The biological characters of eight strains of the Newcastle disease virus, isolated in different parts of the world between 1933 and 1951, have been studied and compared in detail.
Two types of the virus have been distinguished, one from the Newcastle disease prevalent in Europe and Australia, the other from that occurring in the United States of America.
Viruses of the first type are highly virulent, multiply rapidly and successively develop the properties of infectivity, haemagglutination and haemolysis. Their haemagglutination pattern is constant and wide; they profoundly modify human group O erythrocytes, and they are antigenically homogeneous.
Viruses of the second type are of weaker virulence. They grow at a slower rate in the allantoic cavity of the chick embryo, and the development of their characters is retarded; they have a reduced haemagglutination pattern and only a minimum capacity to modify human group O cells. Antigenically strains of the second type are homogeneous.
From haemagglutination inhibition and absorption studies it is concluded that major antigens are shared by both types, and that the first type possesses one or more antigens lacking in the second.
The highly virulent Canadian TWISS strain possesses the characters of the first type and an antigenic constitution with features of both types.
The significance of the results is discussed in relation to the ecology of Newcastle disease.