Keim's team did not find any factors associated with safer milk. She said some of the milk was shipped improperly. She suspects that hand-washing and poor cleaning of breast pumps were responsible for some of the contamination.
She said parents may need to rethink the risk-benefit ratio of formula vs. purchased breast milk.
Keim suggested more help for women who are having trouble producing enough milk, an idea that local lactation experts heartily endorsed.
"Women get terrible breast-feeding help overall," said Diane Spatz, director of Children's Hospital of Philadelphia's lactation program. With proper support, 95 percent of women can breast feed effectively, she said. When they can't make enough milk, the most common causes are insufficient glandular tissue, breast reduction surgery, or augmentation surgery that was done through the nipple.
Parents who need breast milk have few options. There are breast milk banks, but they give milk primarily to premature babies with problems and even then don't have enough. That leaves purchasing milk, sharing with friends or sharing with strangers through online communities.
The study found lower levels of bacteria in milk that had been donated to an Ohio milk bank. That milk would have been pasteurized before use.
The study focused on milk purchased on the Internet and shipped to buyers.
Leaders of sites where women donate their milk, often in person, said the researchers may have chosen the riskiest milk samples.
"The study is sensational and flawed," said Shell Walker, a Phoenix midwife who started Eats on Feets, a site that fosters sharing but does not allow breast milk sales because it "opens up too many ethical and safety questions."
She said her site has hundreds and sometimes thousands of participants at any given time.
A similar sharing site, Human Milk 4 Human Babies, has garnered 2,000 Facebook likes for both its Pennsylvania and New Jersey chapters.
The website for Only the Breast, which advertises that it is for people who buy, sell, and donate breast milk, was not working properly Monday. Another site mentioned in the paper, milkshare.birthingforlife.com, said on its home page that it does not support selling milk. Keim declined to say which sites were used in the study.
Kim Updegrove, president of the board of directors of the Human Milk Banking Association of North America, said the study highlights a fact that milk-sharers tend to downplay.
"Human milk is a body fluid, so it contains all the bacteria and viruses that are in the mom's body at the time the milk is expressed," Updegrove said. "It's no different than blood. Would you go to your neighbor and ask for a pint of blood?"
Milk donated to banks also contains microorganisms, but it is heat-processed by the bank in a way that destroys the germs while preserving the antibodies and other immunizing components that benefit babies.
A full-term baby with a normal immune system might not be sickened by low levels of bacteria in donated milk. "But as a donor," Updegrove said, "you don't actually know the recipient's immune status."
She said formula is better than exposing a baby to possible contamination. "My advice would be to use formula. These children are just too precious."
Jenna Tress, an administrator for the Pennsylvania chapter of Human Milk 4 Human Babies, began donating milk while her first child, now 3, was an infant. "I had basically an entire freezer full of milk," she said. "It was going to kill me to throw it out after all that work."
Her youngest is 1 and Tress is donating again. She meets recipients in person. "It's forging community connections," she said. "I have become friends with a lot of the women I've donated to."
Inquirer staff writer Marie McCullough contributed to this article.