The laboratory differentiation between variola major and variola minor

Three methods have been used to differentiate between the viruses of variola major and variola minor in the laboratory. In this paper these three methods are compared directly, using the international reference strains of the two viruses. Results emphasize the great importance of temperature in determining the growth and spread of these viruses in the chick embryo. On the basis of the results described and of previous experience a simple diagnostic test, applicable to infective material obtained directly from the patient, is recommended. This test will enable a differential diagnosis to be made within two days of the receipt of suitable specimens. It is based on the fact that variola major virus will produce pocks on the chick chorio-allantois in eggs incubated at 38.25 degrees C, whereas variola minor will fail so to do.