A new protocol for the induction of chronic mastitis with intramammary infusion of lipopolysaccharide (LPS) in Balb/c mice

Mastitis is inflammation of the mammary tissue and is commonly observed in farm animals. The problem causes severe financial losses in the dairy industry in terms of veterinary costs, milk disposal, and treatment expenses. Bacteria are the main actors in the etiology and cause acute and chronic inflammatory changes in the mammary tissue. Acute inflammatory changes are easily recognized clinically, and treatment is initiated immediately, but subacute inflammation progresses insidiously and leads to chronic inflammation with irreversible fibrotic changes. Standardized experimental models for the induction of acute mastitis in laboratory animals are available. Usually, infusion of bacteria or some bacterial structural components into mammary tissue is easily applied for this purpose. However, there are few studies on the induction of chronic mastitis with fibrotic changes, and the applications are relatively complex. In this study, LPS was infused through the teat duct three times on days 0, 5, and 10 to induce chronic mastitis in mice. Tissues were sampled on days 1, 6, and 15 to evaluate histopathological changes. While severe neutrophil infiltrates, a component of acute inflammation, were observed on day 1, lymphocyte infiltrates increased on day 6, consistent with subacute inflammation. On day 15, lesions representing chronic mastitis, such as fibrosis and lymphocyte infiltration, were observed. A model similar to the lesions in chronic mastitis of dairy cattle was successfully and easily established by LPS infusion in mice.