How machine milking influences mastitis

Machine milking, along with its various associated practices such as udder or teat washing, has significant effects on the etiology, incidence, and progress of mastitis. These effects may operate directly by increasing the new infection rate, or indirectly by increasing exposure to bacteria or reducing disease resistance.

There are four major ways in which machine milking can affect the development and severity of mastitis:

1. Facilitate the transmission of pathogenic bacteria between cows or quarters at milking time.

The milking process offers multiple opportunities for bacteria to be transferred between cows and quarters. Improper udder preparation, such as using a common towel or failing to dry teats, increases contamination and transmission of bacteria. During milking, vacuum fluctuations in the claw lead to milk moving between teat cups. If the cow being milked has one or more infected quarters, this process transfers pathogenic bacteria to the surfaces of other teats. After a cow has been milked, the milking machine liner surfaces carry bacteria originating from the teat surface and milk of that animal. These bacteria are transferred to the next animal when the machine is applied.

2. Aid the multiplication of bacteria at the teat end.

The major factor in determining the intramammary infection rate is exposure of the teat orifice and duct to pathogenic organisms. Machine milking can influence this by modifying conditions at the teat end so that bacterial colonization occurs more readily. These conditions are often referred to as teat orifice "eversion", or "hyperkeratosis". Machine milking may also lead to hemorrhagic blisters at the teat end, teat chapping, and lesions. Such skin abnormalities are readily colonized by pathogenic bacteria, and may lead to intramammary infections.

3. Increase bacterial penetration of the teat duct.

The action of the milking machine can cause bacteria to be propelled directly from the exterior of the teat into the teat sinus. This "impact mechanism" results from vacuum fluctuations. Liner slip is an important event in the generation of vacuum fluctuations. The impact mechanism is the only known means by which vacuum fluctuations are capable of increasing infection rate under both experimental and field conditions.

4. Modify the teat or intramammary environment to enhance bacterial infection or impair host defenses.

Machine milking may cause trauma to the teat rendering it more susceptible to colonization and infection. Trauma to the mucous membranes lining the teat sinus may provide an environment favoring bacterial colonization or multiplication. Local pain may lead to neurohormonal responses which suppress immune function and increase the likelihood of disease.

A common question often raised is "how much mastitis is attributable to machine milking?" While it has been shown that machine milking can exert potent effects on mastitis incidence, the true impact is not known. There a multitude of potential interactions with season of the year, climate, environment and milk yield making it difficult to determine.

Mastitis control requires more than good machine design; it also demands housing practices that keep cows clean and dry, proper pre-milking udder preparation, and teat disinfection after milking. By combining these approaches, it is possible to milk efficiently and minimize machine-induced infection.

From: "Machine Milking and Lactation", Insight Books, Huntington, VT (1992) p. 355-368