Investigating premature lactation in pregnant dairy females

Summary

• Mastitis does occur in heifers that are unbred and in pregnant heifers – the prevalence estimates vary from 8 to 90%, with a moderate rate of infection (20 to 30% being reported relatively consistently). These infections are usually subclinical and are associated with coagulase negative staphylococci (CNS), however, major mastitis pathogens such as Staphylococcus aureus and Streptococcus agalactiae have been identified.
• Risk factors for mastitis especially relevant to the shipping situation include, crowding, environmental contamination of the udder, exposure to dietary change and fly worry.
• Clinical mastitis in heifers is most frequently described as the ‘summer mastitis’ complex.
• This clinical syndrome involving flies, environmental contamination and multiple organism infection with Archanobacterium pyogenes and anaerobic gram negative organisms is extremely difficult to treat and results in a high prevalence of blind quarters as detailed in Mexico (McCarthy pers comm.).
• Heifers should be treated for mastitis with systemic therapies rather than intramammary therapies. Specific recommendations are given in Figure 1.
• Udder oedema, the accumulation of fluid in the mammary region is more specifically related to the immediate pre-calving period than the first 2-6 months of gestation and is unlikely to be a cause of the observed problems. However, concentrations of salt and potassium in the diet should be tightly controlled to reduce the risk of oedema contributing to the clinical syndrome observed.
• Oestrogen is the dominant hormone in mammary development, however, many other orchestrated hormonal changes occur and progesterone also has a very substantial role in development. These contentions are supported by the potential to induce lactation in non-pregnant heifers using these hormones.
• Corticosteroids play little role, identified to date, in premature mammary development. Consequently, stress associated with transport, shipping and group assembly is unlikely to play a major role in causing the mammary distension seen.
• Exogenous sources of oestrogen in feed are well-reported as a cause of premature udder development. Many of these reports have been from Western Australia, but the syndrome of ‘clover udder’ is well recognised in other regions. This syndrome can also be associated with ingestion of damaged lucerne (personal observation).
• Figure 1 shows a flow diagram to differentiate between the different causes of udder distension in heifers and appropriate diagnostic approaches and responses for veterinarians in charge.
• It appears likely that feed factors are leading to premature development of the mammary gland. This is increasing the risk of bacterial penetration of the teat end under circumstances where serious environmental contamination of the udder is inevitable.
• A series of recommendations are given (in Recommendations Page 15) that should minimise the risk of this problem.