Mutations in pathogenicity of some recently isolated strains of fowl-plague virus

Abstract

Summary Two out of four recently isolated field strains of fowl-plague virus failed to yield recognizable mutants during serial cultivation in pigeon and chick embryos. One strain became avirulent for fowls at the 11th pigeon-embryo passage, but capacity to infect fowls was completely or almost completely, lost during the next three passages in either chick or pigeon embryos, and was not regained until 27 to 30 additional passages had been made in chick embryos. The regained capacity of this strain to infect fowls was unaffected by further serial repass age in pigeon embryos. Other lines of this strain, started from material stored before any mutation had occurred, behaved differently under serial cultivation in pigeon embryos. One line remained lethal to rather more than 50 per cent of inoculated birds, while the virulence of a second line fell to a level at which it caused a mortality of about 10 per cent. Of two lines, derived from the series in which infectivity for fowls had been lost and reacquired, one showed an exaltation of virulence after further serial pigeon-embryo passage. A substrain “B” of this line, which had been passaged six times in chick embryos simultaneously with Newcastle-disease virus, was of remarkably low virulence and high antigenic capacity for fowls from that point onwards. The second line, subjected to 125 serial passages in the pigeon embryo was at that stage lethal to about 5 per cent of inoculated fowls. A subs train derived from one of the pigeon-embryo passaged strains by serial intracerebral passage in mice showed greatly reduced capacity to infect and immunize fowls. After 7 repassages in the pigeon embryo there was a sudden access of virulence for fowls. When the first passage in chick embryos of a fourth field strain. Of virus was stored at −20°C for 64 days, and then passaged in chick embryos, material from two successive passages immunized birds without causing any illness; but its full virulence was restored after a further 6 passages in chick or pigeon embryos. After a further 19 passages the virulence showed signs of declining. Virus stored in the original brain suspension for 88 days was still fully virulent after a single passage in chick-embryos, but showed signs of declining after 6 further passages in chick or pigeon-embryos. Mutual interfence was demonstrable in birds inoculated with mixtures of “attenuated” fowl-plague and Newcastle-disease viruses, but when titres of the two viruses were adjusted so that neither virus had a marked advantage in numbers most of the birds developed immunity to both diseases.