Genetic markers and serological identity of wild and attenuated strains of type-1 poliovirus, with special emphasis on strains isolated from patients during an epidemic in the Belgian Congo.

An epidemic of paralytic poliomyelitis occurred in Léopoldville at a time when both virulent and attenuated type 1 polioviruses were present in the population. The attenuated virus was of the CHAT strain, then being given extensive field trials as a vaccine in the Belgian Congo. It was thought that intratypic identification of "wild" and vaccine strains might give information on the hypothetical possibility of a reversion to virulence of attenuated strains. Viruses were isolated from the stools of patients with paralytic poliomyelitis, both those who had not been vaccinated and those who had previously been fed the CHAT attenuated vaccine. Other viruses studied were CHAT virus itself and strains obtained from first, second and third human passages; Mahoney, a virulent strain; W 1063, a "naturally attenuated" strain; and Sickle, a strain, which, in mixture with Mahoney strain, constituted the initial inoculum from which CHAT had been derived. Serologically, the strains were tested for growth in the presence of an agar overlay containing CHAT immune serum. Genetically, the strains were characterized by a study of genetic markers relating to growth under agar of low bicarbonate content (d character), comparative growth in a fresh versus the MS established line of monkey kidney tissue culture, and temperature susceptibility at 37 degrees C and 40 degrees C. The Léopoldville strains and Mahoney were able to produce plaques when grown under overlay containing CHAT immune serum, and they had d+, MS+ and "hot" characters. CHAT and its derivatives were d, MS and "cold"-in other words, exactly opposite in all respects studied. Sickle and W 1063 were intermediate.Evidence is given that CHAT is genetically stable through several human passages, that the poliomyelitis cases in vaccinees were not caused by vaccination, and that the hypothesis of a return to virulence of attenuated strains has not been substantiated.