Breast milk and infectious disease

Definition

The female breast is a “factory” of milk production. It is composed of milk-producing mammary glands and lactiferous ducts that carry milk to the nipple, which is surrounded by fatty tissue. Breast milk provides ideal nutrition for a growing newborn and offers significant advantages for the baby’s immune system. It has been known that breastfed infants contract fewer infections than formula-fed infants. Experts have begun to understand and identify the specific immune components transferred to the infant in breast milk.

Breast milk also contains microorganisms, a few of which can be passed to the infant, leading to infection and clinical disease. Rarely, the considerable benefits of breastfeeding must be weighed against the risk of transmitting infection.

Breastfeeding can also be a source of infection in the breast. Mastitis (infection of the breast tissue) is commonly seen between one and three months after delivery and may cause pain, fever, and malaise in the mother, which makes care of the newborn difficult. In almost all cases, it is recommended that a woman with mastitis continue to breastfeed.

Immunity

During the first months of life, the infant’s immune system is immature and unable to make the proteins and cells necessary to respond to “foreign” invaders. Breast milk offers protection in several ways. Immunoglobulins cross the placenta during pregnancy to help protect the fetus from infections. Some types of antibodies can cross the placenta. IgM, the first antibody to fight acute infection, does not cross the placenta.

All five major antibody types—IgG, IgA, IgM, IgD, and IgE—have been found in human breast milk and are active when ingested by the nursing infant. The most abundant type, known as secretory IgA, binds with potential pathogens, preventing them from invading the infant’s system. Most antibody types are specific for only one pathogen and do not attack irrelevant or commensal (good) organisms. However, antibodies can provide broader protection as well. Other important immune molecules are present in breast milk too. Oligosaccharides (chains of sugars) and mucins (large molecules made of protein and carbohydrates) are able to clump together with invading bacteria, making them harmless.

White blood cells (leukocytes) are abundant in breast milk; most notably in colostrum, the milky fluid that precedes the flow of milk. Neutrophils, macrophages, and lymphocytes are all present and protect the infant from disease. In addition, studies suggest that some hormones and other factors in breast milk may induce the infant’s own immune system to mature more rapidly, allowing breastfed infants to protect themselves sooner than formula-fed infants.

Infection Transmission

Few organisms are readily passed by breast milk to cause clinical infection, and it may be difficult to accurately determine the mode of transmission because breastfeeding requires close contact between mother and infant. Some infections that are spread during the breastfeeding period pass by other means, such as airborne droplets or skin contact. Concern about infection rarely leads to a recommendation against breastfeeding.

Three viruses can be transmitted through breast milk and are of greatest clinical concern. These include cytomegalovirus (CMV), human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS) and HTLV. Transmission is thought to occur through exposure to small amounts of the virus during several daily feedings during the prolonged period of breastfeeding.

CMV is a common cause of congenital infection. Most women are infected before becoming pregnant and develop antibodies that cross the placenta to protect the growing fetus and breastfeeding infant. However, if the woman experiences primary infection during pregnancy or breastfeeding, inadequate immune resources can result, and infection can result.

While breastfeeding by an HIV-positive mother once increased the transmission risk in addition to the risk of perinatal transmission, the risk is significantly reduced for mothers on effective antiretroviral therapy (ART). When a mother is on ART, the risk of transmitting HIV to her child is less than 1 percent. Still, it is not zero, and the nuances of viral suppression and breastfeeding are complex. With the use of ART and viral suppression, the benefits of breastfeeding, even from an HIV-positive mother, may outweigh the risks. However, patients may not have access to ART. 

The human T-cell lymphotropic virus (HTLV) is a cause of adult leukemia and other chronic conditions, and it is endemic to several regions of the world. Transmission occurs more often in breastfed than in formula-fed infants. Mother-to-child transmission can be avoided by not breastfeeding.

Bacterial and other infections are rarely passed to infants through breast milk. Some infections, including gonorrhea, group B strep, syphilis, or tuberculosis, could lead to a brief interruption of breastfeeding while the mother or the mother and infant begin antimicrobial therapy. One should not necessarily stop breastfeeding if using antibiotics.

Infection in the Lactating Breast

Mastitis can occur when bacteria from the infant’s mouth or the mother’s skin enter a duct through a sore, cracked nipple and multiply in breast milk, an ideal growth medium. This condition may lead to a localized, minor infection or a more serious deep-breast abscess. Symptoms include tenderness and swelling of the breast, fever, chills, and other flu-like symptoms.

Breast infections require antibiotic treatment. Prevention includes good hygiene, handwashing, and proper breastfeeding techniques to avoid cracked nipples. Most women with mastitis should continue to breastfeed; doing so does not harm the infant. Also, emptying the breast through feeding speeds healing.

Impact

Breastfeeding provides important protection against disease. The immunologic benefits are well documented and beyond question. However, for those few circumstances where disease transmission is of concern, more work is needed to develop vaccines and other interventions. 

Bibliography

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"Hepatitis B or C Infections - Breastfeeding." CDC, 4 Apr. 2023, www.cdc.gov/breastfeeding/breastfeeding-special-circumstances/maternal-or-infant-illnesses/hepatitis.html. Accessed 12 Oct. 2024.

Huggins, Kathleen. The Nursing Mother’s Companion. 5th ed. Boston: Harvard Common Press, 2005.

Jackson, Kelly M., and Andrea M. Nazar. "Breastfeeding, the Immune Response, and Long-Term Health." Journal of the American Osteopathic Association, vol. 106, no. 4, 2006, pp. 203-207.

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Lawrence, Robert, and Ruth Lawrence, editors. Breastfeeding: A Guide for the Medical Profession. St. Louis, Mo.: Mosby, 1999.

Mestecky, Jim, et al., editors. Immunology of Milk and the Neonate. New York: Plenum Press, 1991.

Riordan, Jan, editor. Breastfeeding and Human Lactation. 4th ed., Sudbury, Mass.: Jones and Bartlett, 2010.