The news that criminal gangs have penetrated refugee communities from the area hit by the Nepali earthquake and are offering their children for sale as domestic servants in the UK and other wealthy destinations is shocking, but not surprising. This kind of transaction is happening all over the world.

For those concerned about the emergence of modern forms of slavery, it is a commonplace that the numbers of adults and children trafficked into sexual or labour exploitation, including domestic servitude, is huge and growing.

Compared with the 200 years of the Transatlantic Slave Trade, during which around 14 million black Africans were forcibly trafficked across the Atlantic to the Caribbean and the Americas, there are now probably in excess of 30 million adults and children at any one time who could be regarded as modern slaves. At this order of magnitude, there remains considerable doubt about precise numbers.

Although international conventions have emerged since the creation of The League Of Nations, and slavery is now illegal in most states, this brings problems of its own. Unlike the Transatlantic Slave Trade, managed as an international enterprise, the modern slavery business is hidden and conducted, not in the open by states, but covertly by criminal gangs. It is now the second or third most profitable global trade, after the drugs and arms trades, depending on how you count turnover.

These gangs exploit the increasing mobility and vulnerability of labour to move people to where modern slaves might be put to work, whether as domestic servants, picking vegetables, packing meat products, working in brothels, farming cannabis or even being available for organ harvesting.

Slavery is now the second or third most profitable global trade after the drugs and arms trades

States are waking up slowly to this trade; cases are increasingly (although still in small numbers) being brought before the courts. However, despite legislation addressing the modern slave trade, most professionals whose job should include spotting the victims of trafficking and forced labour, and the vast majority of the public, remain unaware of this trade in human beings, having little understanding of how to spot a victim.

Some cases achieve a very high media profile, although the public, through misreporting or misunderstanding, often don't understand the significance of what they are reading. One such case was that of the Morecambe Bay cockle pickers in the UK in 2004 in which 23 Chinese workers drowned when they could not run faster than the incoming tide in a notoriously dangerous coastal area in Northern England. For most people, however, the issue came to be understood as an issue of illegal immigration given that the workers were smuggled into the UK before they were trafficked into forced labour.

Another was the case of Victoria Climbie, a little girl from West Africa, brought to the UK allegedly to improve her life chances, but who ended up being beaten to death by adults who imprisoned her. This became a case primarily of child cruelty and professional incompetence. The fact that she had been trafficked into the UK was lost sight of.

Current visa arrangements mean that if a servant (who may in time be one of these sold children) protests their actual conditions of employment − abuse, severe exploitation and rape − they face deportation to perhaps being re-trafficked.

Border Agency officials are getting wiser now but this has not stopped the flow of children and young adults being trafficked into the UK and other so-called "developed" countries. Even where Border agencies are suspicious of the stories told by unaccompanied children or by those bringing them across borders, and children are taken into care, many still escape from care homes rapidly and are found later as prostitutes on the streets of cities, or disappear into private domiciliary settings.

One such case recently highlighted was of a 13-year-old girl brought to a northern UK city and incarcerated in domestic servitude for 15 years, beaten and abused by older adults, until she was accidentally discovered by a professional worker investigating, ironically, some other form of wrongdoing.

In situations of chaos, as with the migrant exodus from Syria, gangs find easy pickings amongst children separated from parents. There is now considerable concern that as many as 10,000 or more children who have either come unaccompanied or been separated from their carers have fallen into the hands of trafficking gangs.

The case of the Nepali children raises specific, pressing issues even in this grotesque context. It is common for children in poorer countries to be trafficked across borders (as Nepali girls are into India for prostitution), or within countries (as with Ghanaian children, sold into working for fishermen on the Volta Lake) often under the misapprehension that they are going to a better life. Parents and guardians, often driven by extreme poverty, collude in this trade by accepting "cash for kids".

Within the UK, the issue of domestic servitude – the final destination for some of these children – was the focus of heated debate as the Modern Slavery Act became law. Home Secretary Theresa May might say that the act is designed to stop modern slavery but at present the government is colluding in a system of domestic servitude whereby diplomats and rich businesspeople bring domestic workers with them on their own passports.

Current visa arrangements mean that if a servant (who may in time be one of these sold children) protests their actual conditions of employment − abuse, severe exploitation and rape − they face deportation to perhaps being re-trafficked. A fundamental review of this provision has told the home secretary that current arrangements are highly exploitative. Perhaps she should deal with this issue first before wringing her hands over the plight of these children.

Gary Craig is the professor of social justice at Durham University, UK and emeritus professor of social justice at the University of Hull.