**4. Vaccines**

*Animal Reproduction in Veterinary Medicine*

tion and spread, rather than treatment [14, 19].

resistant to more than one antimicrobial.

all or most available antimicrobials [36, 37].

lead to the development of antimicrobial resistant bacteria including methicillinresistant *S. aureus* (MRSA) [18]. Treatment of *Staphylococcus aureus* mastitis with antibiotics is of limited success which may dictate the culling of the animal [14, 19]. Until recently, MRSA was a common antimicrobial resistant strain mainly found in human hospitals; however, recent findings indicated that it has also been increasingly isolated from cattle herds [20]. The major problem with MRSA is that it is mostly resistant to multiple commonly used antimicrobials (multidrug resistant) and difficult to control and eliminate [21]. On an average, the cure rate of lactating cow therapy against *S. aureus* mastitis is about 30% or less [22]. Currently, there is no effective vaccine against bovine *S. aureus* mastitis [23], and since treatment is of limited efficacy, control of *S. aureus* mastitis focuses on prevention of contamina-

Antimicrobial resistance is a growing problem in *Staphylococcus aureus* mastitis. Antimicrobial resistance helps bacteria to stay alive after treatment with antibiotics and some of the mechanisms of resistance are the presence of antimicrobial resistance genes that can spread by horizontal transfer from bacteria to bacteria by mobile genetic elements such as plasmids, phages, and pathogenicity islands [24]. This resistance can also occur through random mutations when the bacteria are under stress [25]. In the cases of mastitis, the prevalence of antimicrobial resistant bacteria seems to be increasing at least for some antimicrobials. Studies reported over 50% of isolates that cause mastitis were resistant to either beta lactam drugs or penicillin [26]. In human medicine, methicillin resistant *S. aureus* (MRSA) is a huge problem because MRSA strains are resistant to most of antibiotics making them very difficult or impossible to treat. There have also been reports of cases of bovine mastitis caused by MRSA [27–30]. Some report that these infections are due to the human strain, but others have found MRSA strains of bovine origin [21, 31]. These authors suggested that MRSA strains isolated from bovine probably gain resistance

from human MRSA strain through transfer of resistance genes [32].

Waller et al. [33] evaluated the antimicrobial susceptibility of CNS and found a difference across the species on β-lactamase production. Similarly, Sawant et al. [34] found that 18% and 46 of the *S. chromogenes* and *S. epidermidis* isolates produce β-lactamase, respectively. Sampimon et al. [35] also found a 70% resistance to penicillin in *S. epidermidis*, but more importantly found that 30% of the CNS were

From antimicrobial resistance perspective, environmental mastitis pathogens are very important for two reasons: (1) some members of environmental mastitis pathogens are either normal microflora or opportunistic pathogens in the gastrointestinal tract of dairy cows and frequently exposed to antimicrobials directly through oral or indirectly through parenteral routes; (2) despite strain variation, some of them are highly pathogenic for human (for example, *E. coli* 0157:H7 is normal microflora in the rectum of cattle). Of significant concern is the potential for human infection by antimicrobial-resistant environmental mastitis pathogens such as extended-spectrum beta-lactam resistant *E. coli* directly through contact with carrier animal or indirectly through the food chain. Some of the Gramnegative environmental mastitis pathogens, such as *E. coli*, *Klebsiella pneumoniae*, *Acinetobacter baumannii*, *Pseudomonas aeruginosa*, and *Enterobacter* spp. are the greatest threat to human health due to the emergence of strains that are resistant to

In general, the antimicrobial resistance of mastitis pathogens varies with dairy farms and bacterial species within and among dairy farms [11, 38–42]. However, the antimicrobial-resistance status of human pathogenic environmental mastitis pathogens, especially the resistance status of Gram-negative environmental mastitis

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Several vaccine studies were conducted over the years as controlled experimental and field trials. Some of the most common mastitis pathogens that have been targeted for vaccine development are *S. aureus*, *S. agalactiae, S. uberis*, and *E. coli* [44]. Most of these experimental and some commercial vaccines are bacterins which are inactivated whole organism, and some vaccines contained subunits of the organism such as surface proteins [45], toxins, or polysaccharides.

All coliform mastitis vaccine formulations use Gram-negative core antigens to produce non-specific immunity directed against endotoxin (LPS) [44]. The principle of these bacterins is based upon their ability to stimulate production of antibodies directed against common core antigens that Gram-negative bacteria share. These vaccines do not prevent new intramammary infection but significantly reduced the clinical severity of the infection [46–48]. Experimental challenge studies have demonstrated that J5 vaccines are able to reduce bacterial counts in milk and resulted in fewer and less severe clinical symptoms [47]. Vaccinated cows may become infected with Gram-negative mastitis pathogens at the same rate as control animals but have a lower rate of development of clinical mastitis [48], reduced duration of infection [46], less loss of milk production, culling, and death losses [49, 50]. The Eviracor®J5 *E. coli* vaccine (Zoetis, Kalamazoo, MI), [51, 52], as well as the UBAC® *S. uberis* vaccine (Hipra, Amir, Spain), [53] are similar to vaccination with nonspecific killed whole bacterial cells (bacterin vaccines), achieving only partial reduction in clinical severity of mastitis.

Despite several mastitis vaccine trials conducted against *S. aureus* mastitis [54–65], all field trials have either been unsuccessful or had limited success. There are two commercial vaccines for *Staphylococcus aureus* mastitis on the market, Lysigin® (Boehringer Ingelheim Vetmedica, Inc., St. Joseph, MO) in the United States and Startvac® (Hipra S.A, Girona, Spain) in Europe and Canada [66]. None of these vaccines confer protection in field trials as well as under controlled experimental studies [54, 58, 62, 67]. Several field trials and controlled experimental studies have been conducted testing the efficacy of Lysigin® and Startvac® and results from those studies have shown some interesting results, namely a reduced incidence, severity, and duration of mastitis in vaccinated cows compared to non-vaccinated control cows [54, 62, 68]. Contrary to these observations, other studies failed to find an effect on improving udder health or showed no difference between vaccinated and non-vaccinated control cows [66, 69]. None of these bacterin-based vaccines prevents new *S. aureus* IMI [54, 58, 62, 67]. Differences found in these studies are mainly due to methodological differences (vaccination schedule, route of vaccination, challenge model, herd size, time of lactation, etc.) in testing the efficacy of these vaccines. It is critically important to have a good infection model that mimics natural infection and a model that has 100% efficacy in causing infection. Without a good challenge model, the results from vaccine efficacy will be inaccurate.
