Breastfeeding mastitis

What is mastitis?

Mastitis is a bacterial breast infection. It is most common in breastfeeding mothers. It can develop as early as the first two weeks after delivery but can develop any time during breastfeeding. One estimate is that 3% to 20% of mothers develop mastitis while breastfeeding. It starts quickly, with the breast becoming very tender and painful within a few hours. Parts of the breast may become red, warm, and swollen. Mothers may feel dizzy and weak with nausea, headaches, fever, and muscle aches. If there is a lump in the middle of the infected area, it may be a sign of a severe infection or the start of an abscess. Mastitis generally affects only one breast at a time. Mothers with mastitis should continue to breastfeed if possible and see their health-care providers.

A) Describing mastitis

Mastitis of the left breast. The skin has turned red and the area is very painful.

Mastitis is a bacterial infection of the breast tissue and skin. The bacteria that cause mastitis are some of the 700 species that are normally found in breast milk and on the areolar skin and that make up the breast microbiome. The most common bacteria that cause mastitis are (Angelopoulou 2018):

  1. Staphylococcus aureus
  2. Staphylococcus epidermidis
  3. Streptococcus

The milk of mothers who develop mastitis appears to have fewer types of bacteria overall but more of the types that cause illness (Fernández 2013; Jiménez 2015; Patel 2017).

It is estimated that 3% to 30% of mothers develop mastitis while breastfeeding (Amir 2014; Angelopoulou 2018). Mastitis commonly develops during the first two weeks after delivery but can develop any time during breastfeeding (Amir 2014; Foxman 2002). Mothers with damaged nipples that become infected are at an especially high risk of it.

Mothers who develop mastitis must ensure that it is treated and also try to prevent further episodes.

B) Signs of mastitis

Mastitis develops quickly. Within a few hours, the breast may become:

  • Red (streaks or a red patch) if the skin is lighter 
  • Warm
  • Tender to touch
  • Painful
  • Firm
  • Swollen

The pain caused by mastitis can range from mild to severe.

Mothers with mastitis may also feel very sick, indicating that they have a toxic mastitis. The signs of a toxic mastitis include the following:

  • Dizziness
  • Weakness
  • Shaking
  • Nausea
  • Headaches
  • Muscle aches
  • Fever of 38.5°C (101°F) or more
  • Rapid heart rate
  • Swollen lymph nodes in the armpits

Some mothers find a firm mass or lump in the middle of the infected area. This may be the body’s response to infection. If it is, it will clear as the infection is treated. A lump is a sign of a severe infection and it may be the start of the area liquefying and creating an abscess.

Generally, mastitis affects only one breast at a time. It is extremely rare for mastitis to affect both breasts at the same time. We have only seen this in two patient. If both breasts are swollen and hard, it is more likely to be engorgement.

Mothers with mastitis are strongly encouraged to continue breastfeeding.

Please see your health-care providers as soon as possible if you have signs of mastitis. Delaying treatment prolongs the illness and increases the risk of an abscess (Dener 2003).


Amir LH; Academy of Breastfeeding Medicine Protocol Committee. ABM clinical protocol #4: Mastitis, revised March 2014. Breastfeed Med. 2014 Jun;9(5):239-43
Angelopoulou A, Field D, Ryan CA, et al. The microbiology and treatment of human mastitis. Med Microbiol Immunol. 2018 Apr;207(2):83-94
Dener C, Inan A. Breast abscesses in lactating women. World J Surg. 2003 Feb;27(2):130-3
Fernández L, Langa S, Martín V, et al. The human milk microbiota: origin and potential roles in health and disease. Pharmacol Res. 2013 Mar;69(1):1-10
Foxman B, D'Arcy H, Gillespie B, et al. Lactation mastitis: occurrence and medical management among 946 breastfeeding women in the United States. Am J Epidemiol. 2002 Jan 15;155(2):103-14
Jiménez E, de Andrés J, Manrique M, et al. Metagenomic Analysis of Milk of Healthy and Mastitis-Suffering Women. J Hum Lact. 2015 Aug;31(3):406-15
Patel SH, Vaidya YH, Patel RJ, et al. Culture independent assessment of human milk microbial community in lactational mastitis. Sci Rep. 2017 Aug 10;7(1):7804