Chapter 10


The mammary gland is a vital part of the reproductive anatomy of the mare. There is no doubt that milk is the ideal food for the very young foal, and under natural conditions continues to provide important nourishment for the growing foal up to natural weaning (often at about 8–9 months of age). Furthermore, the mammary gland plays a vital role in providing early immunological protection for the foal through the production of immunoglobulin-rich colostrum. Without effective production of high-quality colostrum containing appropriate antibody IgG, the foal has a dramatically reduced chance of survival1 (see p. 374). Of all the domestic species, the foal is probably the most reliant upon colostral intake for its survival in the first few weeks of extrauterine life.

In spite of the significance of mammary gland function to the survival of the foal, little is known about the specific features in the mare. Diseases and disorders of the mammary gland are usually manifest by changes in size and texture of the gland and alteration in the natural secretions.


The mammary glands are located high in the inguinal region. Their relatively small size and protected position reduce the risks of trauma, sunburn, and infections. In the dry (nonlactating) mare, the glands are small (except that, in older multiparous mares, the teats may be long and the udder skin rather slacker than in young mares) and have an almost insignificant blood supply; the lactating gland is somewhat larger and the blood supply is greater. Inflammatory and other disease conditions invariably increase the blood supply.

The mare has two mammary glands separated completely by a facial sheet. There is one teat per gland.2 Each teat usually has two openings (Fig. 10.1), but occasionally there are three, reflecting the number of mammary lobes present. This has implications for mammary disease and the use of intramammary therapy.

Milk is secreted into the teat sinus from specialized cells that form the bulk of the gland. The glandular tissue itself is generously supplied with myoepithelial cells that have the ability to contract in response to oxytocin in particular.


The mammary gland of the horse is relatively small and the foal feeds frequently during lactation. This provides a significant protection mechanism against infection.

Mammary gland development and the production of milk is a highly complex process requiring fine control and coordination of birth and mammary development; milk production needs of course to be coordinated closely with delivery of the foal. The process of the manufacture of milk is demanding of energy and requires a fully functional mammary cell structure free of inflammation. The process is largely governed by the interrelationships between the hypothalamus, the anterior pituitary gland, the ovaries, and the placenta. Several hormones are responsible for individual aspects of the development of the mammary gland and the change from a quiescent nonsecretory structure to a highly specialized secretory function which, in the first instance, produces colostrum and then quickly shifts to produce milk.

Mammary development begins around 1 month before foaling. This process is believed to be instigated by ovarian and adrenal steroids, prolactin, growth hormone, insulin, thyroid hormones, and oxytocin:

• Estrogen causes development of the duct system.

• Progesterone stimulates the development of the milk-producing cells but inhibits milk production.

• Prolactin concentration increases dramatically in the last week of gestation and remains elevated until some 3 months after parturition.3 This increase is probably instigated by the cessation of release of an inhibitory factor in the hypothalamus.4

• The decline in progesterone at the end of gestation and the rise in prolactin over the last few days are probably the basis of the trigger for milk production to begin.

• Oxytocin is secreted from the neurohypophysis of the pituitary glands in response to stimulation of the udder (usually by nursing).5 Oxytocin causes contraction of the myoepithelial cells and ejection of milk into the duct system and eventually into the teat sinus. Circulating oxytocin released during labor will also have an effect on the mammary glands, so milk may be ejected from the teats during later stages of labor. This may have implications for the quantity and quality of colostrum available to the foal. The so-called ‘milk let-down’ response can also occur in the absence of any detectable oxytocin in the circulation.6

During the last 2 weeks of gestation, the mammary gland actively concentrates immunoglobulin from the blood. This abstraction from the mare can sometimes be detected by a fall in immunoglobulins in the blood of the mare during this time. The protein-rich secretion that is produced first (colostrum) is high in IgG but it is only produced once; once the colostrum has been removed from the udder no more is produced in that lactational period. Within 12–14 hours of birth the IgG concentration falls dramatically. The other immunoglobulins (IgM and IgA) are at low levels in colostrum but are sustained for weeks in normal milk.

Milk contains more calcium and potassium and less sodium than blood, and as parturition approaches these changes become more apparent in the mammary secretion.7 The concentrations of the various electrolytes in mammary secretions are used to provide information on readiness for birth8 (see p. 270).

Milk is an ideal food material for the newborn and very young foal but becomes an inadequate total feed within a few weeks of birth (see Table 10.1). Low levels of iron and some other trace elements mean that a totally milk-based diet will result in significant metabolic deficiencies, and the energy requirement for the growing foal cannot be met solely by the volume of milk produced by the mare.

Lactation is maximal at around 2 months postpartum, the average production in a Thoroughbred mare being around 10–12 kg/day. This production is roughly equivalent to 2–3% of bodyweight and should enable the foal to grow at a rate of around 1 kg/day. In order to produce this amount of milk the mare needs to be fed additional food value of 1.5 times the normal requirements for energy and protein. Mares that are deprived of food or which do not eat for other reasons will inevitably produce less milk in response to the deprivation and negative energy balances. In some cases (ponies in particular) the negative energy balance results in the mobilization of fat and hyperlipemia can develop. This condition carries a poor prognosis and is a relatively common occurrence in late pregnancy or early lactating overweight pony mares. Any overweight mare that is subjected to a reason for negative energy balance may precipitate hyperlipemia. The disease is not the sole preserve of overweight pony mares.


Under natural conditions weaning occurs at around 7–9 months, but most management systems wean foals at around 5–6 months of age. The weaning process is usually done abruptly by separating the mare and foal out of sight and hearing for 2–3 days. After a few days the foal will have forgotten the mare and can then be grouped with other weanlings in a small paddock. There are numerous variations in the weaning process, including gradual weaning or housing mares and foals together and gradually removing individual mares from the group, leaving an old experienced mare in the group for company.

Management of the mare is at least as important as management of the foal. The mammary secretions will, of course, continue to be produced and so the glands invariably become mildly or moderately engorged with milk. This accumulation induces a suppression of activity in the glandular cells and so milk production is gradually ‘turned off’. This mechanism can also be instigated by lack of nursing in a younger foal, so it is important to examine the udder of nursing mares regularly to identify foals that are off suck at an early stage so that lactation is not inhibited.

Normally, the glandular distention that develops at weaning subsides gradually over a few days without evidence of inflammatory response, but occasionally mastitis develops. Failure to detect mastitis can have serious repercussions both immediately and at a future lactation. Therefore, regular attention should be paid to postweaning mares for 2 weeks after weaning. It is probably not advisable to milk-off the mare even if the udder is grossly distended, for fear that either infection will be introduced or because release of the pressure may reinstate lactation. Any serious oversupply of milk and associated congestion may respond best to restriction of food and water for a few days.



Clinical signs


Slow intravenous administration of 250–500 ml of 20% calcium gluconate diluted in glucose saline (1:4) is rapidly curative. The response to treatment confirms the diagnosis.



• Poor body condition; physiological or psychological stress.11

• Early parturition/abortion.

• Fescue toxicosis.12 This pasture grass is associated with a high incidence of agalactia, thickened placenta, prolonged gestation, stillbirths, and occasionally abortion. Infertility at subsequent breeding is also reported.13,14

• Secondary to systemic illness including strangles.

• Specific udder diseases such as mastitis and mammary neoplasia.

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Jun 18, 2016 | Posted by in EQUINE MEDICINE | Comments Off on THE MAMMARY GLAND

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