Active Immunization of White Pekin Ducks Against Duck Virus Enteritis (Duck Plague) with Modified-Live-Virus Vaccine: Immunization of Ducklings

SUMMARY Thirteen serials of an experimental chicken embryo-adapted, duck virus enteritis modified live-virus (dve-mlv) vaccine were tested for viral content in embryonating chicken eggs and for their safety and immunogenicity in susceptible ducklings. Titers of serials varied between 3.2 and 4.6 (av. 4.2) log 10 median embryo lethal doses (eld50)/ml. The dose of 1 ml. of vaccine did not cause disease or mortality among 260 inoculated ducklings. Among 258 challenge-tested ducklings that were vaccinated with 1 ml. of undiluted vaccine, 0.77% died, whereas among 200 nonvaccinated controls that were challenge tested, 88.0% died. Eight of the 13 serials of dve-mlv vaccine were administered in increasing tenfold dilutions to determine the 50 and 90% duck protective dose (dpd50 and dpd90). These were mean log 104.47 dpd50 and 1O3.58 dpd90/0.5 ml., for 7 serials whose immunogenicity was challenge tested at postvaccination days 12 to 14. For the 8th serial, challenge tested at postvaccination day 8, these were log 102.57 dpd50 and 101.23 dpd90. Postchallenge mortality occurred for both vaccinated and nonvaccinated ducklings between the 3rd and 8th days, peaking on the 4th day. Immunity was manifested in two ways: in the positive correlation between increasing concentration of vaccine (i.e., less dilution) and resistance against challenge infection with virulent virus, and in a delayed peak of mortality. Death was also delayed in ducklings which, because they were inoculated with greatly diluted vaccines, had a high total mortality when challenged. The dose of 0.5 ml. of undiluted vaccine provided full protection against challenge infection by postvaccination day 4. One vaccine serial completely inactivated by propialactone (bpl) did not give any protection, whereas the same serial incompletely inactivated by N-acetylaziridine (naa) was less protective than the live-virus vaccine serial.