Breast edema in cows is mostly caused by a rapid influx of large amounts of blood supplied to the uterus during the late stages of pregnancy, or by an initial increase in blood pressure in the mammary vein, which prevents the venous and lymphatic systems from regulating accordingly. As a result, a large amount of liquid components seeping from the blood vessels accumulate subcutaneously, leading to mammary gland edema. Symptoms generally have no systemic symptoms and mostly occur in high-yielding cattle, appearing suddenly from one month before delivery to near delivery

The incidence rate and severity of breast edema in pregnant young cows were higher than those in multiparous cows, and the older the young cows were, the more serious the disease became. Breast edema can cause discomfort in animals and lead to various management problems, such as difficulty in mechanical milking, damage to nipples and breasts, and mastitis. When using imported cattle to examine their breasts with ultrasound, it will be found that the breast structure is uneven, and the echo intensity on the ultrasound of imported cattle is not as high as that of normal breasts.

The exact cause of breast edema is still unclear and is likely due to multiple factors. One of the factors is that during late pregnancy, due to pelvic fetal pressure, venous blood flow and lymphatic fluid outflow from the breast are restricted or silted up, or in other words, the blood flow into the breast increases while the blood flow out of the breast does not correspondingly increase, leading to an increase in venous blood pressure; Using imported cows with B-ultrasound Doppler function, abnormal blood flow in swollen cows' breasts can be detected. Another reason may be that during the late stages of pregnancy, there are significant changes in the secretion and relative proportions of steroid hormones in cows, but little is currently known about this aspect.

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