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CHICAGO — A cottage industry has sprung up facilitating the sale and donation of human breast milk on the Internet, but a study published Monday in the journal Pediatrics confirms the concerns of health professionals over this unregulated marketplace.

The report found that breast milk bought from two popular websites was often contaminated with high levels of bacteria, including, in a few instances, salmonella. The amounts detected in some samples were sufficient to sicken a child.

Research shows that breast milk protects infants from infections and other ailments, and health-care providers in recent years have strongly encouraged new mothers to abandon formula and to breast-feed. But this can be a difficult challenge. Parents who have adopted, for instance, or have had mastectomies — or who simply do not produce enough milk — often rely on donated or purchased breast milk.

“Milk-sharing” websites host classified advertisements from women wishing to buy, sell or donate breast milk.

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“My daughter is two months old and has gained five pounds and grown three inches since birth!” reads one ad. “I have a serious oversupply and I am looking to free up room in my freezer.”

Advertisements from some sellers play up the convenience and price, which can be as low as $1.50 an ounce. But many women wish to donate milk simply to help out fellow mothers in need.

Women who do not wish to rely on an informal network may turn to breast-milk banks, which store and sell breast milk from donors. Thirteen banks in the United States and Canada follow voluntary guidelines set up by the Human Milk Banking Association of North America, which require that donors and donations be screened, and that milk be pasteurized.

But milk banks give priority to premature infants with significant medical complications, and the milk is available only by prescription.

The association’s banks distributed 2.15 million ounces in 2011, but the current annual need is 9 million ounces.

Even when it is available, milk from banks is expensive, as much as $6 an ounce, and the average intake of a 1-month-old is about 25 ounces a day.

All of this encourages new parents, often scrambling to find breast milk on short notice, to shop for cheaper or more convenient sources, and websites have sprung up to fill the demand.

In 2011, there were more than 13,000 postings on the four leading milk-sharing websites, according to Sarah Keim, an assistant professor of pediatrics at Ohio State who was involved in the study.

Keim and her colleagues collected 101 samples from milk-sharing websites, recording the sellers’ assertions about their own health and their methods of handling and storage. Then the researchers analyzed the bacterial content in the samples, comparing them with unpasteurized breast milk from screened donors at banks.

The researchers found that 64 percent of the samples from milk-sharing sites were contaminated with staph, 36 percent with strep and almost three-quarters with other bacterial species. Three of the samples contained salmonella. Seventy-four percent of the samples would have failed milk-bank criteria.

“Most staph and strep are harmless at normal levels,” Keim said. “But some of the levels we found were very high.”

Salmonella “doesn’t belong in milk at all,” she noted.

The Food and Drug Administration does not regulate the buying or selling of breast milk, but the agency discourages both online and person-to-person sharing. Only three states — California, Maryland and New York — regulate milk banks.

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