Host responses associated with chronic staphylococcal mastitis in rabbits

• Differences in the number of cells were detected depending on mastitis lesion.
• Immature lesions had greater cellularity and mature lesions had lower T-cell counts in mammary gland and blood.
• A positive correlation was observed between blood monocytes and tissue macrophages.
• Mammary glands infected by S. aureus ST96 had fewer granulocytes compared with infection by the ST121 lineage.
• Mammary glands infected by ST96 had increased lymphocytes compared with infection by the ST121 lineage.

Staphylococcal infection causes substantial economic losses in commercial rabbit production systems, and is associated with a wide variety of lesions, including chronic suppurative mastitis, which mainly affects breeding females. Most chronic staphylococcal infections in rabbits are caused by the ST121 lineage of Staphylococcus aureus, although other less common lineages, such as ST96 can also be involved. The aims of the present study were to characterise the host immune response in natural cases of mastitis in rabbits caused by S. aureus, to evaluate any relationship between peripheral and local immunity and to investigate the effect of different S. aureus genotypes on these immune responses.Adult multiparous female rabbits that were affected with chronic staphylococcal mastitis (n = 204) were enrolled into the study. Histological and immunohistochemical evaluations of mammary glands were undertaken, as well as flow cytometric analyses of blood. S. aureus isolates from the mammary glands were identified by multilocus sequence typing. Differences in the number of infiltrating cells were detected, depending on the type of pathology, with more immature lesions demonstrating greater cellularity, characterised by greater numbers of T lymphocytes, macrophages and plasma cells. A relationship was seen between the cells in blood and mammary tissues, the most notable being the positive correlation between monocytes and tissue macrophages. When glands were infected with ST96 strains, fewer granulocytes (P < 0.01) and greater numbers of B cells (P < 0.01), T cells (P < 0.001), CD4+ T cells (P < 0.001) and CD8+ T cells (P < 0.01) were detected, compared with mammary glands that were infected by ST121 strains of S. aureus.