Susceptibility and reservoir potential of the dog to spotted fever-group rickettsiae

SUMMARY This study was initiated to determine the degree of susceptibility of dogs to virulent and nonvirulent spotted fever-group rickettsiae and to evaluate dogs as sources of infection for ticks. Dogs were exposed either by inoculation (syringe) or by infective tick bite to the following rickettsial serotypes: (1) Rickettsia rickettsii (Wachsmuth and Sawtooth ♀ 2 strains), (2) R montana (M/5-6 B strain), and (3) R rhipicephaii (3-7- ♀6 strain). Results indicated that dogs inoculated with 1,000 or 10,000 egg infective doses of virulent R rickettsii developed a rickettsemia that was detectable as early as 4 days after inoculation to as late as 10 days. Conversely, none of the dogs inoculated with R montana (M/5-6 B) or R rhipicephali (3-7- ♀ 6) or exposed to ticks infected with these strains developed detectable rickettsemia, fever, or other observable clinical signs. None of the 394 ticks that fed on rickettsemic dogs (R rickettsii) infected by inoculation became infected, and only 3 of 348 ticks (0.9% infection rate) were infected after feeding on dogs which had been infected by tick bite. All ticks fed on dogs exposed to R rhipicephali and R montana were shown to be free of rickettsiae. The largest concentrations of plaque-forming units (pfu) in Vero cell culture from undiluted whole blood were found on day 6 and on day 7 in dogs that were inoculated with 10,000 and 1,000 R rickettsii, respectively, of the Sawtooth ♀ 2 strain. The highest rickettsial concentration observed for the dog infected by tick feeding was on day 9. Conversely, there were no pfu observed in Vero cell culture of undiluted whole blood of the dogs inoculated with 1,000 or 10,000 R rhipicephali (3-7- ♀ 6 strain) or exposed to infective Rhipicephalus sanguineus. On the basis of these data, it would appear that the dog, under natural conditions, is not an efficient reservoir for infecting ticks with either virulent or nonvirulent spotted fever-group rickettsiae.