Cathleen C. Williams

Mastitis is one of the most common and most expensive diseases of dairy cattle in the world. One-third of all dairy cows are estimated to have mastitis. Mastitis costs dairy producers approximately $200 per cow per year. Nearly 70 percent of this loss is a result of reduced milk production, with the remainder coming from replacement costs, discarded milk, treatment and veterinary expenses.

It is doubtful that any dairy herd, regardless of size or management system, is free of this disease. It is often the end result of the interaction of several factors, which could include humans, cows, environment, microorganisms and management. So, what exactly is mastitis?

Mastitis is defined as an inflammatory reaction in the mammary gland. The inflammation in the udder tissue may be the result of bacterial, chemical, thermal or mechanical injury. Mastitis may be infectious – meaning it is caused by microbial organisms – or non-infectious as a result of physical injury to the udder. The inflammation occurs to destroy the irritant, repair the damaged tissue and return the udder to normal function.

While mastitis can be non-infectious, it is usually an infectious disease. An infection occurs when microorganisms enter the mammary gland through the teat end. Most cases of mastitis are caused by bacteria, but other types of microorganisms – including yeasts, mycoplasmas and even algae – occasionally cause intramammary infections. The majority of infections are caused by a few types of bacteria.

An important concept in understanding mastitis is that these common pathogens fall into two major categories: (1) contagious bacteria that are spread from an infected quarter of the udder to other quarters and cows, and (2) environmental bacteria that are commonly present in the cow’s environment and may reach the teat end from that source. This distinction is of practical importance because different control measures are needed for the different groups of microorganisms.

Mastitis infections may be clinical or subclinical. Clinical mastitis is characterized by visible abnormalities in the udder or in the milk. This form of mastitis can vary in severity, depending on the type of microorganism causing the infection. Subclinical mastitis, on the other hand, is far more subtle and cannot be detected by visual observation. It can be identified by conducting tests to detect the presence of microorganisms or the products of inflammation such as somatic cells.

Somatic cells are normal constituents of milk and only indicate a problem when numbers become excessive. Somatic cells are composed of leukocytes (75 percent) and epithelial cells (25 percent). Leukocytes (white blood cells) increase in milk in response to infection or injury while an increase in epithelial cells is the result of infection or injury. The number of somatic cells reflects the severity of mastitis.

Subclinical mastitis is 15 to 40 times more prevalent than clinical mastitis. It usually precedes clinical mastitis, is of long duration, is difficult to detect, reduces milk production, adversely affects milk quality and constitutes a reservoir of microorganisms that lead to infection of other cows in the herd. Information to assess the prevalence of subclinical mastitis may be provided by somatic cell counts of the bulk tank milk and of individual cows.

Although mastitis is a very prevalent disease in the dairy industry, it is important to note that milk is a safe, wholesome and nutritious product. Pasteurization destroys the organisms that cause mastitis, so there is no threat to human health. The dairy industry is one of the most highly regulated food industries. All milk is carefully tested for bacteria and antibiotics to ensure the safety of our food supply.

Mastitis is a disease which cannot be eradicated, but it can be controlled. Herd managers must be concerned with both clinical and subclinical mastitis. An effective mastitis control program is an important component of dairy management. Research at the LSU AgCenter has produced a relatively simple set of practical technologies producers can use to prevent most occurrences of mastitis. With sound mastitis management procedures, economic losses from this prevalent and complex disease may be reduced and allow dairy producers to maximize profit from their labor and investments.