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Mastitis can be caused by a build-up of milk within the breast (milk stasis) or damage to the nipple, which may lead to a bacterial infection.
Many cases of mastitis in breastfeeding women are thought to be caused by milk stasis. This occurs when the milk isn't properly removed from your breast during breastfeeding.
It can be caused by:
Milk stasis can cause the milk ducts in your breasts to become blocked, and can cause milk to build up within the affected breast.
Experts aren't sure exactly why breast milk can cause the breast tissue to become inflamed. One theory is that the pressure building up inside the breast forces some milk into the surrounding tissue.
Your immune system may then mistake proteins in the milk for a bacterial or viral infection and responds by inflaming the breast tissue in an effort to stop the spread of infection.
Fresh human milk doesn't usually provide a good environment in which bacteria can breed. However, milk stasis can cause milk to stagnate and become infected. This is known as infective mastitis.
Exactly how bacteria enter the breast tissue hasn't been conclusively proven.
The bacteria that usually live harmlessly on the skin of your breast may enter through a small crack or break in your skin, or bacteria present in the baby's mouth and throat may be transferred during breastfeeding.
You may be at greater risk of developing infective mastitis if your nipple is damaged – for example, as a result of using a manual breast pump incorrectly or because your baby has a cleft lip or palate, an opening or split in their lip or roof of their mouth.
Mastitis in breastfeeding women is more likely to be caused by an infection if self-help measures to express milk from the affected breast haven't improved symptoms within 12 to 24 hours.
Read more about treating mastitis.
In women who don't breastfeed, mastitis is often caused by a bacterial infection. This can occur as a result of bacteria getting into the milk ducts through a cracked or sore nipple, or a nipple piercing.
This type of mastitis is known as periductal mastitis. It usually affects women in their late 20s and early 30s, and is more common among women who smoke.
Occasionally, mastitis can occur in non-breastfeeding women as a result of duct ectasia. This is when the milk ducts behind the nipple get shorter and wider as the breasts age. It typically occurs in women approaching the menopause.
Duct ectasia is usually nothing to be concerned about, but in some cases a thick, sticky secretion can collect in the widened ducts, and this can irritate and inflame the duct lining.