Immunologic Relationships Among the Spotted Fever Group of Rickettsias Determined by Toxin Neutralization Tests in Mice with Convalescent Animal Serums

Tests of certain variables inherent in the spotted fever toxin neutralization test in mice revealed that a) spotted fever group toxins remained stable for a period of at least 3 hr at either room temperature (26°C) or at 2°C, b) serum antitoxin titers were maximum 15 min after mixing antiserum with toxin and they did not change significantly during 2 hr further incubation at room temperature, c) a relationship existed between the serum neutralization titer and the concentration of toxin neutralized: a 2-fold increase in quantity of toxin reduced the serum titer 4-fold. Three strains of Rickettsia rickettsii associated with spotted fever disease in man behaved alike in reciprocal cross neutralization tests. A strain of R. rickettsii from the rabbit tick Haemaphysalis leporis-palustris, showed a significant serologic difference from one but not from the other two of these strains of R. rickettsii. The rickettsias of boutonneuse fever, South African tick bite fever, Kenya tick typhus and Indian tick typhus, were indistinguishable from one another by the mouse toxin test. These studies with Indian tick typhus confirm recent information concerning its serologic relationship with this group of rickettsias. R. rickettsii and Rickettsia conorii were clearly differentiated from one another by the mouse toxin test. Neither of these two rickettsias produced more than a slight degree of cross neutralization with the other. The rickettsia of Siberian tick typhus could be distinguished by the mouse toxin technique from R. rickettsii and R. conorii as well as from other spotted fever group strains studied. However, a relatively high one-way cross neutralization occurred between the Siberian agent and certain strains of R. rickettsii and the rickettsia from the tick Amblyomma maculatum, but not R. conorii. Two strains of Siberian tick typhus rickettsias, one from the tick Dermacentor nuttalli, and the other of human origin, gave complete cross neutralization with one another and were essentially alike in their cross reactions with other members of the spotted fever group. The maculatum rickettsia exhibited distinct serologic differences from all other spotted fever strains studied. This rickettsia did, however, give notably high nonreciprocal cross reactions with R. conorii strains and with the rickettsia of Siberian tick typhus. Guinea pig antisera against Rickettsia australis and Rickettsia akari showed little or no cross neutralization of toxins of 10 other strains of