Comparative Susceptibility of Larval Stages of Amblyomma aureolatum, Amblyomma cajennense, and Rhipicephalus sanguineus to Infection by Rickettsia rickettsii

The current study compared the susceptibility of larvall stages ofAmbb (F.), Amblyomma aureolatum (Pallas), and Rhipicephalus sanguineus (Latreille) to infection by a Brazilian strain of Ricketisia rickettsii. Guinea pigs experimentally infected by rickettsii were simultaneously infested by larvae of the three tick species. Recovered engorged larvae were allowed to molt to nymphs and held in an incubator at 23 degrees C and 85-90% RH. Subsequent flat nymphs were tested for rickettsial infection by polymerase chain reaction (PCR). Concomitant infestations with sibling ticks on noninfected guinea pigs (control) were done. While 10-60% of the A. cajennense nymphs were shown to be infected by B. rickettsii, both A. aureolatum and R. sanguineus were highly Susceptible to R. rickettsii, since 80-100% of their nymphs were shown to be infected ill the corresponding trials. Most of the engorged larvae (approximate to 70-95%), regardless of being infected or not, successfully molted to nymphs. Mortality rates for engorged larvae tended to be statistically similar (P > 0.05) for ticks recovered from R. ricketesii-infected and noninfected guinea pigs, within each tick species. The only exceptions were the significantly higher mortalities (P < 0.05) for engorged A. cajennense larvae recovered from two infected guinea pigs. Therefore, A. cajennense was less susceptible to R. rickettsii infection than A. aureolatum and R. sanguuineus, while feeding on rickettsemic guinea pigs. These two later species were similarly highly susceptible.