Mastitis in heifers is defined as intramammary bacterial infection (IMI) leading to temporary or permanent colonization of the mammary secretory tissues with bacteria. Intramammary infections can produce clinically visible disease (clinical mastitis) or no outwardly visible signs of disease associated with bacterial presence in the mammary tissues (subclinical mastitis). Clinical disease is manifest as hot swollen quarters with or without systemic signs of fever, depression and toxemia, and elevated SCC. Subclinical disease is detected as elevated SCC in milk from infected quarter or elevated composite SCC in milk across all 4 quarters. Prepartum intramammary infection in heifers may or may not result in loss of the quarter. However, early infections occurring as calves or at the time of breeding are certainly capable of producing a sustained reduction in intramammary secretory tissue that reduces lifetime productivity and profitability. Intramammary heifer infections can occur as early as 1-9 months of age and have been observed as early as 9 months before parturition (Boddie et al., 1985, Fox et al, 1995, 2009). The prevalence of prepartum intramammary infection in heifers varies considerably across farms but is a common event that may involve as many as 35% of all yearling heifers (Fox et al., 1995). Heifer mastitis differs from adult cow mastitis in several ways. Heifers more frequently experience a higher incidence of clinical mastitis within the first few days post-partum compared to adult cows. The causal bacteria in heifers differs from those in adult cattle in that minor mastitis pathogens such as coagulase negative Staphylococcal agents like Staphylococcus epidermidis typically invade heifer mammary tissues. In contrast intramammary infections in adult cattle more often occur with major mastitis pathogens such as Staphylococcus aureus. Heifers with intramammary infections prepartum most often manifest the infection as a subclinical problem marked by elevated somatic cell counts (SCC) preand postpartum. Many of these prepartum infections are sustained as subclinical mastitis problems with a persistently elevated SCC during the first lactation. Still many other pre-partum infections spontaneously selfcure within the first few test dates. These appear as a return of elevated SCC early in lactation to near normal amounts later in the first lactation. A lower percent of the subclinical prepartum infections will eventually progress into clinical mastitis in the first lactation. In all cases first lactation milk yields are reduced and the risk of premature culling is increased. Most importantly, these pools of subclinical, prepartum infections present an enormous biosecurity threat to the lactating herd in that they serve as a reservoir of contagious mastitis pathogens for the lactating herd with the trends for greater demands for heifer replacement that accompany 30-35% replacement requirements in the commercial herd.
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