Sharing is caring, but women exchanging breast milk with other mothers online may be passing on more than just nutrition. In a study appearing on Monday in the journal Pediatrics, doctors tested 101 samples of breast milk purchased over the Internet and found high levels of bacteria lurking in the samples.

"It’s natural for breast milk to have some bacteria in it,” says lead researcher Sarah Keim, a researcher at Nationwide Children’s Hospital in Columbus, Ohio. “There’s tons of beneficial bacteria” -- some thought to be key to developing a healthy immune system -- “but these are not the ones we focused on.”

Breast milk can often contain relatively low levels of benign strains of Staphylococcus and Streptococcus from a mother’s skin, but there are more dangerous types of bacteria that can crop up: Salmonella and fecal coliform bacteria that can cause disease even at lower levels. These kinds of pathogenic bacteria can cause “anything from a mild case of diarrhea to sepsis, meningitis or even death,” Heim says. How much bacteria is necessary to cause disease will depend on the baby in question -- and a sick baby will be most at risk.

Breast milk banks typically will not feed raw breast milk to a baby if it is found to have more than 10,000 colony-forming units per milliliter, or if it contains traces of pathogenic bacteria. Three quarters of the milk the researchers bought over the Internet failed at least one of these standards, Keim says. The harmful bacteria may have come from unclean containers, unsanitary breast milk pumps, a lack of hand washing or improper storing and shipping practices. Nineteen percent of the breast milk samples ordered online arrived in the researchers’ mailbox with no kind of cooling device in the package, and were found marinating at room temperature.

Breast milk sharing advocates immediately pushed back against the study, saying the methods the researchers used would naturally exclude responsible donors, thus rendering the findings inapplicable to most mothers’ experiences.

In a blog post, Eats on Feets criticized the study, arguing that “it only addresses the use of donor milk obtained in a deceitful and unethical manner on behalf of the recipient (the researchers), and shady black market sales (the donors). These practices do not apply to the breastmilk [sic] sharing community.”

Eats on Feets pointed out that Keim and her colleagues worked only with donors that were willing to ship off their milk to an unknown, anonymous buyer. The researchers terminated interactions with sellers who inquired about the hypothetical infant that would be consuming the milk, or those that insisted on talking with the purchasers on the phone or in person. Fifty-seven transactions were terminated this way, and of the 495 inquiries the researchers sent, 191 people did not respond at all.

"Women aren't stupid,” Emma Kwasnica, the founder of the milk exchange network Human Milk 4 Human Babies, told the Verge. “They get a vague, anonymous note asking to buy some milk? They know something weird is going on."

Keim acknowledges that her study has some limitations.

“It’s not clear whether the milk we received would be representative of all milk on internet,” Kwasnica said. “But we felt it was important to ensure anonymity. If we didn’t, that would also bias the sample milk that we received.”

Because the study focused on websites that sold milk for money, it may also be hard to generalize the results in respect to sharing networks, Keim says.

Women who have trouble making milk for their babies should talk to their physicians first, Keim says. The doctor can connect the mother to lactation support groups and other resources in their area. 

“Our focus is getting this information out to the public and to health care providers so people can make the best decisions to feed their babies,” she says.

Meanwhile, if women have a freezer full of extra milk, they may want to consider donating to their nearest milk bank, Keim added.

What can mothers do if they’re looking for a source of breast milk? Some turn to one of the 13 milk banks across the U.S. and Canada. These banks pasteurize their milk and screen donations and donors closely, according to guidelines set by the Human Milk Banking Association of North America. But getting breast milk from officially sanctioned banks requires a prescription, and priority goes to premature babies and babies with medical problems, so mothers with healthy babies may be left holding the bottle.

If a mother of a healthy baby can get her hands on milk from a bank, it’s not cheap. According to Sue Evans, the coordinator for the WakeMed Mothers’ Milk Bank in New York, the processing fee for screened and pasteurized donor milk is $5 per ounce. Some insurance companies will pick up the cost, but for an uninsured mother, those feeding costs add up. Babies need anywhere from 12 to 36 ounces of milk or formula a day, depending on their age and weight. It’s not hard to see the allure of a cheaper option – Keim and colleagues ended up paying an average of just $1.47 an ounce for their Internet milk, according to the Verge.

The U.S. Food and Drug Administration already cautions mothers against using milk obtained directly from other people or through the internet. But the FDA doesn’t actually regulate breast milk marketplaces or sharing networks. And even donor milk banks are operating without clear government guidelines: New York and California are the only states that regulate their milk banks (requiring them to become licensed tissue banks under the eye of the state health department). Part of the problem, Keim says, is that the government can’t quite agree on what to classify breast milk as. Is it a bodily fluid, like blood? A food? A medicine, as some doctors say it is?

Whatever the government ends up deciding, sharing advocates insist that nursing mothers are careful.

“New moms have just gone through the most intensive screening of their lives,” Kwasnica told the Canadian Medical Association Journal in 2011. “They know if they’re positive for HIV or hepatitis or syphilis or gonorrhea, and they can provide ready proof of that information. Not only are women asking to see blood screening records, they’re seeing the healthy baby gurgling away on the other mom’s hip.”

SOURCE: Keim et al. “Microbial Contamination of Human Milk Purchased Via the Internet.” Pediatrics published online 21 October 2013.