**1.1 Antibiotic use in dairy farms and antimicrobial resistance**

Economic losses due to bovine mastitis is estimated to be \$2 billion in the United States alone [1]. Most studies showed that there is no widespread, emerging resistance among mastitis pathogens [2–4] in dairy farms. Some studies showed that the antimicrobial resistance of mastitis pathogens varies with dairy farms and bacterial species within and among dairy farms [4–9]. However, antimicrobial resistance patterns of human pathogenic bacteria and their resistome in dairy farms might be of significant concern.

On average, starting from calving (giving birth) dairy cow is milked (in lactation) for about 300 days and then dried off (stop milking) for about 60 days before they calve again. Under the ideal dairy farming condition, a dairy cow should become pregnant within 60 days of calving, and the lactation cycle continues (**Figure 1**). The goal of a dry period is to give them a break from milking so that milk-producing cells regenerate, multiply, and ready for the next cycle of lactation. The incidence of intramammary infection (IMI) by bacteria is high during the early dry period and transition periods [10]. In general, for a dairy cow, a transition period, also known as the periparturient period, is a time range from three weeks before parturition (non-milking time) until three weeks after calving (milking time). It is a transition time from non-milking to milking.

Dairy cows are susceptible to mastitis during early non-lactating (dry period) and transition periods [11, 12], especially new infection with environmental pathogens (*Streptococcus* spp. and coliform) are highest during the first two weeks after drying off and last two weeks before calving [13] compared to contagious mastitis pathogens such as *S. aureus* [14]. The incidence of intramammary infection is high during the early dry period because of an absence of hygienic milking practices such as pre-milking teat washing and drying [15], pre- and post-milking teat dipping in antiseptic solutions [16, 17], that are known to reduce teat end colonization by bacteria and infection. An udder infected during the early dry period usually manifests clinical mastitis during the transition period [18] because of increased production of parturition inducing immunosuppressive hormones [19], negative energy balance [12], and physical stress during calving [20].

Cows are naturally protected against intramammary infections during the dry period by physical barriers such as the closure of teat opening by smooth muscle (teat sphincter) and the formation of a keratin plug, fibrous structural proteins (scleroproteins) [21, 22], in the teat canal produced by teat canal epithelium [23]. Keratin contains a high concentration of fatty acids, such as lauric, myristic, and palmitoleic acids, which are associated with reduced susceptibility to infection and stearic, linoleic, and oleic acids that are associated with increased susceptibility to infection. Keratin also contains antibacterial proteins that can damage the cell wall of some bacteria by disrupting the osmoregulatory mechanism [23]. However, the

#### **Figure 1.**

*Antimicrobials usage patterns during the lactation cycle. DIM: Days in milk, yellow star: Peak lactation at 60 DIM, green bars: Energy demand that requires the mobilization of body energy reserve at the expense of losing bodyweight, red bumps showed increased usage of antimicrobials.*

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of dairy cows and farm environments.

In addition to the use of antimicrobials for the prevention and treatment of mastitis and other diseases of dairy cattle, some farms also feed raw waste milk or pasteurized waste milk from antibiotic-treated cows to dairy calves. Feeding of

*Current Status of Antimicrobial Resistance and Prospect for New Vaccines against Major…*

time of teat canal closure varies among cows. Some studies showed that 50% of teat canals were classified as closed by seven days after drying off, 45% closed over the following 50–60 days after drying off, and 5% had not closed by 90 days after dry off [24]. Teats that do not form a plug-like keratin seal are believed to be most susceptible to infection. Infusion of long-acting antimicrobials into the udder at drying-off (dry cow therapy) has been the major management tool for the prevention of IMI during the dry period, as well as to clear IMI established during the

