Studies on Newcastle Disease Virus

Abstract

Electron microscope studies of purified preparations of the virus of Newcastle disease have shown that this virus has a filamentous to sperm-like form when dried from saline suspensions (1, 2). If, however, the virus is suspended in water and then dried from this suspension, it has a roughly spherical form, which corresponds to its appearance in the original allantoic fluid (3). An apparent conversion from the spherical form in water to the filamentous form has been brought about by the addition of saline to make a 0.07 to 0.15 solution (3). The change in shape revealed by electron microscopy was brought about without any detectable change in activity of the suspension. Two questions must be answered before these observations can be assigned any basic significance. (a) Could the lack of a detectable change in activity be merely due to inadequacies in the methods of measuring activity? In other words, might we destroy 95 per cent of the virus by the procedure which changes its shape, and not detect this loss of active virus particles? (b) Does this change in shape on transfer to saline represent a change which occurs within the solution, or is it merely the result of drying from different solvents? Some experimental evidence against this latter possibility was previously mentioned (3), but more work needs to be done.We attempt in this paper to evaluate the methods of determining activity (embryo infectivity) and conclude that the changes in shape of the virus cannot be accounted for on the basis of disintegration.The determination of the 50 per cent mortality or infectivity end-point (4) by means of chick embryos has been in use for a number of years, but very few reports have been made on the accuracy of the method used. Indeed, there is no statistical study available which attempts to estimate the expected probable error with a given number of ~nlmals and a given titration end-point slope.In a limited number of duplicate titrations of Eastern equine encephalomyelitis, a variation of as much as 0.6 log was found between two duplicate titrations (5), using five embryos in each of three dilutions. In a more extensive evaluation of the 50 per cent infectivity measurements for influenza vires in the embryo, Knight (6) found that the chances were 19 out of 20 that differences in end-points of 0.62 logarithmic units were significant.* This means that more than 75 per cent of the infectiousness 1 Five embryos in each dilution. 233 on

Studies on the Pathogenesis of Fever With Influenzal Viruses

“…Four or 5 embryos were used per dilution. Allantoic fluids of embryos dying within 2 to 6 days were harvested and tested for the presence of hemagglutinln (14). The LD~0 was calculated according to the method of Reed and Muench (15).…”

Cell Blockade in Newcastle Disease of Chickens and Chicken Embryos

“…It was not thought necessary to run controls inoculated with Newcastle virus at dilutions below the end point, since such infections with this virus are uniformly fatal to the embryo. All interference experiments here reported were with the B strain of Newcastle virus (4). Tables V and VI show that previous infection with either the virus of swine influenza (V-15) or the PR8 strain of influenza A is capable of blocking the action of two strains of Eastern equine encephalomyelitis virus, and that this blocking was effective against 1000 lethal doses of encephalomyelitis virus.…”

Morphology of Viruses

“…The spherical form is apparently the form to be expected in physio logical solutions. These have been found in allantoic fluid (6,74,76) and in tissue culture (79) and have been associated with the infected tissue (79). It was early shown by centrifugation that particles of about 100 mM were associated with both infectivity and hemagglutination (6,69,70).…”

“…These have been found in allantoic fluid (6,74,76) and in tissue culture (79) and have been associated with the infected tissue (79). It was early shown by centrifugation that particles of about 100 mM were associated with both infectivity and hemagglutination (6,69,70). The parti cles may be adsorbed on to red blood cells and studied there (80).…”

“…These may be fulfilled in varying degrees. They are presented in Table 1 and are an extension of those used originally in identifying the Newcastle disease virus (6). Applied to Hemagglutinating Activity or in some Cases Complement Fixation.…”