Insight of diagnostic performance using B-cell epitope antigens derived from triple P44-related proteins of Anaplasma phagocytophilum.

Human granulocytic anaplasmosis (HGA) is caused by Anaplasma phagocytophilum. Indirect immunofluorescence assay (IFA) is generally used for HGA serodiagnosis. A. phagocytophilum immunodominant P44 major outer membrane proteins are encoded by p44/msp2 multigene family, responsible for IFA reactivity. However, because multiple P44-related proteins may involve immunoreactivity in IFA, the available diagnostic antigens remain obscure. In this study, we identified 12 B-cell epitopes on triple P44-related proteins using peptide array that reacted with 4 HGA patients' sera. Then, peptide spot immunoassay using 14 synthetic peptides derived from those 12 epitopes as antigens was applied for the detection of antibody to A. phagocytophilum from patients with fever of unknown origin. The sensitivities and diagnostic efficiencies of this immunoassay were higher than those of Western blot analysis using 3 recombinant proteins previously developed. Thus, the immunoassay using our epitope-derived antigens, which has higher diagnostic performances, may have significant benefit for HGA serodiagnosis.

Low Seroprevalence of Severe Fever with Thrombocytopenia Syndrome Virus Antibodies in Individuals Living in an Endemic Area in Japan.

Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne infection with a high mortality rate. It is caused by the SFTS virus (SFTSV) and is endemic in some areas in western Japan, including the Kagoshima prefecture. In the present study, healthy individuals living in this prefecture were examined to assess for anti-SFTSV seroprevalence. An initial study was performed using the serum samples collected from a total of 646 individuals living in Kagoshima. At the same time, a questionnaire was used to collect information (such as occupation and a history of tick bite). Enzyme-linked immunosorbent assay and indirect immunofluorescence assay were used for the screening. Finally, the seroprevalence of anti-SFTSV antibodies was confirmed using a neutralization assay. Only 2 (0.3%) out of 646 study participants were positive for anti-SFTSV antibodies. No significant difference was observed between individuals who are at a high or low risk of tick bite in terms of seropositivity. Next, a total of 1,000 serum samples collected from general blood donors by the Japanese Red Cross Kyushu Block Blood Center were tested. None of these samples tested positive for anti-SFTSV antibodies. These results suggest a low seroprevalence of anti-SFTSV antibodies in healthy individuals living in an endemic area in Japan.

Extremely Low Genomic Diversity of Rickettsia japonica Distributed in Japan.

Rickettsiae are obligate intracellular bacteria that have small genomes as a result of reductive evolution. Many Rickettsia species of the spotted fever group (SFG) cause tick-borne diseases known as "spotted fevers". The life cycle of SFG rickettsiae is closely associated with that of the tick, which is generally thought to act as a bacterial vector and reservoir that maintains the bacterium through transstadial and transovarial transmission. Each SFG member is thought to have adapted to a specific tick species, thus restricting the bacterial distribution to a relatively limited geographic region. These unique features of SFG rickettsiae allow investigation of how the genomes of such biologically and ecologically specialized bacteria evolve after genome reduction and the types of population structures that are generated. Here, we performed a nationwide, high-resolution phylogenetic analysis of Rickettsia japonica, an etiological agent of Japanese spotted fever that is distributed in Japan and Korea. The comparison of complete or nearly complete sequences obtained from 31 R. japonica strains isolated from various sources in Japan over the past 30 years demonstrated an extremely low level of genomic diversity. In particular, only 34 single nucleotide polymorphisms were identified among the 27 strains of the major lineage containing all clinical isolates and tick isolates from the three tick species. Our data provide novel insights into the biology and genome evolution of R. japonica, including the possibilities of recent clonal expansion and a long generation time in nature due to the long dormant phase associated with tick life cycles.