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Only some infectious organisms have been reported to be transmitted and to be infectious via human milk
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Bacteria: MTB and certain Streptococci
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Transmission of both could be through lesions on the breast
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Presence and transmissibility may be context-specific (e.g., geography)
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Viruses shown to be transmitted in human milk include: 1) HIV-1; 2) CMV; 3) HTLV-1; 4) DENV; and 5) ZIKV
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Mechanism of secretion differs for most of these viruses
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HIV-1 may be translocated across the mammary gland epithelium
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The advent of highly active antiretroviral therapies (HAAARTs) has significantly reduced HIV transmission from the birthing parent to the child, presumably by decreasing the viral load in the lactating parent and human milk
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HTLV is likely translocated across the mammary gland epithelium by leukocytes
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CMV is reactivated in mammary gland cells and shed into the milk
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The mechanisms by which DENV and ZIKV translocate to human milk are not yet known, but these viruses must have some way of crossing the mammary gland epithelium into human milk
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To date, there is no evidence of transmissible, infective SARS-COV-2 in human milk
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Human milk also contains myriad individual biologically active proteins that play extensive roles in protection and communication
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Secretory IGA (sIgA), the most abundant immunoglobulin in human milk, may provide unique vertical transmission of immune system education
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sIgA in milk serves a dual function by protecting newborn infants from infection by neutralizing pathogenic bacteria and at the same time shaping the commensal GI microbial community structure
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Specificity of sIgA in milk reflects prior parental exposures
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sIgA has also been shown to affect host–microbiome interactions by modulating the colonic transit time of bound microbes and binding to antigens originating from the lactating parent’s diet
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Leukocytes are the predominant cells in colostrum and early milk
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Leukocytes are presumed to protect the mammary gland from infection during lactation but also may respond to the health of the infant as concentrations increase during parental or infant infection
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In vitro studies suggest that the leukocytes present respond to viral antigens with increased proliferation rate and altered expression of IL-6, IL-17A, interferon-g, and tumor necrosis factor-a demonstrating potential roles to deliver protection in the form of innate immunity to the infant
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There is evidence suggesting that several metabolically active components of human milk may be involved in interactions that may provide protection (e.g., xanthine oxidase reductase [XOR] enzymatic activity produces reactive oxygen and nitrogen species that have been shown to have antimicrobial and infant immune defense properties)
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WG 2 provided several other examples of how the complex interactions of components within the milk matrix may confer protection
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Active infection in infants from a variety of ailments, including influenza, measles, and respiratory and gastrointestinal infections, has been associated with an increase in human milk leukocyte concentrations, which quickly return to baseline after recovery
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The mechanisms for this response are unclear; however, it has been suggested that 1) active infection in the infant either co-occurs with or results in an infection of the parent and/or 2) the presence of infant infection signals the need for an inflammatory lactating parent response
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Both mechanisms result in increased secretion of white blood cells and cytokines into milk, a cascade of events that could potentially have the dual effect of attacking infectious organisms while also adversely impacting the human milk microbiome
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Antibiotic use by the lactating parent has been shown to change the human milk microbiome
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Antibiotic use during infancy alters the gut microbiome of the infant. Whether an altered infant gut microbiome can have a clinically relevant impact on the human milk microbiome remains to be determined
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