1. Introduction

Bovine mastitis is a limiting disease of the production in dairy herd, commonly caused by Staphylococcus aureus. The inflammation of the glandular tissue constitutes a mechanism of natural resistance of the mammary gland infection in dairy cattle [1]. The appearance of inflammation in the mammary gland to increase the number of somatic cells (CCS), with an increased proportion of the leukocytes in milk is related to the agents present in the infection and the severity of the inflammatory response in the mammary gland [2, 3]. In addition, the inflammation provokes physical and chemical changes in the milk affecting the quality and its nutritional composition [4]. The significant increase in CCS in milk may be influenced by S. aureus herd infection level which stands out as one of the main contagious pathogens that are capable of producing a persistent infection and chronic inflammation in the mammary gland [5, 6]. The pathogenicity of S. aureus strains are related to the different virulence factors of the agent, considered primary in the development of mammary gland infection. The production of the alpha toxin shows a cytotoxic and cytolytic activity responsible for cases of gangrenous mastitis due to S. aureus [7]; the capsular exopolysaccharide is responsible for interfering with phagocytosis and complement activation [8]. In addition, colonization of udder skin in cows before parturition increases the risk of developing an intramammary postpartum infection, when the S. aureus infection level in the herd is high. The environment and body sites of animals are a frequent source of infection from cows with persistent S. aureus infection [9]. The potential risk for an epidemic colonization by S. aureus in the dairy herd occurs from the skin contamination of the nipple, and the udder lesions present favor the development of intraglandular infection. The first phase of lactation in cows is considered the most risky to new infections by S. aureus in cattle herds [10, 11]. Other environmental pathogens such as Escherichia coli and Streptococcus agalactiae increase the infection risk in fresh cows during postparturition period. The design of the milking parlors has a determining influence on the infection rate in the dairy herd and the elevated milk pipe lines producing large vacuum fluctuations that increase intramammary infection risk in dairy cows [12]. In comparison, the low pipe lines in the milking parlors reduce the proportion of new infections in the mammary glands. The risk of a cross infection increases at the milking time, when milking techniques are inappropriate or the milking equipment malfunctions. These factors increase the bacterial contamination of the nipple affecting the resistance mechanisms of the mammary gland. The reduction of the risk of bacterial colonization in the skin is achieved with the application of the nipple disinfectants, decreasing the skin contamination by S. aureus and other pathogens, without appreciably affecting the physical characteristics of the nipple skin. Keratin in the nipple duct is a natural barrier that reduces bacterial colonization and limits the development of the mammary gland infection [13, 14]. In the presence of mastitis, the chemical composition of keratin changes, and there is an increase in the proportion of polyunsaturated fatty acids and a decrease in the content of short-chain fatty acids [15, 16]. On the other hand, when the keratin is removed experimentally from the teat canal, the rate of infection with Str. agalactiae increases; consequently, there is a high excretion of bacteria in milk and an elevated somatic cell count in milk. This shows the importance of keratin as a natural barrier that contributes to reduce mammary glandular infection [17]. On the other hand, in infection of S. aureus in the mammary gland, significant changes of glandular tissue structure occur in which hyperplasia, stratification, and keratinization of the milk cistern and reduction of the glandular lumen are observed [18]. These changes are caused by the inflammatory

In vitro Evaluation of the Phagocytosis Activity of Neutrophils… DOI: http://dx.doi.org/10.5772/intechopen.83834

reaction and local leukocyte infiltration, with the formation of plaques of cellular debris and keratin present in the amorphous material. S. aureus experimental infection in dairy cows is affected by the lactation stage and the number of milk somatic cells [6]. The S. aureus strain type and the infective dose influence the development and the infection persistence in the mammary gland. In dairy cows with lessthan 4 months of lactation, when the number of somatic cellsisless than 500,000 cells/mL, resistance to infection occurs. In turn, otherstudiesindicate that the pathogenicity of S. aureusstrains contributesin a decisive way to the development of mammary gland infection and its potentialspread in the herd [19]. The persistence of infection by S. aureus in the udder of the cows increasesthe levels of antibodies present in the milk; this correlates positively with the number of lactations and the number of somatic cells in the cows [20]. The diagnosis and timely treatment decrease the evolution of clinical signs and the severity of damage to the mammary gland caused by S. aureus, Str. agalactiae, and Str. uberis, resulting in a reduction in the incidence of bovine mastitisin the dairy herd [21]. The diagnostic tests allow early identification of physical-chemical milk changes caused by mastitis. The detection of agents, antigens, and antibodies favors the identification of an intramammary infection and its clinical evolution in the herd [22]. The diagnosis of the situation of subclinical mastitisin the herd, under a causal model based on clinical diagnosis, establishes an important association between the environmental factors and the causal conditions in the transmission of infectious agentsin the population.In this case,the S. aureus infection is able to affectseriously the udder health and milk production. To evaluate herd infection level and to characterize Staphylococcus aureus phenotypes, dairy cows from small family farmsin the Central Mexico region were studied.
