Effect of Rickettsia felis Strain Variation on Infection, Transmission, and Fitness in the Cat Flea (Siphonaptera: Pulicidae)

Abstract

Rickettsia felis is a human pathogen transmitted by the cat flea, Ctenocephalides felis (Bouché) (str. LSU), as well as an obligate symbiont of the parthenogenic booklouse Liposcelis bostrychophila (Badonnel) (str. LSU-Lb). The influence of genetic variability in these two strains of R. felis on host specialization and fitness and possible resulting differences on infection and transmission kinetics in C. felis is unknown. Utilizing an artificial host system, cat fleas were exposed to a R. felis str. LSU-Lb-infected bloodmeal and monitored for infection at 7-d intervals for 28 d. Quantitative real-time PCR was used to determine rickettsial load and infection density in newly exposed cat fleas, and transmission frequency between cat fleas. The effect of persistent R. felis infection on cat flea F1 progeny was also assessed. At 7 d postexposure 76.7% of the cat fleas successfully acquired R. felis str. LSU-Lb. In R. felis str. LSU-Lb-exposed cat fleas, the mean infection load (6.15 × 106), infection density (0.76), and infection prevalence (91/114) were significantly greater than R. felis str. LSU infection load (3.09 × 106), infection density (0.68), and infection prevalence (76/113). A persistent R. felis str. LSU-Lb infection was detected for 28 d in adult cat fleas but neither female:male ratio distortion nor vertical transmission was observed in F1 progeny. While infection kinetics differed, with higher intensity associated with R. felis str. LSU-Lb, no distinct phenotype was observed in the F1 progeny.

Keywords: Ctenocephalides felis; Liposcelis bostrychophila; Rickettsia felis strain; cat flea; infection kinetics.

Fig. 1

Overall mean Rickettsia felis str. LSU-Lb infection in fleas. (A) Overall Rickettsial load in R. felis str. LSU Lb-infected fleas compared to R. felis str. LSU. (B) Overall Rickettsial infection density in R. felis str. LSU Lb-infected fleas. Overall R. felis str. LSU Lb-infected fleas had significantly greater rickettsial loads (females and mixed sex adults) and densities (mixed sex adults), compared with R. felis str. LSU-infected fleas. Bars represent means (±SEM) of Rf17kDa copy number and Rf17kDa/Cf18S ratio. Significant differences (P < 0.05) between strains are marked with an asterisk.

Fig. 2

Rickettsial load and infection density in fleas exposed to a R. felis str. LSU- or R. felis str. LSU-Lb-infected bloodmeal 7–28 d postexposure (dpe). (A) Rickettsial load in adult fleas infected with R. felis str. LSU or R. felis str. LSU-Lb collected at different time points. (B) Rickettsial infection density in adult fleas infected with R. felis str. LSU or R. felis str. LSU-Lb collected at different time points. Bars represent means (±SEM) of Rf17kDa copy number and Rf17kDa/Cf18S ratio. Means with different letters are significantly different (P < 0.05).

Fig. 3

Mean Rickettsia felis str. LSU-Lb infection in donor and recipient fleas at different time points. (A) Rickettsial load in R. felis str. LSU Lb-infected donor and recipient fleas collected at days 1, 7, 14, and 21 post cofeeding. (B) Rickettsial infection density in R. felis str. LSU Lb-infected donor and recipient fleas collected at days 1, 7, 14, and 21 post cofeeding. Donor fleas typically had greater rickettsial loads and densities, compared with recipient fleas. Bars represent means (±SEM) of Rf17kDa copy number and Rf17kDa/Cf18S ratio. Means with different letters are significantly different (P < 0.05).