Background Rickettsia are obligate intracellular gram-negative bacteria of the class Alphaproteobacteria and are vector-borne species that cause diseases in humans and animals. New Rickettsia species have been involved as human pathogens; however, the pathogenic potential of other species is unknown. Candidatus Rickettsia colombiensis is a new species in the spotted fever group of unknown pathogenicity that is phylogenetically related to other pathogenic species.

Objetive To evaluate the pathogenic potential of Ca. R. colombiensis in Syrian hamsters (Mesocricetus auratus) and analyzed its cross-reactivity against other SFG rickettsia species in human and Syrian hamster sera.

Methods Ca. R. colombiensis was isolated from Amblyomma dissimile using a shell vial. Subsequently, five male hamsters were inoculated intraperitoneally (IP) and five intradermally (ID) with 1 × 106 Vero cells infected with Ca. R. colombiensis. One control animal was used in each group of the study. The health status of the animals was assessed daily, and necropsies were performed on days 5, 10, 15, and 16 DPI. Serum samples for indirect immunofluorescence and tissues were processed for qPCR and immunohistochemistry.

Results All animals remained healthy during the trial and showed no changes in their physiological parameters. No Rickettsia DNA was detected in any of the tissues. Animals infected with Ca. R. colombiensis showed IgG antibody titers ranging from 1:64 to 1:1024. The control animals were negative. Regarding human sera, 56% (84/150) had IgG antibodies against Ca. R. colombiensis antigen. Sera with titers equal to or higher than 1:64 were 100% seroreactive.

Conclusions Ca. R. colombiensis caused a subclinical infection in hamsters, suggesting the possibility of infecting other mammals. However, the clinical, pathological, and molecular findings are inconclusive in establishing that Ca. R. colombiensis is a pathogenic species. Regarding cross-reactivity, it is possible to serologically diagnose Rickettsial infection using Ca. R. colombiensis as antigens.

Author summary Rickettsiosis is a human and animal disease caused by the bacterial Rickettsia species, which are mainly transmitted by arthropods. The clinical spectrum of rickettsiosis ranges from asymptomatic infection to severe and fatal diseases, such as Rocky Mountain spotted fever caused by Rickettsia rickettsii. In recent years, the number of species belonging to the genus Rickettsia has increased, in part due to the progress in the detection and identification of these agents by the application of modern molecular techniques. Much of this new Rickettsia species, known as Candidatus, has been involved as animal and human pathogens; however, for other Candidatus Rickettsia species, the ability to cause diseases is unknown. Candidatus Rickettsia colombiensis is a new species phylogenetically related to pathogenic species. Here, we evaluated, for the first time, the pathogenic potential of this microorganism in a hamster animal model and analyzed its cross-reactivity against other members of the Rickettsia genus in human and hamster sera to determine the possibility of using Candidatus Rickettsia colombiensis as antigens to diagnose rickettsiosis in regions where immunologic reagents are scarce and high biosafety level laboratories do not exist. In this report, we demonstrate that Candidatus Rickettsia colombiensis causes an inapparent infection in hamsters and cross-reactivity between other members of the Rickettsia genus.

The authors have declared that no competing interests exist

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The study was approved by the Institute of Biological Research of the Tropics Ethics Committee of the University of Córdoba Minutes 010-2022 on October 12, 2022. This study was approved by the University of Antioquia Ethics Committee for Animal Experimentation (minutes of session 151, April 11, 2023).

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