Abstract:
This study investigated the serum haemagglutination inhibition (HI) antibody titres required to protect against drop in chicken egg production in velogenic Newcastle disease virus (NDV) infection. Isa brown pullets were obtained at 12 weeks of age. At 18 weeks, they were vaccinated against Newcastle disease (ND) using LaSota vaccine. Also at the peak of lay (27 weeks of age), they were revaccinated with ND LaSota vaccine. Two more booster vaccinations were also given at one week intervals to achieve hyperimmunity. The HI antibody titres of these birds were monitored weekly. At antibody geometrical mean titres (GMT) titres of 84.4, 42.2 and 21.1, twenty birds were randomly selected, taken to a distant location and challenged intramuscularly with a local velogenic strain of NDV, KUDU 113. After the third challenge, the remaining layers came down naturally with ND and showed drop in egg production. The groups of birds were designated batches A-D representing layers challenged at HI antibody GMT titres of 84.4, 42.2, 21.1 and natural infection respectively. After each challenge, the layers were monitored for clinical signs especially drop in egg production. Gross and histopathological examinations were performed on two layers that were sacrificed on day 3 PC from every challenge and control groups. Cloacal swabs were collected from three layers from every challenged group on days 3, 6 and 9 post challenge (PC) for virus isolation, confirmation and quantification. Serum samples were collected from every challenge and control layers on days 0, 7, 14 and 21 PC for antibody assay using haemagglutination inhibition (HI) test.Results showed reduced water and feed intake, slight cough and drop in egg production in batch D. No death was recorded. The three weeks percentage egg production was significantly lower (p < 0.05) in the infected batch D than their controls. Batches A, B and C showed enlargement of the spleen in their challenged groups while batch D showed atrophy of the spleen and oviduct with atretic ovarian follicles in the infected group. Histopathological results showed inflammation and oedema of the oviduct with infiltration of mononuclear cells. Serology showed increased antibody titres two weeks after hyper-immunisation which started declining two weeks after. Increased antibody titres were also observed on days 7- 21 PC. Virus was isolated from all the cloacal swabs collected on days 3, 6 and 9 PC from all the batches and was confirmed to be NDV using specific antiserum.From the results, it could be concluded that higher antibody titre is required to protect layers from clinical disease of ND than that required to protect them from drop in egg production. In laying chickens, antibody titre of 24 (16) was observed to be the protective titre against drop in egg production in velogenic NDV infection.
