Temporal Changes in Prevalence of Scrub Typhus Rickettsia (Orientia tsutsugamushi) Infecting the Eggs ofLeptotrombidium imphalum(Acari: Trombiculidae)

Eggs from seven colony lines of the chigger mite Leptotrombidium imphalum (Vercammen-Grandjean & Langston) were examined for infection with Orientia tsutsugamushi (Hyashi), the etiologic agent of scrub typhus. The polymerase chain reaction (PCR) using primers OtP 56.809 and OtM 56.1221, which amplify a 291 bp region of the P56 gene of O. tsutsugamushi, was used to detect scrub typhus within single eggs. All seven chigger mite lines produced infected eggs with varying rates of infection (Li1 = 8.1%, n = 124; Li2 = 45.6%, n = 90; Li3 = 30.1%, n = 144; Li4 = 31.7%, n = 145; Li5 = 21.3%, n = 136; Li6 = 41.6% n = 77; Li7 = 22.5%, n = 110). The 3 wk with the highest infection rates for each line using Fourier analysis were as follows: Li1 = 2, 7, 14; Li2 = 4, 6, 12; Li3 = 3, 6, 12; Li4 = 4, 6, 12; Li5 = 5, 7, 14; Li7 = 4, 6, 12. Li6 only had nine measurements over time; therefore, Li6 was excluded from individual analysis. Infection rates of scrub typhus in eggs occurred in a 2-wk 2-d cycle, using Fourier analysis of combined data. Not only did infection rates vary among the progeny of females, but temporal variation also occurred.