What does the research tell us about the diagnoses of subclinical or subacute mastitis?

Background to the use of the diagnoses mammary dysbiosis and subclinical or subacute mastitis

Sodium concentration is highest in colostrum, averaging 48.2 ± 1.7 mmol/l in Malaysian mothers, and decreases by 30% in transitional milk, while potassium levels remain relatively constant.

In a study on Bangladeshi women, a single session of lactation counseling significantly reduced the geometric mean Na/K ratio from 0.49 (uncounseled) to 0.42 (counseled) at 1 month postpartum, decreasing the proportion of milk samples with a Na/K ratio >0.6 from 25% to 10%. The counselling messages included feeding on demand and how to achieve good positioning and attachment.

A review of the research finds that:

1. The diagnosis of subacute (or subclinical) mastitis derives from the dairy industry.1

2. The condition popularly diagnosed as nipple thrush or mammary candidiasis is not supported by evidence.11

3. The concept of dysbiosis is increasingly contested by human microbiome researchers, since it inaccurately assumes a comparative state of eubiosis against which to determine dysbiosis. Microbiome researchers increasingly focus on function of microorganisms, rather than taxonomically cataloguing species. 12-14

4. Identification of Staphylococus and Streptococcus in human milk does not imply causation of pathology.15 The hypothesis that pathogenic biofilms develop in mammary ducts and resist medication penetration is not supported by evidence.12 Lactating women may occasionally express cords of inspissated milk in the context of breast inflammation and this may occasionally contain mucinous solid. This more solid expressed milk and products would be expected to contain very high leukocyte and epithelial counts, and secondary biofilm formation. There is no reason to believe that biofilm formation is causative.

Samuel et al 2020

In 2020, Samuel et al diagnosed 40% of 305 breastfeeding mothers at day 2 post-birth with subclinical mastitis, decreasing to 10% at day 17, and 5% at day 30. Samuel et al applied a milk sodium to potassium ratio greater than 0.6 as diagnostic.5 But in this and earlier related studies, subclinical mastitis remains very poorly defined.

• The authors found that their diagnosis of subclinical mastitis was associated with lower lactose levels, changes in fatty acid, mineral, and trace element composition, an.d elevated interleukin and inflammatory proteins. They found more subclinical mastitis in mothers who underwent caesarean, attributing this to delay in breastfeeding leading to breast engorgement and inadequate breast emptying. They found no difference in infant milk intake, number of feeds per day, infant weight gain and head circumference between those diagnosed with and those without subclinical mastitis.

• The authors observed that the prevalence of subclinical mastitis varied remarkably across countries, proposing that differences in breastfeeding practices impacted.5

The NDC mechanobiological model of breast inflammation argues that Samuel et al not in fact demonstrate the existence of a clinically relevant diagnosis, subclinical mastitis. Their study does, however, demonstrate that multiple factors related to inflammation are variably identifiable in human milk, and corroborates the NDC mechanobiological model of breast inflammation, which proposes that the human breast and milk are dynamically situated at all times on a spectrum of inflammatory or wound-healing activity, which forms part of the mammary gland's immunoregulatory system, and which maintains breast health during a time of highly dynamic tissue activity. Subclinical feedback loops operate to downregulate inflammation within the multiple complex systems of the lactating breast, maintaining breast health when the breast is stressed, and impacted by both biological and behavioural factors.

The interindividual variability of pro-inflammatory factors in mothers’ milk reflects the impact of multiple genetic and clinical breastfeeding factors upon intra-luminal pressures, including frequency of feeds and conflicting intra-oral vectors of force during suckling. For example, the transition from colostrum to transitional to mature milk occurs at different rates between women depending on myriad factors.

There may be an excess of milk secretion relative to the infant’s needs post-birth, which downregulates in response to mechanobiological factors through a pro-inflammatory response, or milk secretion may downregulate due to restrictive feeding practices or compromised poor milk transfer. These same multiple factors will impact on infant weight gain and introduction of formula. The diagnosis of subclinical mastitis is redundant.

