Peripheral Edema, Pleural Effusion, and Ascites

Edema is an abnormal accumulation of extracellular fluid in the interstitial spaces of the tissues or in body cavities that can be generalized or localized. If the fluid accumulation occurs in the pleural cavity, it is referred to as pleural effusion or hydrothorax; if the fluid accumulation is in the abdominal cavity, it is referred to as peritoneal effusion or ascites.

Fluid accumulates more easily in those parts of the body where the connective tissue structure is relatively loose. The accumulated fluid tends to gravitate to the dependent areas of the body. In the cow, generalized edema is detected externally by swelling of the submandibular tissue, the brisket, the ventral abdomen, and occasionally the limbs (Fig. 6.1). External manifestation of general­ized edema in the horse is frequently in the pectoral region between the front limbs, along the ventral abdomen, in the prepuce in stallions and geldings (Fig. 6.2), in the limbs, and sometimes in the head. Stocking up, or limb edema restricted to the lower limbs, is commonly detected in stabled horses with no underlying disease. Large amounts of fluid may accumulate before clinical signs become evident. External evidence of pulmonary edema (i.e., a frothy, possibly blood-tinged fluid in the nares or expec­torated) is rarely detected in large animals (Fig. 6.3). Edema has numerous causes including congestive heart failure (CHF) (Boxes 6.1 and 6.2). Edema is a late sign of CHF; other subtle signs of failure may be present before edema appears.

Mechanisms of Edema

Edema is caused by an alteration in the equilibrium between capillary permeability and the forces that govern fluid move­ments at the capillary level. These forces are as follows:

1. Intravascular hydrostatic pressure

2. Interstitial fluid hydrostatic pressure, which exerts coun­terpressure to keep fluid within the capillary

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3. Intravascular colloid oncotic pressure exerted by plasma proteins, which favors the resorption of interstitial fluid; the major determinant of colloid osmotic pressure in the capillary is albumin

4. Interstitial fluid colloid osmotic pressure exerted by some proteins in the interstitial fluid, which resists resorption of fluid from the interstitial space

5. Vascular surface area capable of fluid transport

6. Vascular permeability to proteins and water

Activation of complement and liberation of cytotoxic agents such as oxygen radicals, leukotrienes, hydrogen peroxide, platelet-activating factor, and lysosomal enzymes contribute to the endothelial and epithelial damage, causing permeability edema. Subsequent increase in colloid osmotic pressure causes fluid accumulation in the interstitial space. The most common causes of increased capillary permeability are trauma, infection, endotoxemia, and hypersensitivity (allergic) vasculitis. Topical administration of counterirritants can also cause local increase in capillary permeability. Equine purpura hemorrhagica, the most common vasculitic disease in horses, may in its mildest form have symptoms of mucosal petechiae and plaques of edema or, in severe cases, serum exudation from and necrosis of skin surfaces.

Increased hydrostatic pressure can cause either localized or generalized edema. In horses and ruminants the most common causes of increased hydrostatic pressure are CHF, venous thrombosis, liver disease causing obstruction of the portal venous system, lymphadenopathy, a cranial mediastinal mass, compression bandages, limb immobilization, and topical administration of counterirritants. CHF occurs when there is concomitant pul­monary and systemic vascular congestion. The compensatory salt and water retention increases ventricular diastolic, venous, and capillary pressures, which can result in the formation of generalized edema.

When the plasma protein concentration decreases from normal to values pericardial effusion

Chronic right-sided or biventricular heart failure due to cardio­myopathy, infectious myocarditis, ionophore toxicity

Starvation

Hemodilution

Copper deficiency

Vasculitis

Trauma

Caudal vena caval thrombosis

Anaplasmosis

Gossypol toxicity

Cassia occidentalis

Phalaris species toxicity

Oxytropis (locoweed) toxicity

B, Bovine.

(Ca, P, Na, K, Cl) concentrations, fractional excretion of electrolytes (see Chapter 34 for method), complete urinalysis; serum concentrations of Na may be decreased, and SUN may also be elevated in chronic heart failure

c. Liver enzyme concentrations and liver function (aspartate aminotransferase [AST], sorbitol dehydrogenase [SDH], alkaline phosphatase, γ-glutamyltransferase [GGT], bilirubin, and bile acid concentrations); values may be elevated in CHF

d. Check for antibodies to equine infectious anemia virus by agar gel immunodiffusion (AGID) or Coggins test

e. Check for antibodies against Streptococcus equi M protein if signs are compatible with purpura hemorrhagica

f. Determine titers to equine viral arteritis by serum neutralization if there is ocular inflammation, nasal discharge, abortion, or fever with edema

g. A. phagocytophilum antibodies as demonstrated by immu­nofluorescence if physical examination reveals icterus and fever with edema

h. Vitamin E levels if cardiomyopathy suspected

5. Perform two-dimensional (2D), M-mode, and Doppler echocardiography if there is a cardiac murmur that is not localized to the left heart base, that radiates, is greater than 2/6 intensity, or is of significant duration.

6. Analyze fluid to rule out peritonitis, pleuritis, and pericarditis; if physical examination and CBC findings are compatible, examine:

a. Pleural fluid

b. Pericardial fluid

c. Peritoneal fluid

7. Isolate virus from nasopharyngeal swabs, buffy coat, or semen for equine viral arteritis if clinical signs are compatible.

8. Oral D-xylose or glucose absorption test should be done in horses if there is hypoproteinemia and if starvation, renal disease, hepatic disease, pleuritis, peritonitis, infectious gastrointestinal disease, and hemodilution have been ruled out; these tests are not useful in ruminants unless intraab- omasal instillation of D-xylose or glucose is accomplished.