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Wait a Tick: R rickettsii Attachment Time

Clinician's Brief (Capsule)

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This study evaluated the minimum feeding period required by Amblyomma aureolatum to transmit Rickettsia rickettsii, causative agent of Rocky Mountain spotted fever (RMSF). A aureolatum ticks are the main vector of RMSF in the metropolitan area of São Paulo, Brazil. Guinea pigs were infested with fed or unfed adult A aureolatum ticks or unfed nymphs, all inoculated with R rickettsii. Results showed unfed nymphs and unfed adult ticks needed to be attached >10 hours to transmit disease, whereas adult ticks that had fed on rabbits just prior to attachment on the guinea pigs needed >10 minutes for transmission.

Minimum attachment times as low as 2 hours have been noted for Dermacentor andersoni ticks transmitting Rickettsia rickettsii.

A previous study on guinea pigs demonstrated a process called reactivation in which R rickettsii in fasting ticks loses pathogenicity and virulence, but allowing infected fasting ticks to feed for >48 hours induces R rickettsii to revert to a virulent state. This reactivation may be responsible for the different time requirements for disease transmission between fed and unfed ticks.

The elevated infection and fatality rates for humans diagnosed with RMSF in São Paulo are likely the result of ticks feeding on domestic dogs in the rainforest, then making contact with humans. Tick-removal procedures in RMSF-endemic areas may have implications for RMSF transmission, as the typical expectation that tick-borne bacteria require tick attachment >2 hours may discourage humans from immediately removing ticks.

Commentary

The question of the minimum tick-attachment time needed for transmission of different pathogens is critical in evaluating infection risk. A 36-hour minimum attachment time is commonly referenced regarding Borrelia burgdorferi transmission by Ixodes scapularis; however, minimum attachment times as low as 2 hours have been noted for Dermacentor andersoni ticks transmitting R rickettsii. The species of tick and host, life stage of the tick, previous feeding history, sex of the tick, attachment site, and other factors could potentially affect this estimate. Fever, seroconversion, and lesion development were used as indicators of successful transmission in this study. The males’ previous feeding history, unfed since molting vs partially fed, had a significant impact on minimum attachment time, with a minimum of >10 hours noted in the former case (similar to nymphal transmission) and as little as 10 minutes attachment required in the latter case. This is significant in understanding tick-borne disease transmission and recommending prevention techniques.—John J. Schaefer, DVM, MS, PhD, DACVM

This capsule is part of the One Health Initiative.

References

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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