The three weeks before and after calving are known as the periparturient period (transition period) in dairy cattle ^[1][2][1,2]. The periparturient period is characterized by negative energy balance, low dry matter intake, metabolic stress [3], physiological and hormonal changes, increased oxidative stress, immune suppression, and abnormal regulation of inflammation in dairy cattle ^[4][5][6][4,5,6]. During the transition period, excessive lipid mobilization, physiological changes, oxidative stress, inflammation, and immune dysfunction are key biological function processes that expose dairy cattle to various economic diseases, including mastitis ^{[7][8][9][10]}[7,8,9,10].

Dry matter intake and energy demands decrease to maintain successful lactation in dairy cattle. Moreover, more oxygen utilization is required for cellular respiration, leading to oxidative stress ^[11][12][13][11,12,13]. In addition to fulfilling energy requirements, dairy cattle utilize the fat of their adipose tissues. Excessive fat mobilization may lead to overproduction of reactive oxygen species (ROS) and reactive nitrogen species (RNS) [14]. High concentrations of nonesterified fatty acids (NEFA) [15] and β-hydroxybutyric acid (BHB) are among the critical factors that also provoke ROS overproduction [16]. In addition, a high body condition score (BCS) (body condition score > 3.5/5) is another key factor in determining susceptibility to oxidative stress (OS) during the periparturient phase in dairy cattle [17]. Consistently, it has been documented that excessive loss of BCS causes high concentrations of NEFA and overproduction of OS [18].

Se, vitamin E, and folic acid (B9) have been targeted to improve metabolism, relieve oxidative stress, and enhance the immunity and anti-inflammatory status of perinatal dairy cattle ^[19][20][21][23,24,25].

Oxidative stress is considered one of the primary variables during the periparturient period, which is associated with the susceptibility of dairy cattle to various diseases such as mammary edema and mastitis ^{[12][13][16][22][23][24][25]}[12,13,16,33,41,42,43]. Increased oxidative stress compromises immunity and elevates the susceptibility of dairy cattle to mastitis during the perinatal phase ^[22][26][33,44]. Consistently, Aiken et al. reported that the expression of antioxidant-associated enzymes was significantly low in periparturient dairy cattle ^[23][41]. Guan et al. documented higher concentrations of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) and a lower concentration of malondialdehyde (MDA) in periparturient cows with low SCC and vice versa ^[27][45], which is supported by other published studies ^[28][29][46,47]. OS is associated with abnormal regulation of immunity and inflammation before and after parturition, which has been proven in dairy cattle to coliform mastitis ^{[30][31][32][33][34]}[48,49,50,51,52].

1.2. Metabolic Alternation and Its Association with Mastitis in Periparturient Dairy Cattle

Negative energy balance and physiological and metabolic changes aggravate immunosuppression in dairy cattle during the periparturient period ^[35][36][53,54]. High milk production stress, hormonal changes, pregnancy burden, and endocrine and metabolic changes may lead to NEB during the periparturient period in dairy cattle ^[37][55]. Negative energy balance triggers the increase in lipid fat mobilization, followed by elevation of NEFA and BHB levels, which promotes the abnormal production of ROS and results in oxidative stress in perinatal dairy cattle ^[27][28][29][45,46,47]. Higher levels of NEFA and BHB are associated with oxidative stress, which disrupts the immune system and enhances the inflammatory status, including lower expression of IL-10, increased blood neutrophil-to-lymphocyte ratio (NLR) and platelet-to-lymphocyte ratio (PLR), and higher levels of IL-6, TNF-α, and p-selectin glycoprotein ligand-1 (PSGL-1) in cattle with the high level of SCC ^[27][45]. Consequently, higher levels of NEFA and BHB also suppress bovine peripheral blood mononuclear cells (PBMCs), disrupt the function of polymorphonuclear leukocytes (PMNLs), and inhibit the production of interferon-γ, which exposes dairy cattle to mastitis during the periparturient period ^[38][39][40][56,57,58]. Consistently, other studies have also documented that a high concentration of NEFA may cause severe inhibition of interferon-gamma (IFN-γ) levels, which has antibacterial activity, thus increasing the chances of mammary infections in periparturient dairy cattle ^[15][19][15,23].

