**1. Introduction**

358 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

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Bovine mastitis is the most prevalent and costly disease, affecting dairy farms worldwide. Economic losses associated with mastitis derive mainly from a decrease in milk production and to a lesser extent, from the culling of chronically infected cows, cost of veterinary treatment, and penalties on milk quality (Seegers *et al.***,** 2003). Mastitis is caused by a wide spectrum of pathogenic agents that penetrate the teat canal and multiply in the udder cistern. The majority of mastitis cases are produced by a relatively small group of bacteria, including *Staphylococcus aureus*, *Streptococcus uberis, Mycoplasma spp* and *Escherichia coli* (Calvinho & Tirante, 2005). Bovine mastitis is characterized by inflammation of the mammary gland. The inflammation severity depends on the causative agent and the host response (Bannerman *et al*., 2004; Barkema *et al*., 2006; Burvenich *et al*., 2003; Petzl *et al*., 2008). Resident and recruited cells together play an essential role in immediate defense against local infection (Rainard & Riollet, 2006). Extensive neutrophil recruitment from the circulation to the lumen of the mammary gland is a hallmark of the early immune response to mammary infection (Thomas *et al.*, 1994; Sordillo & Streicher, 2002; Oviedo-Boyso *et al*., 2007). When designing mastitisprevention and control programs, it is worthy to take on account the adoption level of mastitis-prevention management practices and control programs as well as the etiology of the intramammary infections (IMI), the herd-level prevalence of contagious mastitis pathogens, and the general factors that influence milk production.
