PhD

During 1991, six outbreaks, clinically suspected to be duck plague (DP) with 33 per cent morbidity and 26 per cent mortality were investigated. Duck plague virus was isolated from each outbreak. The isolates were able to produce the lesions and death of the duck embryos but failed to kill the chicken embryos during initial passages.
One of the strains named DP – S was partially attenuated by 10 passages in chicken embryos following 20 passage in duck embryos. Though the attenuated strain did kill ducks, its pathogenicity index was reduced from 1.9 to 1.23. The isolate DP – S under transmission electron microscope revealed virions of herpes virus morphology.
Two DP vaccines – commercial vaccine and lab – adapted vaccine having virus titres 0.74 and 3.5 log 10 ELD 50/ml respectively, were separately inoculated into four group of ducklings respectively, two groups receiving single dose and two receiving double dose of corresponding vaccines at an interval of four weeks. Another group of ducklings was kept as control without vaccination.
Three ducks in each group were challenged with virulent DPV at four, eight and 20 weeks post – vaccination. The birds in all the five groups were screened at regular intervals for studying the immune response by virus neutralization (VN), leucocyte migration – inhibition (LMI) and passive haemagglutination (PHA) test.
The challenged and survived birds were screened for the carrier status of DPV by examination of their rectal swabs for virus isolation.
In an organized farm, 180 ducks were given commercial vaccine at one year of age and were screened for VN antibodies, LMI response and PHA titres before and eight weeks post – vaccination. Randomly selected two birds were challenged six weeks post – vaccination.
The findings of the study are briefly listed as under:
* Six duck plague outbreaks were investigated, the virus isolated,
and characterized. It was partially attenuated in duck and chicken
embryos.
* The commercial vaccine could elicit very poor immune response as
compared to laboratory adapted vaccine. The immunity could not
last long even upto eight weeks in single vaccination and 20 weeks
in double vaccination.
* Single vaccination is not effective as compared to double vaccination
given four weeks apart.
* The assessment of antibody – mediated (AMI) and cell – mediated
(CMI) immune responses indicated that both AMI and CMI are
involved in protection of ducks against duck plague.
* The vaccinated or vaccinated and infected birds did not show carrier
status as attempts to isolate the virus from rectal swabs collected
after vaccination and challenge were unsuccessful.
* The PHA has been standardized for diagnosis of duck plague.