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The present study was designed to
(1) compare the stability of avian influenza virus (AIV), Newcastle disease virus (NDV), feline calicivirus (FCV) and porcine parvovirus (PPV) at 40˚°C, 60˚°C and 80˚°C and to assess the influence of egg yolk ingredients on the thermal inactivation of AIV, NDV and FCV;
(2) examine and compare the thermal inactivation between three subtypes of swine influenza virus (SIV; H1N1, H1N2 and H3N2) and one avian influenza virus (AIV; H7N1);
(3) determine the influence of egg yolk powder and water concentration on virus inactivation during acetone extraction; and
(4) evaluate the removal of virus from ethanolic extracts by microfiltration and compare the distribution of viral genome with virus infectivity in fractions after partitioning.
The results revealed that
(1) FCV was the virus most sensitive to heat inactivation at 40˚°C, followed by AIV and NDV. PPV was found to be the virus most resistant to heat inactivation. In two cases (AIV and NDV) the virus titres were reduced within 1 min of heat treatment at 60˚°C by a factor of ≥5 log10, nevertheless FCV was only reduced by a factor of 2.25 log10. After 6 h of heat treatment of PPV at 60˚°C no virus was detectable (≤0.5 log10 TCID50/ml). The mean starting virus titres of all viruses species (AIV, NDV, FCV) were rapidly reduced to ≤0.5 log10 TCID50/ml within 1 min and PPV after 6 h of heat treatment at 80°C, respectively. These experiments showed that the virus titres were reduced by heat treatment at 80˚°C by a factor of ≥5 log10.
2) Comparison of the viral persistence between different strains of influenza viruses (SIV and AIV) showed that the swine influenza strain H1N2 was the most heat resistant one at 40˚°C, followed by SIV strain H1N1, SIV strain H3N2 and AIV strain H7N1. These experiments indicate that the selection of virus strains used for inactivation experiments can be of importance for the validation of production processes with regard to the inactivation capacity of individual production steps.
3) During treatment at 60˚°C viruses in the presence of egg yolk showed a higher stability than the control virus without egg yolk. It is suggested that proteins and other components present in egg yolk provide partial protection of virus particles during heat exposure. During acetone extraction the egg yolk quantity in relation to the volume of the virus inoculum influenced the stability of the viruses (AIV and NDV). The mean starting virus titres of all concentrations of egg yolk powder were reduced by ≥ 4 log10 for the residue and the extract. Inactivation of NDV and AIV was stronger when the water concentration was high.
4) No infectious influenza virus could be detected after partitioning of virus-spiked ethanolic extracts in the filtrate or on the filter parts. However, after microfiltration AIV genomes or genome equivalents were detected in both fractions.