Minimal Duration of Tick Attachment Sufficient for Transmission of Infectious Rickettsia rickettsii (Rickettsiales: Rickettsiaceae) by Its Primary Vector Dermacentor variabilis (Acari: Ixodidae): Duration of Rickettsial Reactivation in the Vector Revisited

Abstract

It has been reported that starving ticks do not transmit spotted fever group Rickettsia immediately upon attachment because pathogenic bacteria exist in a dormant, uninfectious state and require time for 'reactivation' before transmission to a susceptible host. To clarify the length of reactivation period, we exposed guinea pigs to bites of Rickettsia rickettsii-infected Dermacentor variabilis (Say) and allowed ticks to remain attached for predetermined time periods from 0 to 48 h. Following removal of attached ticks, salivary glands were immediately tested by PCR, while guinea pigs were observed for 10-12 d post-exposure. Guinea pigs in a control group were subcutaneously inoculated with salivary glands from unfed D. variabilis from the same cohort. In a parallel experiment, skin at the location of tick bite was also excised at the time of tick removal to ascertain dissemination of pathogen from the inoculation site. Animals in every exposure group developed clinical and pathological signs of infection. The severity of rickettsial infection in animals increased with the length of tick attachment, but even attachments for less than 8 h resulted in clinically identifiable infection in some guinea pigs. Guinea pigs inoculated with salivary glands from unfed ticks also became severely ill. Results of our study indicate that R. rickettsii residing in salivary glands of unfed questing ticks does not necessarily require a period of reactivation to precede the salivary transmission and ticks can transmit infectious Rickettsia virtually as soon as they attach to the host.

Keywords: Rickettsia rickettsii; grace period; reactivation; transmission dynamics.

Fig. 1.

Titers of R. rickettsii (no. of rickettsial DNA copies per 10,000 copies of tick DNA) in salivary glands of unfed and feeding female D. variabilis ticks: (A) variability of rickettsial titers in salivary glands of individual ticks prior to feeding and during the initial 48 hours of tick attachment; (B) rickettsial titers in salivary glands of individual ticks in comparison to the corresponding hemolymph samples. Line – linear regression + 95% CI: rPearson = 0.3248 (95% CI 0.1709–0.4633), P < 0.0001; (C) mean (±SE) titers of R. rickettsii in tick salivary glands; brackets identify time points with significantly different (PANOVA) means.

Fig. 2.

Summary of clinical and pathological signs of rickettsial infection in guinea pigs exposed to individual R. rickettsii-infected D. variabilis ticks for preset time intervals or needle-inoculated with salivary glands of unfed ticks. (A) Bite-sites (inoculation site) remained intact after tick removal; (B) bite-sites were extricated simultaneously with tick removal. *Overall score = the total number of observed clinical and pathological signs: 0–; 1–; 2–; 3–; 4–; 5–; 6–; 7–. **Detection of rickettsial DNA in ear-skin biopsies and/or internal tissues signifies dissemination of R. rickettsii from the site of tick bite or needle-inoculation.

Fig. 3.

Peak temperature (— median) in guinea pigs following attachment of R. rickettsii-infected D. variabilis female ticks for preset periods of time, or subcutaneous inoculation (black stars) of salivary glands from unfed ticks: (A) bite-site intact (black circles); (B) bite-site is removed with the ticks (black triangle). Dotted line: fever threshold.

Fig. 4.

Rickettsia rickettsii DNA copy number of detected in individual 2-mm skin biopsies taken at the site of tick bite (0 mm) and at the distances of 10, 20, and 30 mm simultaneously with removal of feeding tick at predetermined time intervals after attachment.