Symbiotic relationship of endogenous bacteria including pathogens for infectious disease development in ticks

2021 Fiscal Year Final Research Report

• Project/Area Number: 17K08835
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Bacteriology (including mycology)
• Research Institution: University of Shizuoka
• Principal Investigator: OHASHI NORIO 静岡県立大学, 食品栄養科学部, 教授 (10169039)
• Project Period (FY): 2017-04-01 – 2022-03-31
• Keywords: Rickettsia japonica / Japanese spotted fever / Rickettsia sp. LON / Rickettsia sp. G4 / Coxiella-like symbiont / Haemaphysalis / Tick

Outline of Final Research Achievements

Rickettsia japonica that causes Japanese spotted fever, nonpathogenic rickettsiae, and Coxiella-like symbionts (CLE) are a group of endogenous bacteria in ticks. It is known that these bacteria are passed on to the tick's offspring by vertical transmission. We found that (i) the nonpathogenic Rickettsia sp. LON harbored by Haemaphysalis longicornis ticks becomes absent in ticks with the unique parthenogenetic reproductive system, (ii) pathogenic R. japonica and nonpathogenic Rickettsia sp. G4 possessed by Haemaphysalis histricis ticks are both a copy number of 100-1,000,000 per tick, (iii) Haemaphysalis tick species and CLE species are co-evolved, and (iv) 4 out of 24 haplotypes of H. histricis ticks possess pathogenic R. japonica. These results are expected to contribute greatly to the control of tick-borne infectious diseases in Japan in the future.

Free Research Field

微生物学

Academic Significance and Societal Importance of the Research Achievements

本研究の学術的意義としては、(i) 日本紅斑熱を引き起こすリケッチア(Rj)のマダニ内のコピー数を明らかにしたこと、(ii) マダニ内の病原性Rjと非病原性R-G4を識別できる検出法を開発したこと、(iii) 単為生殖系のマダニではリケッチアが消えていくことを見出したこと、(iv) マダニ種とCLE種が共種分化していることを発見したこと、(v) マダニのあるハプロタイプが病原性Rjを保有していることが判ったこと、などである。社会的意義としては、公衆衛生上、日本紅斑熱の多発地域での啓発活動に役立つ知見が得られたことと考える。