Walking Dead - Peter Dickinson

Foxe, a researcher working with laboratory animals, is sent to the Caribbean island of Hog’s Key, in the Southward Islands, to work on the effects of an intelligence-enhancing drug in rats. The Islands are ruled by Dr Onesiphorus Trotter, believed by the islanders to have voodoo-like powers (the set-up is a bit exaggerated from that of Haiti under Doc Duvalier.)

The discovery of a body in Foxe’s laboratory allows Dr. Trotter to have him arrested and sent to his personal prison and there force him to use his fellow prisoners as his experimental animals.