Etiology:
Edema generally results from four causes:
- 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.
- 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
- 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
- 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