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Common Causes of Mastitis and Who is at Risk

Woman wearing floral shirt holds top of breast and has look of pain on face.

Mastitis is the most common postpartum infection; it occurs in 25-30% of breastfeeding mothers. It can sometimes be a recurring problem and it can lead to more serious conditions, too. Although there are some surprising facts of mastitis, here are some explanations of some very common questions that most families ask.

What is the timing?

Mastitis can occur anytime. Surprisingly, it can occur during pregnancy, as well as in individuals who have never been pregnant or lactated. It can occur years after weaning, too! But it most frequently occurs during the second week or sixth week postpartum.

What causes mastitis?

Tiny glandular cells called alveoli produce milk. These cells are surrounded by muscle cells that propel the milk into the ducts (and smaller ductules) that carry the milk to reservoirs called lactiferous sinuses, and eventually to the nipple which is the external opening of the gland.

If milk is not transferred, it stays in the ducts. That’s called milk stasis. That’s a problematic situation, because the mammary gland is designed to secrete and transport milk, but not to store milk.

Hence, when the milk stays in the breast, it frequently causes a duct to be obstructed. Soon, the first sign of serious trouble can be seen: a small, hard, warm, tender lump. Usually within 12-24 hours of ductal obstruction, “inflammatory mastitis” begins.

As the “itis” suffix implies, the internal ducts and ductules become inflamed causing further local effects: a breast that is swollen, hard, tender, and hot. These signs and symptoms — subtle at first — often (but not always) appear in the upper outer quadrant of the breast where the glandular tissue is most dense and progressively intensify; redness may also appear.

At this point, the white blood cell count in the milk increases — a normal response to inflammation. However, despite this protective mechanism, the bacterial count remains largely unchanged. If the obstructed duct is not treated, the inflammation is followed by “infectious mastitis” within about 24-48 hours, and the bacterial count is elevated along with an even higher white blood cell count.

Systemic effects of infectious mastitis — fever, chills, flu-like symptoms and malaise — occur when the inflamed tissue becomes infected by bacteria. Most frequently, the invading organism is Staphylococcus aureus.

Although small amounts of Staphylococcus are normally present on the external nipple skin, the organism should not be present in the internal ductule system. Nipple trauma (e.g., cracked nipples) makes the breast more vulnerable to mastitis, because the organism can go directly into the ductule system through the broken nipple skin. Organisms (including bacteria or other pathogens) thrive in environments that are warm, moist and have food available. Warm, moist milk contains lactose, and it’s an ideal medium for bacterial growth.

Although it’s easy to associate poor hygiene with infection, there’s no evidence that poor hygiene is the cause of mastitis.

Who is at high risk for mastitis?

Women who have previously had mastitis are much more likely to experience it again. And women who have a greater-than-normal milk supply are also at higher risk for mastitis.

Some conditions increase the risk for mastitis. For example, diabetes increases blood glucose (sugar) levels, which provide food for bacteria to thrive on.

Mothers can do little to change their personal histories, but all mothers can avoid or minimize future episodes of mastitis by taking some simple preventive steps. Read my post on how to prevent and treat mastitis.

Do you know the warning signs and causes of mastitis?

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