CD40LG and plague: To investigate the role that black rat immune responses may play in plague persistence in Madagascar and facilitate future serological investigations of reservoir hosts in Madagascar and elsewhere, we (i) analyzed anti-F1 IgM and IgG responses in wild rats challenged with different doses of Y. pestis, (ii) examined antibody persistence and (iii) evaluated the use of the anti-F1 RDT as a tool for investigating plague foci.