybean oil (6-12 fl oz containing 240 mg/mL) or administration of vegetable oils alone, such as peanut oil, sunflower oil, or soybean oil (cattle: 60 mL; sheep: 10-15 mL), also relieve acute frothy bloat when given PO. The incidence of frothy bloat in feedlot cattle may be reduced by including ionophores (such as monensin) in the ration or administering these as controlled-release capsules.

Ruminoreticular Antacids:

Ruminal alkalinizing agents are principally used to treat ruminal lactic acidosis (pH <6) due to grain engorgement or soluble carbohydrate overload. (See also subacute ruminal acidosis, Subacute Ruminal Acidosis.) The resultant systemic dehydration and acidosis necessitate immediate correction of fluid and electrolyte balance and restoration of a viable microbial population. Often, the latter involves removal of ruminoreticular contents and replacement with fresh ruminoreticular fluid. Antacids that may be given PO, bid-tid, include magnesium hydroxide (cattle: 100-300 g; sheep: 10-30 g) and magnesium carbonate (cattle: 10-80 g; sheep: 1-8 g). Antacids should be mixed in ~10 L of warm water to ensure adequate dispersion through the ruminoreticular contents. Administration PO of activated charcoal (2 g/kg) is believed to protect the ruminoreticular mucosa from further injury by inactivating toxins.

Ruminoreticular Acidifying Agents:

Ruminal acidifying agents are used to treat ruminal stasis or simple indigestion as well as acute ammonia poisoning. In ruminal stasis, the intraruminal pH often increases to >7.5 because of the constant inflow of bicarbonate-rich saliva in the absence of active ruminal fermentation and formation of volatile fatty acids. In acute ammonia intoxication, the elevated intraruminal pH increases the activity of urease and facilitates the absorption of free ammonia (pK_a of ammonium is 9.1). Administration of weak acids in cold water returns the pH of ruminoreticular content toward physiologic levels, promotes the uptake of volatile fatty acids, depresses the absorption of ammonia, and inhibits excessive urease activity. Acetic acid (4-5%) or vinegar (cattle: 4-8 L; sheep: 250-500 mL) is the most common acidifying agent used.

Modulators of Ruminoreticular Motility:

A number of factors may exert a detrimental influence on forestomach motility and result in ruminoreticular hypomotility or atony. The most effective strategy for reestablishing motility is to restore the normal ruminoreticular environment, which often requires transfaunation. The use of parasympathomimetic agents (eg, neostigmine, physostigmine, carbachol, or bethanechol) is seldom appropriate. All these drugs have cholinergic effects, which are potentially hazardous. Neostigmine (cattle: 0.02 mg/kg, SC; sheep: 0.01-0.02 mg/kg, SC) generally produces the fewest side effects but tends to increase frequency, rather than strength, of ruminoreticular contractions. This is particularly true in ruminal atony. The stimulatory effect of neostigmine is not always reliable, and some inhibition of motility can be seen. This may be due to the adrenergic component associated with ganglion stimulation by cholinergic agents. Metoclopramide has been reported to be useful in correcting disorders of ruminoreticular motility, but few definitive studies are available; it does produce powerful abomasal contractions.

Conditioned responses to the presence of feed and feeding itself are 2 physiologic means by which ruminoreticular motility can be notably enhanced.

Several pharmacologic agents such as anticholinergics, adrenergics, opiate analgesics, CNS depressants, and several toxic compounds (eg, cyanide) can result in ruminoreticular paresis.