Evaluation of the precipitation-in-gel reaction in the diagnosis of smallpox.

Specimens of vesicular or pustular fluids and of scabs from patients with smallpox as well as emulsions of variola-infected chorioallantoic membrane (CAM) were tested for virus titres and by the precipitation-in-gel (PIG) reaction. They were also tested after exposing them directly to sunlight and after keeping them at temperatures of -20 degrees C, 4 degrees C, and 25 degrees C. It was found that when extracts of fresh specimens were diluted to the point where the PIG reaction became negative there was still a titre of 10(4)-10(5) infectivity in the swab extracts and 10(3)-10(4) infectivity in the scab extracts. It was also found that the PIG reactions were all negative on specimens that were kept for 14 days at 25 degrees C, and that several were negative after only 7 days; the loss in infectivity titre, however, was only slight in all the specimens tested. It is concluded that the laboratory diagnosis of smallpox by virus inoculation of CAM is more reliable than by the PIG test.