1.3. Association of Immune System Suppression and Increased Inflammatory Status with Mastitis in Periparturient Dairy Cattle

Hormonal, digestive, and immunological changes in periparturient dairy cattle interfere with immune function, causing immunosuppression and subsequent mastitis ^[41][42][43][59,60,61]. The periparturient period is regarded as an infection-prone period for dairy cattle due to immunosuppression ^[44][45][62,63]. Moreover, during the first week of lactation, the bactericidal ability of phagocytes and blood PMNLs is significantly reduced, which predisposes dairy cattle to mammary gland infections ^[46][64]. C

2. Role of Se and Vitamins E and B9 in Overcoming Oxidative Stress, Immune Suppression, Metabolic Imbalance, and Inflammatory Status Associated with Mastitis

2.1. Antioxidant Properties of Se and Their Role in Mastitis Alleviation in Periparturient Dairy Cattle

It has been reported that the reduction of hydrogen peroxide and lipid hydroperoxides is catalyzed by plasma glutathione peroxidase (GPx-3), which is a selenocysteine-containing extracellular antioxidant protein ^[47][69]. In addition, GPx plays a key role in the antioxidant defense system of dairy cattle ^[48][70]. Milk lactoserum has shown undefined antibacterial activity in Se-supplemented dairy cattle ^[49][71]. Although the exact mechanism for this antimicrobial activity remains unknown, an increased level of GSH-Px has been found in the blood of dairy cattle treated with Se ^[50][51][72,73]. To prevent oxidative stress, Se has been reported to regulate key antioxidant-associated genes (TOAX, GPX, CAT, SOD, and GSH) ^[52][74]. Consistently, our previously apublished studies have experimentally proved that superoxide dismutase-1 (SOD1) and catalase (CAT) protect cells from oxidative damage induced by heat stress in bovine granulosa cells ^[53][54][75,76]. Due to its immunological and antioxidant properties, Se has been widely targeted in mastitis control research in dairy cattle. Mastitis is more common in high-yielding periparturient dairy cattle due to oxidative stress, which promotes changes in the expression of genes associated with proinflammatory factors ^[23][41]. Miranda et al. ^[55][77] reported that low levels of Se and GPx activity promote oxidative stress in the mammary gland, which is associated with a decrease in mammary epithelial cell numbers. However, in mammary epithelial cells, a balanced level of Se lowers the concentration of hydrogen peroxide ^[55][77]. Thus, by lowering hydrogen peroxide levels in mammary epithelial cells, the oxidative status can be eased, resulting in a reduction in apoptotic cells. Twelve out of twenty-five selenoproteins in animals have shown strong immunological and antioxidant ability ^[56][57][85,86], suggesting that they could be useful options in preventing mastitis in dairy cattle ^[58][87]. The level of Se in dairy cattle is associated with the sensitivity of the mammary gland to bacteria ^[59][78]. Ali-Vehmas et al. ^[60][88] have documented that Se treatment significantly enhances the antibacterial activity, and GSH-Px level of milk. Moreover, the SCC level and mastitis-causing bacteria such as Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Streptococcus agalactiae, and Streptococcus uberis (S. uberis) growth were significantly reduced in response to Se treatment in the milk of dairy cattle ^[60][88]. Oxidative stress was restricted by enhancing the level of GSH-Px by selenium in dairy cattle ^[61][62][89,90]. Moreover, it has been documented that Se supplementation reduces the chances of udder infections in dairy cattle ^[61][89].

