Abstract
Peste des Petits Ruminants (PPR) is an acute highly contagious viral disease of small
ruminants. The disease is endemic in Bangladesh since 1993 and causes huge economic
losses due to its high morbidity and mortality. Although a vaccine is currently being used
in the country to control PPR but its efficacy is questionable; outbreaks of PPR are noticed
frequently despite vaccination. Moreover the vaccine virus currently being used in
Bangladesh is almost 20 years old; isolation history of this strain is not known and genetic
analyses of the vaccine virus revealed remarkable variations with field strains. To address
this problem a vaccine based on local isolate with known biological and genetic
properties is required. Therefore, the present study was undertaken to develop a vaccine
candidate from the local isolates. PPR virus was isolated from natural PPR outbreaks of
goats, confirmed by RT-PCR and used as viral inoculum for serial passaging in Vero cells
for adaptation and attenuation. A total of 60 serial passages were completed and RT-PCR
and real time RT-PCR were done at every 5 passages for confirmation of the presence of
PPR virus in tissue culture fluid (TCF). To assess the adaptation and attenuation of the
virus at 60th passage level, cytopathology, virus titration and sequencing of F gene were
done. To determine the optimum time point for harvesting of infectred Vero cells for
maximum yield of attenuated virus, growth kinetics of the attenuated virus was studied.
Live animal experimentation was performed to deterimine the immunogenicity and
protective efficacy of the attenuated virus. Cytopathology produced by PPR virus in
infected Vero cells included syncytia formation, development of both intranuclear and
intracytoplasmic inclusion body and finally cell degradation. As the number of passage
progressed, syncytia development was rapid, both the number and size of the syncytia
increased. At 60th passage level, the first syncytia appeared at 24 hour post inoculation
(hpi) with the fusion of 3 cells together, with gradual increase of the number of cells in
the syncytia up to 15 cells at 60 hpi. The titre of PPRV also increased with the number of
passages from 2.5 to 6.5 Log10 TCID50/ml. On growth curve assay, the peak virus titre
was observed at 72 hpi at 60th passage. Partial F gene sequence revealed 14 nucleotides
and 4 amino acids substitution of PPRV at 60th passage. Both fresh and lyophilized TCF of
60th passage were found immunogenic (average CP value 15.73) and 100%protective
against PPRV challenge in live goats. The 60th serially passaged PPR virus can be used as
a potent PPR vaccine candidate.