Prevalence, incidence and risk factors of heifer mastitis.

Traditionally heifers, as calves and as primiparae, have been thought of as a group as free of mastitis. Without appreciable lacteal secretion, there is reduced nutrient fluid available to support growth of intramammary pathogens. Contagious mastitis is primarily transmitted at milking time and the milking process affects the patency of the teat orifice which can increase the risk of development of environmental mastitis. Logically therefore prepartum heifers should be free of intramammary infections. During the last 20 years there have been numerous investigations describing the nature of mastitis in heifers and thus the dogma that heifers are free of this disease has been challenged. The purpose of this manuscript is to review that literature describing heifer intramammary infections that cause both subclinical and clinical disease. Mammary quarter infection prevalence ranges between 28.9-74.6% prepartum, and 12.3-45.5% at parturition. Generally, the pathogens that cause mastitis in heifers are the same as those that cause infections in the older cows. In all but one study reviewed, coagulase-negative staphylococci (CNS) are the most prevalent cause of subclinical intramammary infections in heifers. Coagulase-positive staphylococci (CPS) in some studies are the second most prevalent pathogens, while in other studies the environmental mastitis pathogens are more prevalent. The risk factors for subclinical mastitis appear to be season, herd location, and trimester of pregnancy; all suggesting that management can have an impact in control of this disease prepartum. With respect to clinical mastitis, the most prevalent mastitis pathogen has been reported to be CNS in one study and CPS, or environmental mastitis pathogens, in other studies. The heifer is most at risk for clinical mastitis during the periparturient period. Risk factors found are related to diet, mammary gland factors such as edema and leaking of milk, and factors associated with the change in management and introduction of the heifer to the milking herd.