Replication kinetics and shedding of very virulent Marek's disease virus and vaccinal Rispens/CVI988 virus during single and mixed infections varying in order and interval between infections

Field studies of the detection, persistence and spread of the Rispens CVI988 vaccine virus and the extent of co‐infection with Marek's disease virus

“…In the present experiment, significant viral loads in dust were observed in very young chickens prior to the onset of the shedding of virus in significant quantities as reported in previous studies . This is likely caused by contamination of the dust samples with material from the previous batch of chickens.…”

“…The Rispens viral load in dust was highest at 24 days post vaccination, at approximately 10 6 VCN/mg of dust. This is higher than observed in an experiment carried out in housed ISA Brown chickens where the load of Rispens in dust 21 days post vaccination was approximately 3.2 × 10 4 VCN/mg . Those results indicate that early vaccine success can be measured from feather samples in chickens aged ≥ 14 days and in dust samples from housing of chickens aged ≥ 21 days.…”

“…Apathogenic MDVs have been isolated from the field in early studies and found to have a protective effect against MD . The Rispens virus has a lower replication and shedding rate than virulent MDV, but our findings and those of Jackson et al suggest that less virulent MDVs with lower replication rates may continue to be present and circulate in the poultry population.…”

“…Much higher levels of co‐infection may be seen under conditions of experimental challenge following vaccination. In a recent study involving MDV challenge 10 days after day‐old vaccination with Rispens, approximately 35% of feather samples and 75% of dust samples were positive for the pathogenic MDV in the post‐challenge period . Our data suggest that vaccinal protection against infection is high, possibly because of the long period of time between vaccination and challenge.…”

“…The potential for co‐infection of MDV has made the measurement of Rispens vaccination efficacy difficult because both Rispens and MDV are GaHV‐2 (serotype 1) viruses, requiring tests that can differentiate between them. With the recent development of qPCR tests that can achieve this task, it has been possible to perform experimental studies involving co‐infection with both viruses …”