As the Nile sun bursts through the bars of the cell the unknown boy shares with other shackled inmates, it casts strips of light over a scene reminiscent of a lithograph from the slave era. He cranes his neck towards the rays but is pulled back by the chains that have lacerated his ankles, and returns to the shadows.

The prison warders explain he’s a “lunatic” who has been locked up for years. No one knows his name or his history. He is naked and shackled to a prison floor in Juba central prison in South Sudan. There are many others like him in the cells. Some spit at anyone who comes in range, others stare vacantly at the walls. The legacy of a 20-year civil war, in which almost 2m people died, is an epidemic of acute mental illness with hardly anyone to treat it.

In the overwhelming heat of Juba’s jail most of the inmates have matted hair, fleas and frequent bouts of dysentery. For the regular prisoners, the mentally ill are figures of fun. They laugh and tease them. The smell of neglect in the communal cell is overwhelming. The men have no plates but are served slops on the concrete where they urinate.
Conditions are no better in the women’s prison across the road. Some of the most disturbed women are shackled to parts of lorry engines that render them immobile. Most bewilderingly of all, two women are chained to trees in the central courtyard. In solitary confinement are the most dangerous women. 24 hours a day they remain alone behind bars.

Many appear to be in prison because they are victims rather than perpetrators of crime: they have nowhere else to go. They are the lost souls of Sudan.