In the United States and many other countries at the end of lactation (at drying off), all cows regardless of their health status, are given an intramammary infusion of long-acting antimicrobials (blanket dry cow therapy) to prevent IMI by bacteria during the dry period [3, 25]. Because of increased concern on the use of blanket dry cow therapy for its role in driving antimicrobial resistance, selective dry cow therapy (intramammary infusion of antimicrobials into only quarters that have tendency or risk of infection) has been under investigation [26, 27]. Some recent studies showed that the use of bacteriological culture-based selective dry cow therapy at drying-off did not negatively affect cow health and performance during early lactation [26, 27]. In general, dairy farms are one of the largest users of antimicrobials including medically important antimicrobials [28]. Some of the antimicrobials used in dairy farms include beta-lactams (penicillins, Ampicillin, oxacillin, penicillin-novobiocin), extended-spectrum beta-lactams (third-generation cephalosporins, e.g., ceftiofur), aminoglycosides (streptomycin), macrolides (erythromycin), lincosamide (pirlimycin), tetracycline, sulfonamides, and fluoroquinolones [28–30]. Antimicrobials are also heavily used in dairy farms for the treatment of cases of mastitis [3, 25, 31] and other diseases of dairy cows such as metritis, retained placenta, lameness, diarrhea, pneumonia, [32–36] and neonatal calf diarrhea [37]. Over 90% of dairy farms in the US infuse all udder quarters of all cows with antimicrobial regardless of their health status [7, 25, 38]. According to dairy study in 2007 that was conducted in 17 major dairy states in the United States, 85.4% of farms use antibiotics for mastitis, 58.6% for lameness, 55.8% for diseases of the respiratory system, 52.9% for diseases of reproductive system, 25% for diarrhea or gastrointestinal infections and 6.9% for all other health problems [3, 25]. Cephalosporins were the most widely used antibiotics for the treatment of mastitis, followed by lincosamides and non-cephalosporin beta-lactam antibiotics [3, 25]. The two most commonly used antibiotics for dry cow therapy are Penicillin G/dihydrostreptomycin and cephalosporins [3, 25]. Antimicrobials were administered for the prevention and treatment of mastitis and other diseases of dairy cattle mainly through intramammary infusion and intramuscular route (USDA APHIS, 2009a). Antimicrobials infused into the mammary glands can be excreted to the environment through leakage of milk from the antimicrobial-treated udder or absorbed into the body and enter the blood circulation and biotransformed in the liver or kidney and excreted from the body through urine or feces into the environments [39–42]. Similarly, antimicrobials administered through parenteral routes for the treatment of acute or peracute mastitis or other diseases of dairy cows will enter the blood circulation and biotransformed in the liver or kidney and excreted from the body through urine or feces into the environments [39–42]. Therefore, both parenteral and intramammary administration of antibiotics has a significant impact on other commensals or opportunistic bacteria in the gastrointestinal tract

*DOI: http://dx.doi.org/10.5772/intechopen.94227*

previous lactation [24].

*Current Status of Antimicrobial Resistance and Prospect for New Vaccines against Major… DOI: http://dx.doi.org/10.5772/intechopen.94227*

time of teat canal closure varies among cows. Some studies showed that 50% of teat canals were classified as closed by seven days after drying off, 45% closed over the following 50–60 days after drying off, and 5% had not closed by 90 days after dry off [24]. Teats that do not form a plug-like keratin seal are believed to be most susceptible to infection. Infusion of long-acting antimicrobials into the udder at drying-off (dry cow therapy) has been the major management tool for the prevention of IMI during the dry period, as well as to clear IMI established during the previous lactation [24].

In the United States and many other countries at the end of lactation (at drying off), all cows regardless of their health status, are given an intramammary infusion of long-acting antimicrobials (blanket dry cow therapy) to prevent IMI by bacteria during the dry period [3, 25]. Because of increased concern on the use of blanket dry cow therapy for its role in driving antimicrobial resistance, selective dry cow therapy (intramammary infusion of antimicrobials into only quarters that have tendency or risk of infection) has been under investigation [26, 27]. Some recent studies showed that the use of bacteriological culture-based selective dry cow therapy at drying-off did not negatively affect cow health and performance during early lactation [26, 27]. In general, dairy farms are one of the largest users of antimicrobials including medically important antimicrobials [28]. Some of the antimicrobials used in dairy farms include beta-lactams (penicillins, Ampicillin, oxacillin, penicillin-novobiocin), extended-spectrum beta-lactams (third-generation cephalosporins, e.g., ceftiofur), aminoglycosides (streptomycin), macrolides (erythromycin), lincosamide (pirlimycin), tetracycline, sulfonamides, and fluoroquinolones [28–30]. Antimicrobials are also heavily used in dairy farms for the treatment of cases of mastitis [3, 25, 31] and other diseases of dairy cows such as metritis, retained placenta, lameness, diarrhea, pneumonia, [32–36] and neonatal calf diarrhea [37]. Over 90% of dairy farms in the US infuse all udder quarters of all cows with antimicrobial regardless of their health status [7, 25, 38]. According to dairy study in 2007 that was conducted in 17 major dairy states in the United States, 85.4% of farms use antibiotics for mastitis, 58.6% for lameness, 55.8% for diseases of the respiratory system, 52.9% for diseases of reproductive system, 25% for diarrhea or gastrointestinal infections and 6.9% for all other health problems [3, 25]. Cephalosporins were the most widely used antibiotics for the treatment of mastitis, followed by lincosamides and non-cephalosporin beta-lactam antibiotics [3, 25]. The two most commonly used antibiotics for dry cow therapy are Penicillin G/dihydrostreptomycin and cephalosporins [3, 25]. Antimicrobials were administered for the prevention and treatment of mastitis and other diseases of dairy cattle mainly through intramammary infusion and intramuscular route (USDA APHIS, 2009a). Antimicrobials infused into the mammary glands can be excreted to the environment through leakage of milk from the antimicrobial-treated udder or absorbed into the body and enter the blood circulation and biotransformed in the liver or kidney and excreted from the body through urine or feces into the environments [39–42]. Similarly, antimicrobials administered through parenteral routes for the treatment of acute or peracute mastitis or other diseases of dairy cows will enter the blood circulation and biotransformed in the liver or kidney and excreted from the body through urine or feces into the environments [39–42]. Therefore, both parenteral and intramammary administration of antibiotics has a significant impact on other commensals or opportunistic bacteria in the gastrointestinal tract of dairy cows and farm environments.

