Rickettsia parkeri as a Probable Agent of Mild Spotted-Fever Group Rickettsiosis Identified by Seroreactivity in Villeta, Colombia

Spotted fever group (SFG) rickettsioses are emerging zoonotic diseases of increasing relevance in Latin America, yet the specific species involved in human infections remain poorly defined in many endemic regions. This study aimed to determine the most probable antigen among SFG-seroreactive febrile patients from Villeta, Colombia. A panel of 25 convalescent-phase serum samples previously identified as positive for SFG Rickettsia spp. antibodies was analyzed by indirect immunofluorescence assay using antigens of Rickettsia rickettsii, R. amblyommatis and R. parkeri. Antibody titers were compared to identify differential seroreactivity patterns. Overall, 44% (11/25) of the samples showed differential antibody titers against one of the tested antigens. Among these, nine (36%) exhibited higher titers to R. parkeri and two (8%) to R. amblyommatis, while none showed exclusive reactivity to R. rickettsii. The remaining 56% (14/25) presented similar titers across antigens, consistent with indeterminate or cross-reactive SFG responses. Antibody titers ranged from 1:128 to 1:4096, with R. parkeri showing the strongest reactivity. These findings suggest R. parkeri or a highly related Rickettsia species as the predominant probable antigen in Villeta, highlighting its potential role in mild rickettsial infections and emphasizing the need for eco-epidemiological studies to identify local vectors and reservoirs.