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Two infection studies in chickens were done to investigate the humoral immune response against fowl poxvirus (FPV) and reticuloendotheliosis virus (REV) after intradermal infection with different passages of a field isolate and with the vaccine strain HP B. The field isolate in a low passage carried the near-full-length REV provirus and induced antibodies to REV, but not to FPV. The vaccine strain carried only remnants of the long terminal repeat and induced antibodies against FPV, but not against REV. The field isolate lost the provirus after 36 passages in vitro, and it induced few antibodies against FPV and no antibodies against REV. Intravenous challenge with the low passage field isolate caused some antibody development against FPV in the birds that had previously been infected with the field isolate, but it caused no antibodies against REV in the previously vaccinated birds. REV proviral DNA was found in peripheral blood mononuclear cells of most birds that had been infected with the low passage field isolate. However, FPV DNA was found only once. The findings showed that the integrated REV provirus had an effect on the pathogenesis of fowlpox and that the tested vaccine strain is effective against FPV strains carrying REV provirus. Investigation of sera from FPV diseased flocks and flocks vaccinated against FPV showed a similar proportion of sera with antibodies against FPV. Sera from all diseased flocks but only from two of 10 vaccinated flocks had antibodies against REV. This indicated that the integrated REV provirus is common in FPV field strains.