John D. Sutter is a columnist for CNN Opinion who focuses on climate change and social justice. Follow him on Snapchat , Facebook and email . The opinions expressed in this commentary are his.

(CNN) Mauritania, the West African country long thought to be home to the world's highest percentage of enslaved people, no longer holds that lamentable title, according to a report released late Monday.

The Walk Free Foundation's slavery index says the percentage of people living in modern slavery in Mauritania dropped from 4% in 2014 to about 1% this year. The country, which CNN featured in the 2012 documentary "Slavery's last stronghold," now has the world's seventh-highest incidence of slavery.

North Korea ranks worst on the index. Nearly one in 20 people there are thought to be enslaved.

"Slavery is not a thing of the past, and we must stop thinking that it is," the Walk Free Foundation said in a statement issued to CNN. "The very nature of modern slavery means it is clandestine and hidden from view, but that doesn't mean it isn't everywhere. Every country in the world is affected."

Globally 45.8 million people are held in slavery, according to the report, which is based on random surveys in 25 countries, including Mauritania, and statistical modeling. That's a 28% uptick since the group's last slavery index report was released in 2014. But Fiona David, the Walk Free Foundation's executive director of global research, cautioned against reading too much into that increase or the decrease in slavery's prevalence in Mauritania. The group's ability to estimate the prevalence of slavery is improving over time, she said, so the trends may result in part from better stats.

"It's too early for us to say whether or not there's been an absolute increase," she said.

The group uses computer models to estimate the prevalence of slavery in countries where it was unable to do surveys. That process of risk assessment has "gone from horse and cart to having a car on the road" in terms of its sophistication, David said. "What we can categorically say," she added, "is we have a better picture (of the prevalence of slavery worldwide) than we've ever had before."

The Walk Free Foundation defines slavery as "situations of exploitation that a person cannot refuse or leave because of threats, violence, coercion, abuse of power or deception."

The nonprofit worked with Gallup to conduct in-person surveys in Mauritania for the first time, David said. Of the 1,000 people interviewed in Mauritania, 37 -- or 3.7% -- said they had been subjected to forced labor or were forced into marriage, according to the report. The percentage was adjusted to ensure the random sample represented the demographics of Mauritania, David said, arriving at this year's 1% figure.

When CNN reported on slavery in Mauritania in 2012, experts, including Gulnara Shahinian, then the U.N. special rapporteur on modern forms of slavery, estimated 10% to 20% of the population was living in modern forms of slavery. With echoes of pre-Civil War America, slavery is still hereditary in parts of the vast country, with mostly darker-skinned black Africans being enslaved by lighter-skinned Arabs.

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It's difficult to get information out of the country. Top government officials continue to deny slavery exists in Mauritania. They often couch anti-slavery programs in terms of "vestiges" of the practice.

"While Mauritania has been the focus of extensive interest and reporting in the past, it has not had the benefit of a national survey until now," the report says. "The extent of slavery in Mauritania is still high; however more reliable methods indicate that it is not as high as previously thought."

Researchers cannot rule out the possibility that the prevalence of slavery in Mauritania is in fact decreasing because of growing awareness. The Mauritanian government last year established special anti-slavery courts. Those courts made their first convictions in May -- marking only the second time that a slave owner had been convicted in Mauritania since the practice of owning slaves was criminalized in 2007.

"It's very encouraging to see the government making new legislation," David said. "And it's very encouraging to see (anti-slavery activist) Biram Dah Abeid was released after 18 months' imprisonment."

Sarah Mathewson, Africa program coordinator for Anti-Slavery International, said slavery in Mauritania is "shrouded in secrecy" and therefore hard to measure.

"It is incredibly difficult to ascertain how many people remain in slavery in Mauritania," she said in an email. "To my knowledge there has never been any comprehensive, nationwide study to identify numbers of people in slavery alongside the local organizations best placed to find cases; the Mauritanian government hasn't allowed that to happen, and because the local organizations do not have the funds to carry out such studies ... we could confidently say that hundreds of thousands continue to be affected, as we have met entire villages of people that might remain under the control of their masters."

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Among the other key findings of the report:

• "Though information on North Korea is difficult to verify, pervasive evidence exists that citizens are subjected to state-sanctioned forced labor, including through forced labor as political prisoners and as workers on overseas contracts."

• "Uzbekistan has the second highest estimated proportion of prevalence of modern slavery by population. While some steps have been taken to address forced labor in the cotton industry, the Uzbek government continues to subject its citizens to forced labor in the cotton harvest each year."

• "In 2016, we estimate 18.3 million people are in some form of modern slavery in India. This estimate reflects extensive surveying conducted in 2016 in 15 states. While many impressive efforts are being taken by the Indian Government to address vulnerability, survey data suggest that domestic work, construction, farming, fishing, manual labor and the sex industry remain sectors of concern."

David highlighted some positive developments as well, including the UK passing a law that requires large companies to report on what they're doing to root out slave labor in their supply chains.

That's a model for other countries, including the United States and China, to follow, she said.