Painful inflammation of the mammary glands, or mastitis, is common among cows raised for their milk, and it is one of dairy farms’ most frequently cited reasons for sending cows to slaughter. There are about 150 bacteria that can cause the disease, one of which is E. coli.(12) Symptoms are not always visible, so milk’s somatic cell count (SCC) is checked to determine whether the milk is infected. Somatic cells include white blood cells and skin cells that are normally shed from the lining of the udder. As in humans, white blood cells—also known as “pus”—are produced as a means of combating infection. The SCC of healthy milk is below 100,000 cells per milliliter; however, the dairy industry is allowed to combine milk from all the cows in a herd in order to arrive at a “bulk tank” somatic cell count (BTSCC).(13) Milk with a maximum BTSCC of 750,000 cells per milliliter can be sold.(14) A BTSCC of 700,000 or more generally indicates that two-thirds of the cows in the herd are suffering from udder infections.(15)

Studies have shown that providing cows with cleaner housing, more space, and better diets, bedding, and care lowers their milk’s SCC as well as their incidence of mastitis.(16) A Danish study of cows subjected to automated milking systems found “acutely elevated cell counts during the first year compared with the previous year with conventional milking. The increase came suddenly and was synchronized with the onset of automatic milking.”(17) Instead of improving conditions in factory farms or easing cows’ production burden, the dairy industry is exploring the use of cattle who have been genetically manipulated to be resistant to mastitis.