Effect of alphavirus infection on mouse embryos

Seven strains and mutants of the alphaviruses, Semliki Forest virus and Sindbis virus, differed in their lethality for mouse embryos and their mothers. The A7 strain and the neurovirulence mutant M103 of Semliki Forest virus were selected for detailed comparison. A7 produced 100% lethality of mouse embryos but was avirulent for their mothers. M103 did not kill embryos or their mothers but did induce postnatal immunity. This immunity could be induced in utero or by suckling to an immune mother. Infectious virus could be recovered from the brain, blood, and embryos of mice infected with A7. Mice infected with M103 developed a viremia of similar titer, but virus could not be recovered from brain or embryos. Electron microscopy showed multiplying virus in trophoblast and fetal tissue after infection with A7, but no virus could be detected in such tissue after infection with M103. Cultures of ectoplacental cone trophoblast cells incubated in the presence of A7 or M103 showed multiplying A7 in the giant cells, but no M103. It is concluded that A7 can traverse both the blood-brain barrier and the placenta, whereas M103 can traverse neither.