2.2. Role of Folic Acid (Vitamin B9) in Mastitis Alleviation of Periparturient Dairy Cattle

In recently published studies, it has been well studied that folic acid plays a key role in the metabolism [2], enhanced immunity, and antioxidant status of periparturient dairy cattle ^[19][23]. During the periparturient period, folic acid deficiency may compromise the immunity of dairy cattle ^[19][23]. With a key role in immunity and anti-inflammation, folic acid has been targeted in bovine mastitis alleviation in periparturient dairy cattle ^[63][64][65][27,112,113]. Folic acid supplementation by suppressing MAPK and NF-κB activation maintains the anti-inflammatory status and prevents mastitis ^[65][113]. Consistently, a study documented that folic acid supplementation (120 mg/500 kg body weight) for 21 days downregulated all genes associated with immune function and inflammation (PIM1, SOCS3, ATP12A, KIT, LPL NFKBIA, DUSP4, ZC3H12, ESPNL, TNFAIP3) ^[19][23] that were found to be upregulated in S. aureus-induced mastitis during the periparturient period in dairy cattle ^[66][67][114,115]. In addition, in our previous study, we found that folic acid supplementation significantly regulated glutathione metabolism signaling and its related genes (LAP3, GSR, G6PD, GSTA4, GCLC, GPX3, PGD, IDH1, GGT1, GPX7, MGST1, and MGST2) in periparturient dairy cattle [2]. Furthermore, itwe documented that folic acid could enhance the antioxidant ability of dairy cattle and improve their resistance to mammary gland infection during the periparturient period. Consistently, Mi et al. ^[68][26] recently proved experimentally that S. aureus induced mastitis in MAC-T cells by downregulating the expression of progenitor renewal associated noncoding RNA (PRANCR). However, in folic-acid-treated MAC-T cells, the expression was higher, showing that folic acid could be the best therapeutic agent in mastitis prevention ^[68][26]. Treatment with 5 μg/mL FA significantly reduced apoptosis in Mac-T cells and produced a strong defense against MRSA treatment by improving cytosolic DNA sensing and tight junction signaling ^[63][69][27,116]. They found the upregulation of ZBP1, IRF3, IRF7, and IFNAR2 within the cytosolic DNA-sensing pathway in FA-treated MAC-T cells. ZTP1 was reported to be associated with milk SCC ^[68][26] and also plays a key role in the activation of anti-pathogen mechanisms and inflammation ^[70][71][117,118].

2.3. Role of Vitamin E in Periparturient Dairy Cattle Mastitis Alleviation

Vitamin E, a fat-soluble vitamin, protects the cell membrane from the action of lipid peroxidation chain reaction ^[72][73][119,120]. Consistently, a study documented that the polyunsaturated fatty acids in cell membranes of immune cells made them vulnerable to the action of lipid peroxidation by ROS ^[72][74][119,121]. In brief, vitamin E neutralizes peroxyl radicals and prevents polyunsaturated fatty acid oxidation (PUFA). Peroxyl radicals react with α-tocopherol instead of lipid hydroperoxide in the presence of vitamin E, halting the chain reaction of peroxyl radical generation and preventing further oxidation of PUFAs in the membrane ^[75][122]. A considerable decrease in the level of vitamin E has been observed during the transition period in dairy cattle ^[76][135]. It has been reported that vitamin E supplementation enhances the anti-inflammatory ability, immunity, and antioxidative ability and reduces mammary infections during the perinatal period in dairy cows ^[77][136]. Vitamin E in combination with Se has shown more profitable outcomes in preventing intramammary infections, including mastitis, in dairy cattle ^[78][79][141,142]. Morgante and coauthors ^[79][142] also found that somatic cell counts in milk were considerably reduced in response to vitamin E and Se therapy, demonstrating their efficacy in preventing mastitis. Parenteral vitamin E (2100 mg) injections for two weeks before and on calving day have consistently been shown to reduce the incidence of mastitis in dairy cows ^[79][142].