Abstract
The immune responses of commercial layer chickens against Newcastle disease(ND) were compared among different administration methods and times of vaccination during 4 weeks of age. A total of 372 day-old chickens were divided into 4 groups of 93 birds each. Each of 3 groups was received a commercially available B$_1$live vaccine via drinking water, eye instillation or spray method at one, 14 and 28 days of age. One group was used as an unvaccinated control. At two and 4 weeks after each time of vaccination, 15 birds from each group were collected randomly out and challenged with virulent ND virus at the dose of $10^5E1D_{50}$ per bird. Ten to 15 birds from each group were bled at two weeks intervals from day old to 8 weeks of age for hemagglutination inhibition antibody titer, The protection rate was generally low regardless of the times of vaccination although two or more times vaccination gave higher protection than once vaccination. The low protection was considered due to low titer of the vaccine used since the vaccine titer was less than $10^{3.5}EID_{50}$ per bird. Spray method gave better protection compared to eye instillation or drinking water method which resulted in lowest response. Majority of birds showed clinical signs of ND between 3 and 6 days after challenge. Death occured one or two days after onset of symptoms. Major clinical signs observed were depression(94%), anorexia(84%), diarrhoea(29%), difficult breath(15%) and torticollis(10%). Hemorrhagic lesions on post mortem were seen in duodenum(51%), trachea(35%), illeum(13%), ceacal tonsil(11%), proventriculus(10%) and some other odrgans.