Parents in a Houston suburb were arrested Monday on suspicion of withholding their Nigerian nanny's pay for two years while she cared for their five children, a snapshot of a nationwide problem that ensnares many live-in domestic workers whose isolation and immigration status make them vulnerable to abuse.

The couple from Katy, Texas, are charged with forced labor, withholding documents, visa fraud, and conspiracy to harbor an illegal immigrant. The nanny says that Chudy and Sandra Nsobundu physically and verbally abused her, seized her passport, and had not paid her for two years. She was initially promised $100 per month.

The nanny escaped the home last October with help from an anti-trafficking group.

Such groups say that awareness is on the rise: at the the Polaris Project, for example, which runs the National Human Trafficking Resource Center Hotline and BeFree Textline, nearly 6,000 calls were recorded last year, a 10 percent increase from 2014. The majority, however, were for sex trafficking victims.

Live-in domestic servants are uniquely vulnerable to abuse because their work takes place out of sight, experts say – which also makes it difficult to estimate how many are in the United States. Roughly 2 million people work in "in-home" professions, from housecleaning to elder care, according to a 2013 report from the Economic Policy Institute. Those who are live-in workers, or immigrants, are especially difficult to tally, and especially vulnerable.

The Fair Labor Standards Act does not apply to live-in staff, making it hard for them to insist on protections like minimum wage and time off. Many are immigrants whose work visas only cover them while they work for one specific employer, making them nervous that losing their job or reporting abuse will lead to deportation, advocates say. In many instances, employers seize workers' passports, making it yet more challenging to leave.

In a survey of more than 2,000 domestic workers, the National Domestic Workers Alliance found that 23 percent were paid below minimum wage, including 67 percent of live-in workers. Although live-in workers do not have to pay for housing, many are supporting their families elsewhere in the US or abroad.

The survey also reported that 65 percent did not have health insurance, and 20 percent had gone hungry in the past month. Many did not have contracts, but of those who did, nearly one third said their employers had broken it in the past year. Nearly 90 percent of them had not complained because they feared losing their jobs.

"People think this happens elsewhere in the world, not in the rich, liberal Northeast," Quinn Kepes told the Boston Globe, which estimated that Massachusetts had as many domestic workers as Boston has finance professionals. Ms. Kepes is program director at Verite, a nonprofit that investigates worker abuse. "These women, and it’s mostly women, have nowhere to go. They’re socially and culturally isolated, and they have nowhere to turn."

This report includes material from Reuters and the Associated Press.