Effect of immunosuppression and passage on virulence of lentogenic vaccine strain of newcastle disease virus
Kibe, Julius Kariuki
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Newcastle disease virus (NDV) causes one of the most serious viral diseases of poultry. The virus is a member of the new Avularvirus genus in the family Paramyxoviridae. NDV has a negative stranded RNA genome which is approximately 15 Kb. NDV has a worldwide distribution, affects chicken, non-chicken poultry and wild birds. Mortality and morbidity of Newcastle disease (ND) in chicken may reach 100% in non immune birds. In Kenya, 70% of the thirty million poultry are indigenous chicken. These chickens are an important source of dietary protein and income from meat and eggs. This source of livelihood is constantly under threat due to frequent outbreaks (ND). The disease is controlled by vaccination of poultry using either live attenuated or inactivated vaccines. Following phylogenetic studies done in Australia and Ireland after NDV outbreaks, it was shown that wild non virulent strains of NDV that had reverted to virulence were responsible for the outbreaks. This supposedly occurred after the non-virulent viruses had circulated in poultry for some time. It has also been demonstrated that non virulent NDV strains turned virulent after passage in heterologous host cell lines, intramuscular inoculation in chicken and passage through intracerebral route in chicken. In Kenya, use of none virulent vaccine strains is widespread. It is not known if these vaccine strains revert to virulence after natural passage in chicken in the field thus establishing endemic ND foci.Neither is it known whether immunosuppression, as can be caused by poor nutrition, stress and unfavorable weather conditions in the field could contribute to this selection for virulence. This study was set to determine whether a lentogenic NDV would change to virulence after passage in leucocyte cultures of chicken and ducks. Forty NDV -free chickens and a similar number of ducks were used in the immunosuppression experiment. Half the bird in each group was used on passage experiments. One group in each species was immunosuppressed with dexamethasone while the other one was not. All the birds were treated with antibiotic to reduce bacterial contaminants in subsequent spleen cultures. Spleens were sampled each fifth day, leukocytes separated and cultured in 4 oz medical flat bottles and infected with a live attenuated Komarov NDV vaccine strain. Leukocyte cultures from immunosuppressed birds were treated with 20~ of dexamethasone per culture bottle. The cultures were incubated under CO2 atmosphere and progeny virus harvested amplified once in chicken embryonated eggs and then passaged into fresh leukocyte cultures. Virus virulence was tested by mean death time, (MDT) intracerebral pathogenicity, index ICPI and plaque assay in every passage. The result showed an increase in (ICPI) from 0 to 0.61 and 0.55 respectively for immunosuppressed chicken and ducks and was observed more in immunosuppressed than in the non immunosuppressed birds. Reduction in (MDT) was also observed from 154 hours to 81 hours and 88 in immunosuppressed ducks and chicken respectively. No plaque formation was observed in cultures infected with viruses passaged in both species of birds. These results show that passaging of the vaccine strain of NDV led to changes towards virulence. Further studies may need to be done in order to determine whether full virulence can be achieved. It was concluded that passaging of the vaccine strain of NDV in immunosuppressed chicken and duck leucocytes increased the virus ICPI and decreased its MDT, with increased potential of reverting to full virulence. It is recommended that use of live NDV vaccines in Kenya be reviewed with a view to using inactivated, or recombinant ND vaccine option and that causes of immunosuppression in poultry rearing environments be reduced or avoided.
- MST-Zoological Sciences