**4. Clinical manifestation of mastitis**

Depending on clinical signs, mastitis can also be divided into clinical and subclinical mastitis. Clinical mastitis is characterized by visible inflammatory changes (abnormalities) in the mammary gland tissue such as redness, swelling, pain, increased heart, and abnormal changes in milk color (watery, bloody, and blood tinged) and consistency (clots or flakes) [9]. Clinical mastitis can be acute, peracute, subacute, or chronic. Acute mastitis is a very rapid inflammatory response characterized by systemic clinical signs which include fever, anorexia, shock, as well as local inflammatory changes in the mammary gland and milk. Peracute mastitis is manifested by a rapid onset of severe inflammation, pain, and systemic symptoms that resulted in a severely sick cow within a short period of time. Subacute mastitis is the most frequently seen form of clinical mastitis characterized by few local signs of mild inflammation in the udder and visible changes in milk such as small clots. Chronic mastitis is a long-term recurring, persistent case of mastitis that may show few symptoms of mastitis between repeated occasional flare-ups of the disease where signs are visible and can continue over periods of several months. Chronic mastitis often leads to irreversible damage to the udder from the repeated occurrences of the inflammation, and often these cows are culled.

Subclinical mastitis is the inflammation of the mammary gland that does not create visible changes in the milk or the udder. Subclinical mastitis is an infection of mammary gland characterized by non-visible inflammatory changes such as a high somatic cell count coupled with shedding of causative bacteria through milk [9]. During this inflammatory process, the milk samples showed a rapid increase of somatic cells, characterized by increased number of neutrophils in the secretion [146, 148]. Despite increased recruitment of somatic cells into infected mammary glands, evidenced by an increased number of neutrophils, infection usually does not clear but became subclinical. Intramammary infections during early lactation may become acute clinical mastitis characterized by gangrene development due congestion and thrombosis (blockage) of blood supply to the tissue but most new infection during late lactation or dry period become acute or chronic mastitis [149, 150].

The increase in somatic cell count during subclinical infections leads to a decrease in useful components in the milk, such as lactose and casein [151]. Lactose is the sugar found in milk, and casein is one of the major proteins in milk and decreases in these two components affect the quality and quantity of milk yield [9]. During mastitis, there is an increase in lipase and plasmin, which have a detrimental effect on the quantity and quality of milk due to the breakdown of milk fat and casein [9]. Subclinical infections can reduce milk production by 10–12% when just one-quarter is infected [152]. These subclinical infections cause some of the greatest unseen economic [20] losses because of their detrimental impact on production and milk quality without showing visible signs of infection [152].
