| Marek's disease (MD) is a lymphoproliferative disease of chickens caused by the highly cell-associated alphaherpesvirus, serotype 1 Marek's disease virus (MDV1). Since R L Witter and Okazaki et al developed the HVT vaccine against MD in the 1970s, vaccination has been a routine procedure to protect birds against MD. Although the vaccines of CVI988, SB1+HVT, Z4+HVT afford good protection against MD, vaccine breaks continue to occur along with variation in poultry industry and the evolution of the MDV. In this case a novel vaccination method, revaccination, has been exploited.Empirical evidence from the field provides increasing support for the use of revaccination with MD vaccines and the revaccination has become common in many countries. However, it is not known if the vaccine virus replicates after revaccination and how revaccination affects the immunity to MD. An experiment was conducted to investigate the dynamics of productive replication of MD vaccine virus given at revaccination comparied with the replication of MD vaccine virus given in a single vaccination. SPF White Leghorn chickens were vaccinated with serotype 3 MDV (HVT strain FC126) at day 1 or day 7 and revaccinated at day 7. Blood samples were collected and peripheral blood mononuclear cells (PBMCs) were isolated at 3 or 4 days intervals after vaccination. PBMCs samples were analysed by the flow cytometry using the monoclonal antibody specific for MDV serotype 3 to identify productively infected cells. The results indicated that the numbers of productive infection's PBMCs reached a peak at day 7 after SPF chickens were inoculated with HVT at either 1 or 7 days of age,and two peaks appeared at day 7 and day 14 in the revaccinated group. To determine if the revaccination affected the replication of very virulent MDV (vvMDV), chickenswere challenged with the RB1B strain at 25 days of age and PBMCs samples were prepared every 3 or 4 days postchallenged. The flow cytometric analysis showed that the peaks for productive replication of RB1B in the HVT revaccinated group were significantly lower than those in the group vaccinated one. These results suggested that productive replication of the vaccine virus given by revaccination could play an important role in augmenting immunity to MD.It has been reported that the protection against MD is improved by using revaccination in the field and the practice has become common in many countries, although little is known about its mechanism. To explian the phenomenon, experiments were carried out in SPF chickens. In the protective efficacy experiment, nine groups of SPF chickens were inoculated with the following vaccines: CVI988 Rispens, HVT or bivalent vaccine HVT+Z4 at either day 1 and/or day 7, all vaccinated groups were challenged at 14 days with wMDV Md5 and RB1B. Additional group of chickens provided the challenge-only control. The results showed that protection index (PI) for CVI were 88.2 after a single vaccination, and 94.1 with revaccination. The Pis for HVT were 70.6 in single vaccination, and 93.8 with revaccination. The Pis for HVT+Z4 were 83.1 in a single vaccination to 89.1 with revaccination, Pis of the revaccinated groups were significantly higher than those for the single vaccinated groups (P<0.05).Both humoral and cell-mediated immunity make important contributions to protection against MD. Experiments were carried out to evaluate cell-mediated immune responses induced by revaccination with MD vaccines. SPF White Leghorn chickens were immunized with CVI988, HVT and HVT+Z4 at either 1 day or 7 days of age and revaccinated at 7 days. All vaccinated groups were held in isolators. Blood samples were collected at 7, 14, 21, 28 and 35 days after the last vaccination. PBMCs were isolated and used in lymphocyte proliferation assays (MTT). The results showed that PBMCs proliferation values from all the groups, the revaccinated were significantly higher than those from the singly vaccinated ones. Changes in T-cell subpopulations were determined at 14, 21, 28, 35 days by flow cytometry using monoclona...
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