Increasing Risk of Tick-Borne Disease through Growth Stages in Ticks

Rickettsia and Coxiella spp. are pathogens transmitted by ticks to humans. However, the developmental stage of the tick carrying the greatest risk of infection is unknown. Detection of pathogen-specific genes proves that ticks carrying Rickettsia or Coxiella spp. constitute a reservoir of infection. However, conventional PCR methods are unable to quantitate the pathogens within ticks. In the present study, we collected ticks in the endemic area of Japanese spotted fever, caused by Rickettsia japonica, and determined the rate of tick-borne pathogens carried by the ticks. As a method of evaluation, next-generation sequencing was used to estimate the proportion of pathogens in 10 adult and 10 larval ticks. Ticks were identified Haemaphysalis longicornis (H.L) from the results of the sequencing of PCR products amplified using tick identification-specific primers. The gene detection rates were 10/10 for Rickettsia sp. and 10/10 for Coxiella sp. among the adult ticks. For the larval ticks, the ratios were 7/10 and 5/10 for Rickettsia sp. and Coxiella sp., respectively. The largest proportion of Coxiella sp.-specific DNA reached 96% in one adult tick. The proportion of Rickettsia sp. genes ranged from 1.76% to 41.81% (mean, 15.56%) in the adult ticks. The proportions of Coxiella and Rickettsia spp. genes in the larvae ranged from 0% to 27.4% (mean 5.86%) and from 0% to 14.6% (mean 3.38%), respectively. When the percentage of Rickettsia sp., out of all pathogens detected via next-generation sequencing, was analyzed between the adult and larval stages of the ticks, a significant difference was observed at p = 0.0254. For Coxiella sp., a highly significant difference (p < 0.0001) was found between the adult and larval stages of the ticks. In conclusion, the detection rates and proportions of Rickettsia and Coxiella spp. genes were highest in adult H.L ticks. The risk of contracting tick-borne infections may increase with bites from adult ticks, especially those harboring Coxiella sp.