Edema:

• Edema is a condition characterized by excessive accumulation of fluid in interstitial tissue spaces or body cavities.
• It may occurs as localized or generalized disorder. Generalized edema with severe form causing swelling of subcutaneous tissue is known as Anasarca.
• Edema may be inflammatory or non-inflammatory in origin.

Terms used in Edema in different body cavities:

• Hydroperitoneum: Accumulation of fluid in peritoneal cavity, generally known as ascites.
• yHydHHydrothorax: Accumulation of fluid in thoracic cavity
• Hydropericardium or pericardial effusion: Accumulation of fluid in pericardial sac
• Hydrocephalus: Accumulation of fluid in ventricles of brain
• Hydrocele: Local accumulation of fluid in tunica vaginalis of testicles.

Etiology:

Edema generally results from four causes:

1. Increased hydrostatic pressure:

• It occurs due to impaired venous return.
• Since the usual cause of this edema is impaired cardiac function, it is called cardiac edema.
• Symmetric ventral edema in CHF, symmetric pulmonary edema in acute heart failure
• Due to low cardiac output, blood supply becomes inadequate and metabolites accumulate in tissue spaces and exert additional attraction of fluid from capillaries. Sodium excretion is reduced and starts to accumulate in ECF resulting water retention.

2. Decreased plasma oncotic pressure:

• Decreased total protein concentration, especially hypoalbuminemia result in symmetric ventral edema.
• Hypoalbuminemia results from increased loss of albumin, decreased production as in chronic hepatic failure and decreased intake of albumin.
• Plasma oncotic pressure decreases in following condition:
  • Chronic blood loss especially in heavy parasitism
  • Protein losing gastroenteropathies as in Johne’s disease, amyloidosis in adult cattle, heavy infestation with nematode parasites in ruminants
  • Glomerulonephropathies; amyloidosis in adult cattle
  • Chronic liver damage causing failure of plasma protein synthesis
  • Prolonged malnutrition with low dietary intake of protein

3. Increased capillary permeability:

• It is seen in following conditions:
  • Endotoxemia
  • Allergic reactions as in urticarial
  • Toxic damage to vascular endothelium or vasculitis- in anthrax, gas gangrene, mulberry heart disease in pigs, equine infectious anemia, equine viral arteritis

4. Obstruction to lymphatic flow:

• It is caused by tumors in lymphatic vessel, granulomatous lesion
• Congenital inherited obstruction in Aryshire breed of calves
• Sporadic lymphangitis of horse (bigleg)
• Edema of lower limb of horses immobilized due to injury or illness

Pathogenesis:

Oedema is result of increase in forces that tends to move fluids from intravascular compartment to interstitial spaces. It is usually due to disturbances in fluid exchange.

Normally fluids are maintained at balance by two opposing forces; vascular hydrostatic pressure and plasma osmotic pressure

Fluids usually moves from intravascular compartment to interstitial space at arteriolar end of microcirculation under influence of hydrostatic pressure

Fluid returns to intravascular compartment at venular end under influence of osmotic pressure

Balance between these forces results in exchange of small amount of fluid in interstitial space which is usually cleared through lymphatics and no edema occurs

Edema results when there is increased hydrostatic pressure or fall in plasma osmotic pressure or obstruction to blood flow

Clinical Findings:

• Edematous swellings are soft, painless and cool to touch.
• Edema pits on pressure.
• In case of ascites, there is distension of abdomen, on percussion fluid thrill wave can be detected
• In case of pulmonary edema, there is difficulty in breathing and lung collapses.
• In case of pericardial effusion, muffled heart sounds can be heard on auscultation.
• Cerebral edema is manifested by neurologic signs with altered mentation.

Laboratory Findings:

• Cytological examination of edema reveals no inflammatory cells if edema results from increased hydrostatic pressure, decreased plasma osmotic pressure
• Blood test reveals decreased albumin or total protein, renal problems

Treatment:

• Treatment of edema is aimed at removing primary causes of edema.
• Edema associated with CHF needs to be treated with diuretics, digitalis
• Hypoalbuminemia requires administration of plasma substitute or plasma but it is expensive measures.
• Parasitic infestation requires administration of broad-spectrum anti-helmintics
• Obstructive edema requires removal of physical causes through surgical intervention, if required.
• Animals should be fed salt-free diet.
• Abdominocentesis is required in case of ascites.
• Diet containing protein of high biological value should be fed to animals.
• Administration of supplements such as vitamins, cobalt, iron, copper in case of blood loss.
• Antihistaminics or corticosteroid should be administered in case of allergic edema.