Betts et al 2021

In a 2021 audit of medical records in a breastfeeding medicine clinic, 8 of 25 patients who were unsuccessfully treated with antifungals prior to presentation were diagnosed with subacute mastitis. Betts et al observed that for 7 of these, symptoms resolved within 42 days on a 4–6-week course of a macrolide antibiotic; in the eighth case, with probiotic treatment. Unfortunately, this retrospective review is methodologically flawed and does not support the validity of the diagnosis of subacute mastitis nor the efficacy of the antibiotics.2

Ito et al 2024

This was a cross-sectional survey of 106 donated milk samples from well women without clinical mastitis or infections. Subclinical mastitis was diagnosed if Na:K ratio was 0.6 or higher. 30% of the participants were considered to have subclinical mastitis. Lactoferrin concentrations and true protein concentrations were significantly higher in the milk samples from the subclinical mastitis group.

The authors write: "In normal human milk, the sodium concentration ranges from approximately 5 to 6 mmol/L, and sodium concentration regulation in human milk is carefully controlled, separating milk from other fluids via tight junctions between the mammary alveolar cells. However, during mastitis, tight junctions open due to inflammation. This enables intercellular fluid and plasma to enter milk through paracellular pathways between alveolar cells."

This study corroborates the NDC mechanobiological theory of breast inflammation, demonstrating that inflammatory processes occur on a spectrum in breastfeeding women and are a normal part of the mammary immune response, which keeps the breast in dynamic homeostasis. The NDC mechanobiological model notes that mechanical pressures apply strain on tight junctions, which is known to trigger an inflammatory response in the breast, which will result in increased Na:K ratios.

Castro-Navarro et al 2025

This study extensively analyses the immunological composition of breast milk in 32 lactating women each week for six weeks postbirth. The authors compared the immune profiles of the milk produced by 10 women experiencing clinical mastitis and 8 women experiencing subclinical mastitis (SCM) during the week of sign/symptom development and in the week prior, comparing this with 14 healthy controls.

Interestingly, signs or symptoms used by the researchers to diagnose a clinical mastitis included the sign of nipple pain alone. Subclinical mastitis was diagnosed by milk analysis in a healthy woman who had no symptoms (though from then on she was viewed as unhealthy). The diagnosis of subclinical mastitis was made if the somatic cell count was higher than 400,000 cell/ml, and/or the sodium-to-potassium ratio higher than 1.0.

Analysis of milk from participants in the week prior to the diagnosis of

• Clinical mastitis showed elevated concentrations of pro-inflammatory cytokines

• Subclinical mastitis did not show milk changes relative to women in the 'healthy women' category.

Analysis of milk from participants at the time of diagnosis of

• Clinical mastitis showed high levels of changes in the milk's immune profile (including pro-inflammatory cytokines and chemokines)

• Subclinical mastitis showed intermediate levels of changes in the milk's immune profile.

Immune profile changes only occurred in milk from breasts affected by clinical signs. This study doesn't provide evidence that milk's immune profiles can be used to improve mastitis risk prediction.

This study corroborates the NDC model of breast inflammation, demonstrating that the diagnosis of subclinical mastitis is used to describe an immune profile which lies on the more active end of the spectrum of pro-inflammatory changes, as part of the self-regulating immune activity of the lactating breast. This study corroborates the NDC perspective that the diagnosis of subclinical mastitis fits the category of overmedicalisation and places women at increased risk of unnecessary treatments, without benefit.

Recommended resources

Diagnoses of subacute or subclinical mastitis and mammary dysbiosis don't help breastfeeding women, but promote antibiotic overuse and inappropriate treatments

References

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2. Ito M, Tanaka M, Date M. Immunological factors and macronutrient content in human milk from women with subclinical mastitis. Journal of Human Lactation. 2024;41(1):26-33 DOI: 10.1177/08903344241297585.
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