Keim said the researchers logged all sorts of information about the milk they got to find patterns of problems that might signal contamination. Some of the milk was shipped with gel packs or dry ice to keep it cold, but that didn't seem to matter. The temperature of the milk when it reached the researchers didn't make a difference. The kind of container and its condition also didn't seem to play a role, nor did promises of healthy, fresh, or organic milk in ads placed by sellers.
"There was nothing that was helpful," Keim said. "You just don't know what you're getting."
In addition to bacteria and viruses, milk can contain traces of drugs and other environmental contaminates, like cigarette smoke.
Profit may also have a hand in how safe the milk is. In the 1950s, when blood banks paid people for blood and plasma, studies found that purchased samples were seven to 10 times more likely to carry diseases like hepatitis than donated samples. The theory was that people who needed to sell blood to make money were also less likely to be healthy than those who donated to patients.
"With the monetary piece in this, we're a little worried that people might be incentivized to do things that aren't 100 percent honest and safe," Keim said.
She also said they're in the process of retesting their samples to find out exactly what's in them "because we suspect some of them might not have been 100 percent human milk."
The study is published online Oct. 21 and in the November print issue of Pediatrics.
Milk-sharing advocates point out that women have been helping each other nurse for generations. Before the Internet came into the picture, they say, mothers often relied on other women as wet nurses. And they say there's never been a documented case of a baby getting sick from shared milk.
That's true, Marinelli said, adding, "But do you think a mom who is buying milk off the Internet and her kid gets sick is going to necessarily tell the doctors what she did?"
And she said that most of the kinds of bacteria found in the study would probably cause symptoms like diarrhea, vomiting, listlessness, and in severe cases maybe an all-over infection -- and mothers might not even realize that the milk caused the problem.
For her part, Katie Sweet, who said she was still waiting for the first shipment of purchased breast milk to arrive, said the study left her feeling disappointed. She said she planned to contact her doctor and a friend who is a nurse practitioner to figure out how to proceed.
"I think if you work so hard to seek out milk it would be devastating to have a reaction like that," she said.
To learn more about the use of donor breast milk, head to the U.S. Food and Drug Administration.
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