V. Evaluation of Cross-Immunity against Type 1 Dengue Fever in Human Subjects Convalescent from Subclinical Natural Japanese Encephalitis Virus Infection and Vaccinated with 17D Strain Yellow Fever Vaccine
Epizootiologic studies conducted during the past few years showed the existence of widespread natural infection of the southern flying squirrel, Glaucomys volans, with epidemic typhus rickettsiae, Rickettsia prowazekii. The ecological findings strongly implicated transmission of the etiologic agent by an arthropod vector. Studies were conducted under controlled laboratory conditions to determine whether ectoparasites naturally associated with flying squirrels (squirrel fleas, lice, mites and ticks) were capable of acquiring, maintaining and transmitting the infection. Also studied were the cat flea, oriental rat flea and the human body louse. Flying squirrels inoculated with the GvF-16 strain of R. prowazekii circulated rickettsiae in their blood for 2–3 weeks, thus providing ample opportunity for arthropods feeding on them to become infected. The results with Dermacentor variabilis ticks indicated that the rickettsiae did not consistently survive in this insect and were not passed to the eggs of adult females that had been infected subcuticularly. Mites became infected by feeding on infectious blood but failed to sustain the infection. Also, mites fed on an infected flying squirrel did not transmit the infection to a normal squirrel. Squirrel, cat, and oriental rat fleas readily became infected by feeding on a rickettsemic host or on infectious blood through membranes, but failed to transmit the infection to susceptible flying squirrels. In the studies with flying squirrel lice, however, transmission of epidemic typhus from infected to uninfected flying squirrels was demonstrated. Infection of the human body louse with the GvF-16 flying squirrel strain of R. prowazekii was similar to that previously observed with classical human strains, viz., multiplication of the rickettsiae and excretion in the feces.