Interactions of the newcastle disease virus with mouse tissues

Summary

• (1) Adaptation of three NDV strains to the brain of newborn mice failed to result in the development of new genetic markers such as the propagability in adult mouse brain and inability of growing in the allantoic cavity of the chicken embryo.

• (2) SMB adaptation of the RO strain resulted in significant increase of the neurotoxicity for adult mice. While about 10⁴ LD₅₀ of the allantoic line RO virus killed the i.cerebrally inoculated adoult mouse, only 10^0.5 LD₅₀ of the SMB line virus was required to obtain the same result.

• (3) Allantoic line virus inoculated in toxic dose showed persistence or first cycle multiplication in the brain of adult mice, the SMB virus did not exhibit any evidence of replication of infectious particles in the same tissue. High speed centrifuged supernatants of the SMB virus did not evoke neurotoxicity and did not contain infectious particles either. RDE treatment of toxic brain extracts did not result in release of infectivity.

• (4) Extracts of toxic brains which did not contain infectious particles were able to produce a short lasting delay in the incubation period of the living virus as well as a low titer of hemagglutination inhibitory antibodies in intraperitoneally immunized adult mice, which antibodies were found inactive in virus neutralization tests. The delay in the incubation period apparently was due to non-specific factors since virus multiplication was not suppressed. Immunization of adult mice with toxic brain extracts did not protect against the neurotoxic effect of the suckling mouse brain line virus. Specific NDV immune sera mixed with the virus neutralized the toxic effect.

• (5) The heat inactivated virus did not interfere with the living virus in the brain of suckling mice when they were given simultaneously; when the inactive virus or a non-pathogenic strain of a living virus was administered prior to the active virus, interference occurred.

• (6) Electron microscopic studies on ultrathin sections showed abundance of mature virus particles in suckling mouse brains. No such particles were seen in brains of adult mice with severe toxic symptoms.