A Nonspecific Heat-Labile Factor in the Serum Neutralization Test for Newcastle Disease Virus

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Abstract
Summary: From the further studies on the neutralization tests against Newcastle disease virus in human sera reported here, it has been found that the positive results previously published (1) were due to a nonspecific heat-labile factor present in the sera in varying amounts according to the conditions of serum preservation. Heating to 56 C for 30 minutes completely destroyed the virucidal activity of all sera except those from specifically immunized animals. The neutralizing activity of immune sera was lowered by inactivation but could be enhanced by addition of fresh normal serum. This virucidal factor for NDV was found in large amounts in the normal sera of man, monkey, rabbit, and guinea pig in correlation with the hemolytic activity of the complement. It was absent in the sera of the ferret, hamster, chicken, and mouse, although complement was detectable in the sera of the first three animals but not in that of the mouse. It may be associated quantitatively with one of the four components of complement, although there remains the possibility that another serum component may be the responsible factor. Both the complement and the neutralizing ability for NDV of normal sera can be preserved for over a year if the serum is kept frozen either at −10 C or in the dry ice refrigerator. These activities are completely lost, however, after storage at 4 to 6 C for varying time periods.

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