ROME — Precious survived a trip across North Africa and two sinking boats before making it to Italy, only to be confronted with the reality facing young Nigerian migrants like her: A “madam” gave her condoms, pantyhose and a G-string, and put her to work.

Nigerian teenagers and young women selling sex is a common sight for motorists in Italy. Working along roadsides and secondary highways in cities big and small, they are a haunting reminder that while Italy has been successful in curbing immigration from Libya, it has largely failed to help a fraction of the migrants trafficked as sex slaves.

Between 10,000 and 30,000 Nigerian prostitutes are estimated to be walking Italian streets, often to pay off the debts they incurred to get there.