Night Wind to Bahia — Thomas Thorpe

On land, the castaways find themselves stranded on the shores of a country struggling for independence. A weak Regency government copes with warring landlords, slave revolts and a cholera epidemic. When escaped slaves abduct the Darmon women, their trail leads to a site of massacre. Arrested and taken to Salvador, they're visited by an emissary who bargains for help with a dangerous blood transfusion experiment to protect the future leader--Dom Pedro II.