Mammary involution and relevant udder health management in sheep

Abstract

Objective of the paper is to describe mechanisms leading to mammary involution in ewes and changes taking place in the mammary gland during that period. Mammary involution is the regression of mammary tissue to non-secreting state and takes place as initiated, gradual or senile. In mutton-type production systems cessation of lactation is abrupt, whilst in dairy-type production systems it is progressive or abrupt. The period from cessation of milk removal until the beginning of subsequent lactation period is termed ‘dry-period’ and is distinguished into stage of active involution, stage of the ‘steady-state’ involution and stage of redevelopment and lactogenesis. The ‘dry-period’ is important in health management of sheep for optimum milk production during the subsequent lactation period, as it is necessary for renewal of mammary epithelial cells. Mammary involution is influenced by decreased activity of galactopoietic hormones and local mechanisms in response to milk accumulation in the gland. Milk accumulation plays a major role in triggering apoptosis of epithelial cells. There are two pathways leading to apoptosis, an intrinsic and an extrinsic. During involution, significant histological changes take place in the mammary gland, mainly reduction (up to 3.6% up to the 4th day) of the epithelial area of the gland and increase of its stromal part. The mammary gland is particularly susceptible to infections after cessation of lactation; progressively, that changes; a keratin plug is formed at the teat orifice, leucocytes accumulate in the gland and concentrations of immunoglobulins and lactoferrin increase. Udder health management at the end of a lactation period aims to cure infections that have occurred during the previous lactation period and prevent new intramammary infections during the ‘dry-period’. Culling ewes with at least one mammary gland permanently damaged or ones chronically affected or others with incidents of relapsing mastitis or not fully respondent to mastitis treatment during the preceding lactation period contributes to decrease of veterinary expenses for mastitis control in the flock, elimination of sources of potential infection for other animals in the flock and decrease of flock bulk somatic cell counts in the subsequent lactation period. Culling should be complemented with intramammary antibiotic administration at the end of a lactation period. In many clinical studies from around the world, administration of antimicrobial agents at the end of a lactation period has been found beneficial. Antibiotic administration at drying-off may be performed to all animals in a flock (‘complete’) or only to those considered to be infected (‘selective’). In all cases, maintenance of the prescribed withdrawal periods is essential to safeguard public health. The procedure should always be applied as part of a strategic udder health management plan in a flock; implementation improves the welfare of animals and affords significant financial benefits to the farmer. Correct udder health management in ewes at the end of a lactation period will contribute to improved mammary health for the forthcoming lactation period. © 2019 Elsevier B.V.