*2.2.1.1 Virulence factors of S. aureus*

*Animal Reproduction in Veterinary Medicine*

**2.2 Contagious mastitis pathogens**

cutaneous lesions in dolphins.

*2.2.1 Coagulase-positive Staphylococcus aureus*

Coagulase-positive *Staphylococcus aureus* is one of the most common contagious mastitis pathogens in dairy cows, with an estimated incidence rate of 43–74% [47, 48]. *Staphylococcus aureus* is grouped under the family *Staphylococcaceae* and genus *Staphylococcus*. It is a gram-positive, catalase and coagulase-positive, non-spore forming, oxidase negative, non-motile, clusterforming, and facultative anaerobe [49]. The coagulase test is not an absolute test for the confirmation of the diagnosis of *S. aureus* from the cases of bovine mastitis, but more than 95% of all coagulase-positive staphylococci from bovine mastitis belong to *S. aureus* [50]. Other coagulase-positive species include *S. aureus* subsp. *anaerobius* causes lesion in sheep; *S. pseudintermedius* causes pyoderma, pustular dermatitis, pyometra, otitis externa, and other infections in dogs and cats; *S. schleiferi* subsp. *coagulans* causes otitis externa (inflammation of the external ear canal) in dogs; *S. hyicus* is coagulase variable (some strains are positive and some others are negative), species that causes mastitis in dairy cows, exudative epidermitis (greasy pig disease) in pigs; and *S. delphini* causes purulent

*S. aureus* can infect many host species, including humans. In humans, *S. aureus* causes a wide variety of illnesses ranging from mild skin infection to a life-threatening systemic infection. It has been reported that certain strains of *S. aureus* with specific tissue tropism can be adapted to infect specific tissue such as the mammary gland [51]. Furthermore, a study by McMillan [52] showed distinct lineages of *S. aureus* in bovine, ovine, and caprine species. *S. aureus* strains can be host specific, meaning that they are found more commonly in a specific species [51]. Some studies showed that *S. aureus* that causes mastitis belong to certain dominant clones, which are frequently responsible for clinical and subclinical mastitis in a herd at certain geographic areas, indicating that the control measures may need to be directed against specific clones in a given area [53–55]. However, because *S. aureus* is such a big problem in human health, cross-infection has been an important research topic. Several studies have reported cases of cross-infection in several different species [56–58]. In the dairy industry, there have been reports of human origin methicillin-resistant *S. aureus* infecting bovine mammary glands [59, 60]. These studies add to the unease that strains can gain new mutations or virulence factors and adapt to cross the interspecies boundary relatively

Although the incidence of *S. aureus* mastitis can be reduced with hygienic milking practices and a good management system, it is still a major problem for dairy farms, with a prevalence of 66% among farms tested in the United States [62]. The prevalence of *S. aureus* mastitis varies from farm to farm because of variation in hygienic milking practices and overall farm management differences on the application of control measures for contagious mastitis pathogens. Good hygiene in the milking parlor can significantly reduce the occurrence of new *S. aureus* mastitis in the herd, but it does not remove existing cases within a herd [63]. Neave et al. concluded that it is nearly impractical to keep all udder quarters of dairy cows free of all pathogens at all times. Since this early observation by Neave et al. [63], many studies have confirmed that management practices can reduce new cases of intramammary infection (IMI) [9, 64] but cannot eliminate existing infections. In the United States, the prevalence of clinical and subclinical *S. aureus* mastitis ranged

from 10 to 45% [65] and 15 to 75%, respectively.

**152**

rapidly [61].

*Staphylococcus aureus* has many virulence factors that can be grouped broadly into two major classes. These include (1) secretory factors which are surface localized structural components that serve as virulence factors and (2) secretory virulence factors which are produced by bacteria cells and secreted out of cells and act on different targets in the host body. Both non-secretory and secretory virulence factors together help this pathogen to evade the host's defenses and colonize mammary glands.