In addition to the use of antimicrobials for the prevention and treatment of mastitis and other diseases of dairy cattle, some farms also feed raw waste milk or pasteurized waste milk from antibiotic-treated cows to dairy calves. Feeding of

*Animal Reproduction in Veterinary Medicine*

time from non-milking to milking.

On average, starting from calving (giving birth) dairy cow is milked (in lactation) for about 300 days and then dried off (stop milking) for about 60 days before they calve again. Under the ideal dairy farming condition, a dairy cow should become pregnant within 60 days of calving, and the lactation cycle continues (**Figure 1**). The goal of a dry period is to give them a break from milking so that milk-producing cells regenerate, multiply, and ready for the next cycle of lactation. The incidence of intramammary infection (IMI) by bacteria is high during the early dry period and transition periods [10]. In general, for a dairy cow, a transition period, also known as the periparturient period, is a time range from three weeks before parturition (non-milking time) until three weeks after calving (milking time). It is a transition

Dairy cows are susceptible to mastitis during early non-lactating (dry period) and transition periods [11, 12], especially new infection with environmental pathogens (*Streptococcus* spp. and coliform) are highest during the first two weeks after drying off and last two weeks before calving [13] compared to contagious mastitis pathogens such as *S. aureus* [14]. The incidence of intramammary infection is high during the early dry period because of an absence of hygienic milking practices such as pre-milking teat washing and drying [15], pre- and post-milking teat dipping in antiseptic solutions [16, 17], that are known to reduce teat end colonization by bacteria and infection. An udder infected during the early dry period usually manifests clinical mastitis during the transition period [18] because of increased production of parturition inducing immunosuppressive hormones [19], negative

Cows are naturally protected against intramammary infections during the dry period by physical barriers such as the closure of teat opening by smooth muscle (teat sphincter) and the formation of a keratin plug, fibrous structural proteins (scleroproteins) [21, 22], in the teat canal produced by teat canal epithelium [23]. Keratin contains a high concentration of fatty acids, such as lauric, myristic, and palmitoleic acids, which are associated with reduced susceptibility to infection and stearic, linoleic, and oleic acids that are associated with increased susceptibility to infection. Keratin also contains antibacterial proteins that can damage the cell wall of some bacteria by disrupting the osmoregulatory mechanism [23]. However, the

*Antimicrobials usage patterns during the lactation cycle. DIM: Days in milk, yellow star: Peak lactation at 60 DIM, green bars: Energy demand that requires the mobilization of body energy reserve at the expense of losing* 

*bodyweight, red bumps showed increased usage of antimicrobials.*

energy balance [12], and physical stress during calving [20].

**184**

**Figure 1.**

raw waste milk or pasteurized waste milk from antibiotic-treated cows to calves increases pressure on gut microbes such as *E. coli* to became antimicrobial-resistant [43–45]. Aust et al. [43] showed that the proportion of antimicrobial-resistant *E. coli,* especially cephalosporin-resistant *E. coli* isolates, was significantly higher in calves fed waste milk or pasteurized waste milk from antimicrobial treated cows than calves fed bulk tank milk from non-antibiotic treated cows. However, pasteurized waste milk from cows not treated with antimicrobials is acceptable to be feed to young calves [43] but it is not known if pasteurization prevents the transfer of antimicrobial-resistant genes to microbes in the calve's gut. Some studies also showed that feeding pasteurized waste milk from antimicrobial treated cows to calves increased the presence of phenotypic resistance to ampicillin, cephalothin, ceftiofur, and florfenicol in fecal *E. coli* compared with milk replacer-fed calves [45]. However, the presence of resistance to sulfonamides, tetracyclines, and aminoglycosides was common in dairy calves regardless of the source of milk, suggesting other driving factors for resistance development [45]. It has been suggested that antimicrobial residues present in waste milk have a non-specific effect at a lower taxonomical level [44]. Collectively, these non-prudent antimicrobials usage practices in dairy farms expose a large number of animals in dairy farms to antimicrobials and also increases the use of antimicrobials in dairy farms, which in turn creates intense pressure on microbes in animals' body especially commensal and opportunistic microbes in the gastrointestinal tract and farm environments. Some of these commensal bacteria in the animal body are serious human pathogens (e.g., *E. coli* 0157:H7). *Staphylococcus aureus* is one of the pathogens with a known ability to develop antimicrobial resistance and established *S. aureus* infections are persistent and difficult to clear. The failure to control these infections leads to the presence of reservoirs in the dairy herd, which ultimately leads to the spread of the infection and the culling of the chronically infected cows [46, 47].

Monitoring antimicrobial resistance patterns of bacterial isolates from cases of mastitis is important for treatment decisions and proper design of mitigation measures. It also helps to determine emergence, persistence, and potential risk of the spread of antimicrobial-resistant bacteria and resistome to human, animal, and environment [48, 49]. The prudent use of antimicrobials in dairy farms reduce emergence, persistence, and spread of antimicrobial-resistant bacteria and resistome from dairy farms to human, animal, and environment.
