Correlation of MDV vaccine virus genome load in feather tips with protection, using an experimental challenge model
Résumé
We previously developed a real-time PCR assay for absolute quantitation of serotype 1 Marek's disease virus in feather tips of chickens, and this has been used clinically to monitor a flock's response following vaccination with CVI988, an attenuated serotype 1 strain. The level of vaccine virus in feather tips associated with protection against challenge by virulent virus is not known. Here, we used an experimental challenge model, in which one dose of vaccine gives over 90% protection against mortality, to investigate correlation between CVI988 level in feathers and protection. Day-old chickens were vaccinated with 1, 0.1 or 0.01 commercial dose of CVI988 vaccine, and then challenged with a virulent strain (RB-1B) 14, 21 or 28 days later. Replication of CVI988 virus was followed in each bird by real-time PCR analysis of feather DNA samples. Since the PCR does not differentiate between CVI988 and RB-1B, samples were taken only prior to challenge, to ensure that the virus being measured was CVI988. Administration of 1 dose of vaccine ensured a uniform, rapid and high replication amongst birds, while replication following administration of 0.1 or 0.01 dose was very variable. However, given time, a low early level of vaccine virus eventually replicated to high levels in some birds. Both dose of vaccine virus administered, and level of vaccine virus in feather tips at 13 days post vaccination, showed significant correlation with protection against challenge. A level of CVI988 vaccine virus of 132 genome copies per 10000 feather tip cells was calculated to be the level required for 90% protection in this experimental model. The potential of this assay, and its limitations for monitoring protection in the field, are discussed.
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