**2. Etiology of mastitis**

Over 135 various microorganisms have been identified from bovine mastitis. The most common bovine mastitis pathogens are classified as contagious and environmental mastitis pathogens [4]. This classification depends upon their distribution in their natural habitat and mode of transmission from their natural habitat to the mammary glands of dairy cows [5]. It is important to mention that all pathogens lists as environmental or contagious may not be strictly environmental or strictly contagious; some of them may transmit both ways. Environmental mastitis pathogens exist in the cow's environment, and they can cause infection at any time. Environmental mastitis pathogens are difficult to control because they are in the environment of dairy cows and can transmit to the mammary glands at any time, whereas contagious mastitis pathogens exist in the infected udder or on the teat skin and transmit from infected to non-infected udder during milking by milker's hand or milking machine liners. Environmental mastitis pathogens include a wide range of organisms, including coliform bacteria (*Escherichia coli, Klebsiella* spp.*, Enterobacter* spp., and *Citrobacter* spp)*,* environmental Streptococcus spp. (*Streptococcus uberis, Streptococcus dysgalactiae, Streptococcus equi, Streptococcus zooepidemicus, Streptococcus equinus, Streptococcus canis, Streptococcus parauberis*, and others), *Trueperella pyogenes,* which was previously called *Arcanobacterium pyogenes* or *Corynebacterium pyogenes* and environmental coagulase-negative *Staphylococcus* species (CNS) (*S. chromogenes, S. simulans, S. epidermidis, S. xylosus, S. haemolyticus, S. warneri, S. sciuri, S. lugdunensis, S. caprae, S. saccharolyticus*, and others) [4, 6–9] and others such as *Pseudomonas*, *Proteus*, *Serratia*, *Aerococcus*, *Listeria*, Yeast and *Prototheca* that are increasingly found as mastitis-causing pathogens on some farms [10, 11].

Contagious mastitis pathogens primarily exist in the infected mammary glands or on the cow's teat skin and transmit from infected to non-infected mammary glands during milking by milker's hand or milking machine liners. *Mycoplasma* spp. may spread from cow to cow through aerosol transmission and invade the udder subsequent to bacteremia. The most frequent contagious mastitis pathogens are coagulase-positive *Staphylococcus aureus, Streptococcus agalactiae, Mycoplasma bovis,* and *Corynebacterium bovis* [11, 12]*.* The prevalence of mastitis caused by these different mastitis pathogens varies depending on herd management practices, geographical location, and other environmental conditions [13]. These different causative agents of mastitis have a multitude of virulence factors that make treatment and prevention of mastitis difficult.
