Inflammation of the mammary gland. This condition is most frequently caused by infection of the gland with bacteria that are pathogenic for this organ. It has been described in humans, cows, sheep, goats, pigs, horses, and rabbits. Mastitis causes lactating women to experience pain when nursing the child, it damages mammary tissue, and the formation of scar tissue in the breast may cause disfigurement.

The mammary gland is composed of a teat and a glandular portion. The gland has defensive mechanisms to prevent and overcome infection with bacteria. Nonspecific defense mechanisms include teat duct keratin, lactoferrin, lactoperoxidase, and complement. Specific defense mechanisms are mediated by antibodies and include opsonization, direct lysis of pathogens, and toxin neutralization. Milk contains epithelial cells, macrophages, neutrophils, and lymphocytes. To induce mastitis, a pathogen must first pass through the teat duct to enter the gland, survive the bacteriostatic and bactericidal mechanisms, and multiply. Bacteria possess virulence factors such as capsules and toxins which enable them to withstand these protective mechanisms.

When bacteria multiply within the gland, there is a release of inflammatory mediators and an influx of neutrophils. The severity of mammary infection is classified according to the clinical signs. In humans, infection occurs during lactation, with clinical episodes most frequent during the first 2 months of lactation. In acute puerperal mastitis the tissue becomes hot, swollen, red, and painful, and a fever may be present. In the absence of treatment, this may progress to a pus-forming mastitis, with the development of breast abscesses.

In acute puerperal mastitis of humans, suitable antibiotics are administered by the intravenous or intramuscular route, while in the abscess form surgical drainage is provided in addition to antibacterial therapy. Penicillins, cephalosporins, and ery-thromycin are administered locally into the infected gland after milking for 1–2 days. Additional antibiotics are given systemically, that is, intravenously or intramuscularly, in severe cases of mastitis, and also to improve bacteriologic cure rates.