Sources for bacteria found in the study aren’t known but could include donors’ skin, breast pumps used to extract milk, or contamination from improper shipping methods, Keim said.
The study was published online Monday in the journal Pediatrics.
There are many milk-sharing sites online, including several that provide milk for free. Sellers or donors tend to be new mothers who produce more milk than their own babies can consume. Users include mothers who have difficulty breast-feeding and don’t want to use formula and people with adopted infants.
Breanna Clemons of Dickinson, N.D., is a donor who found a local woman who needed breast milk through one of the online sites where milk is offered free.
“A lot of people are like, ‘Ewww, it’s weird,’ but they haven’t been in a situation where they didn’t want their child to have formula,” or couldn’t produce enough milk, Clemons said. She said she shared her medical history with the recipient.
Clemons is breast-feeding her 7-month-old and stores excess milk in her freezer. Every few weeks, she meets up with the recipient and gives her about 20 6-ounce bags. Clemons said the woman has a healthy 9-month-old who “loves my milk.”
Keim said it’s unclear if milk from sites offering donated milk would have the same risks because donors might be different from those seeking money for their milk. And in a comparison, the researchers found more bacteria in breast milk purchased online than in 20 unpasteurized samples donated to a milk bank.
Bekki Hill is a co-founder of Modern Milksharing, an online support group that offers advice on milk donation. She said there’s a difference between milk sellers and donors; milk donors “don’t stand to gain anything from donating so they have no reason to lie about their health.”
Hill, of Red Hook, N.Y., used a donor’s milk for her first two children and plans to do so for her third, due in February, because she doesn’t produce enough of her own.
“Breast milk is obviously the preferred food” for babies, she said.