Reproduction in Domestic Animals

Mastitis in Post-Partum Dairy Cows 253No of cows700600500400300200100Fig. 1. Distribution of the timefrom calving (days in milk) to thediagnosis of clinical mastitis, basedon data from 28 dairy herds inNew Zealand (McDougall et al.2007)0–7 7 21 35 49 63 77 91 105 119 133 147 161 175 189 203 217 231 246Time from calvingCalvingCellular FactorsPolymorphonuclear neutrophils (PMN) belong to theinnate immune response of the mammary gland toinfections (Paape et al. 2003; Burvenich et al. 2007).Around parturition, many functions of the PMN areimpaired. The number of immature neutrophils incirculation increases and the number of mature neutrophilsin the blood and milk are at the lowest. Theproduction of reactive oxygen species (ROS) to killbacteria is reduced from 1 week before parturition overthe first 2 weeks after calving (Hoeben et al. 2000;Mehrzad et al. 2002). The change in the respiratoryburst activity has been found to be parallel withperipartum increase of concentrations of 3b-hydroxybutyricacid (3-BHB), bovine pregnancy-associated glycoproteinand bilirubin (Hoeben et al. 2000). The rapidrise of the concentration of blood cortisol induceschanges in the function of PMN, supporting theirextended life span in the blood and increasing releasefrom the bone marrow. At that time, neutrophils favourtissue remodelling over defence against infections astheir primary task (Burton et al. 2005). At parturition,large numbers of leucocytes are recruited to the reproductivetract and placenta. The ability of the cells tomarginate on and migrate through endothelium toinfected peripheral tissue in other sites than uterusdecreases. After parturition, cortisol down-regulates itsown receptors in neutrophils and the system returns tonormal function (Burton et al. 2005). The effect ofsteroid hormones on bovine PMN function was studiedby Lamote et al. (2004), who showed that 17b-oestradioltreatment decreased the number of viable cells butprogesterone had no effect. In periparturient cows, a lossof expression of critical neutrophil adhesion moleculeshas been seen (Monfardini et al. 2002), and this loss hasbeen associated with the elevated cortisol levels (Weberet al. 2004). The same was demonstrated by externalglucocorticoid administration (Burton et al. 2005). Theproportion of PMN expressing, for example, the adhesionreceptor L-selectin, which is necessary for penetrationto the sites of infection, is diminished (Diez-Fraileet al. 2004). Neutrophil extracellular traps have recentlyshown to have a role in killing of bacteria and also to befully capable to function in the milk environment. Theimpaired efficiency of this system during the periparturientperiod may be one more explanation for theimmunosuppression of the dairy cows at that time(Lippolis et al. 2006).Lymphocytes can recognize antigens through specificreceptors and are divided to T and B lymphocytes.CD4+ T lymphocytes cells activate lymphocytes ormacrophages to secrete cytokines, which then canfacilitate either cell-mediated or humoral immuneresponse (Sordillo 2005; Rainard and Riollet 2006).The proportion of CD4+ cells in blood and mammarygland declines post-partum, and their cytokine productionis different from that in mid-lactating cows. Inperiparturient cows, the percentage of T cells has beenshown to substantially decline from that in mid-lactatingcows (Shafer-Weaver et al. 1996). Macrophages are thedominant cell type in milk of healthy, lactating gland.During infection, macrophages initiate the immuneresponse by releasing cytokines and other substancesaugmenting local inflammatory process (Rainard andRiollet 2006). Bovine macrophage numbers are highestin the mammary gland during the last week of gestation,but their phagocytic capacity is decreased (Sordillo2005).Humoral FactorsInnate and specific soluble factors represent an importantpart of the defence in the mammary gland;complement, lactoferrin, lysozyme and antimicrobialpeptides are the most common (Rainard and Riollet2006). Lactoferrin is most active during the steady stateof involution (Smith and Schanbacher 1977). Duringthat time, lactoferrin efficiently prevents growth ofbacteria with a high demand of iron such as coliforms(Todhunter et al. 1991). Complement is present in highconcentrations in colostrum, and seems not to be alimiting factor in the defence of the mammary gland atthat time (Rainard 2003). The most important factors ofthe specific immune response are opsonizing immunoglobulins(Ig) produced by antigen-activated B lymphocytes.IgG 1 is the primary isotype present in the healthymammary gland, but IgG 2 increases during inflammation.The concentration of IgGs in the bovine serum islower around parturition and in particular, the lack ofÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag