Background Bartonella bacilliformis is the aetiological agent of Carrión's disease, a biphasic and highly lethal illness formerly restricted to the South American Andes that is now spreading to adjacent areas. Reliable serodiagnostic approaches and vaccines are urgently needed. In this study, we aimed to identify immunodominant proteins of B bacilliformis and to establish novel and reliable serodiagnostic tools. MethodsWe used a reverse vaccinology approach in combination with an analysis of heterologous genomic expression libraries to identify immunodominant proteins, on the basis of the genome sequences of B bacilliformis strains KC583 and KC584. Antigens were screened with serum samples collected from Peruvian patients with B bacilliformis infections and from German healthy blood donors without history of travel to South America. We further analysed immunoreactive proteins of B bacilliformis with immunoblotting and line blots. We used selected target proteins to develop a diagnostic ELISA. To assess the performance of this ELISA, we did receiver operating characteristic analyses to assess the area under the curve, cutoff values, sensitivities, and specificities with 95% CIs. FindingsWe used serum samples obtained between Dec 23, 1990, and May 5, 2018, from 26 Peruvian patients with B bacilliformis infections and serum samples taken between Aug 28 and Aug 31, 2020, from 96 healthy German blood donors. 21 potentially immunodominant proteins were identified and recombinantly expressed, and their reactivity was assessed with immunoblotting and line blots. Of these 21 antigens, 14 were found to be immunoreactive. By using serum samples of Peruvian patients with Carrión's disease and of healthy German blood donors, we identified three antigens (porin B, autotransporter E, and hypothetical protein B) as suitable immunodominant antigens, and we applied them in a diagnostic ELISA using two different antigen combinations (porin B plus autotransporter E and porin B plus autotransporter E plus hypothetical protein B). For the combination of porin B and autotransporter E, with optical density measured at 450 nm (OD 450 ) cutoff value of 0•29, sensitivity was 80•8% (95% CI 60•7-93•5) and specificity was 94•8% (88•3-98•3) for all Peruvian patient samples. For a combination of porin B, autotransporter E, and hypothetical protein B, with an OD 450 cutoff of 0•34, sensitivity was 76•9% (56•4-91•0) and specificity was 93•8% (86•9-97•7) for all Peruvian patient samples. Interpretation This novel ELISA could represent a useful serodiagnostic tool for future epidemiological studies of B bacilliformis in endemic areas. Additionally, the immunodominant antigens we have identified could provide a first basis for future vaccine development to prevent the highly lethal Carrión's disease. Funding DRUID (Novel Drug Targets against Poverty-Related and Neglected Tropical Infectious Diseases) Initiative and Robert Koch Institute.

Louse-borne relapsing fever (LBRF) caused by B. recurrentis is a poverty-related and neglected infectious disease with an endemic focus in the Horn of Africa. Re-emergence of the disease occurred in Europe during the refugee crisis in 2015 and sporadic outbreaks were frequently reported in Eastern Africa where poor settings lack affordable diagnostics. Currently, there are no validated in vitro assays available for the serodiagnosis of LBRF. The aim of this study was to develop novel and reliable immunoassays by investigating clinically suspected and culture-confirmed serum samples from LBRF patients and a broad panel of serum samples from patients with other spirochetal, bacterial, and parasitic diseases. We identified two immunoreactive antigens (complement-inhibiting protein CihC and the glycerophosphodiester phosphodiesterase GlpQ of B. recurrentis) as the most promising target candidates leading to the evaluation of two immunoassays (line immunoblot and ELISA) for IgM and IgG. To optimize the IgM immunoassay, we conducted a bioinformatic approach to localize the relevant immunogenic regions within CihC. By utilizing a N-terminal CihC fragment, the sensitivity and specificity of both immunoassays (CihC and GlpQ) were high (IgM: sensitivity 100%, specificity of 89.9%, IgG: sensitivity 100%, specificity 99.2%). In conclusion, our findings indicate the diagnostic potential of CihC and GlpQ as valuable markers for the serodiagnosis of LBRF even at early time points of infection. Here, we provide strong evidence for the utilization of these immunoassays as reliable tools in clinical practice.