As detailed in their report, Cow’s milk contamination of human milk purchased via the internet, researchers in Ohio tested 102 samples of breast milk anonymously purchased online.
They tested 200 microliter (μL) measures of each sample for human and bovine DNA.
"To better understand the presence of bovine DNA" in these samples, four laboratory mixtures of human milk with fluid cow's milk or reconstituted Similac Advance infant formula were also prepared.
Of the 102 breast milk samples tested, 11 contained bovine DNA.
Ten contained levels of bovine DNA consistent with human milk mixed with around 10% cow's milk.
“Tens internet samples had bovine DNA concentrations high enough to rule out minor contamination suggesting a cow’s milk product was added,” said the study.
The "altruistic donation" and sale of breast milk online - driven by a reluctance to use infant formula - "has become popular" in the US in recent years.
An estimated 13,000 posts offering breast milk are currently published on US websites, such as Eats on Feets, Human Milk 4 Human Babies, and Only the Breast, each year.
The US Food and Drug Administration (FDA) "recommends against" feeding infants breast milk "obtained directly from individuals or through the internet."
Such breast milk "is unlikely to have been adequately screened for infectious disease or contamination risk" and is not likely to have been "collected, processed, tested or stored in a way that reduces possible safety risks to the baby," says the FDA.
The discovery of bovine DNA in samples is, according to the study, also a cause for concern.
It represents a "potential risk to infants with allergy or intolerance to cow's milk," it said.
"Cow's milk can be problematic for infants with allergy or intolerance."
"Because buyers cannot verify the composition of milk they purchase, all should be aware that it might be adulterated with cow's milk."
"Pediatricians should be aware of the online market for human milk and the potential risks," it concluded.
Source: Pediatrics DOI: 10.1542/peds.2014-3554
Title: Cow’s milk contamination of human milk purchased via the internet
Authors: S A Keim, M M Kulkarni, K McNamara, S R Geraghty, R M Billockd, R Ronau, J S Hogan, J J Kwiek.