Evaluation of Vaccination during Disease Outbreak as a Disease Control Option: Case Study on Newcastle Disease

Abstract

This study investigated the response to Newcastle disease vaccination in previously vaccinated and unvaccinated poultry flocks experimentally infected with Newcastle disease virus. A total of 90 day old cockerel chicks procured from a local hatchery were used for the study. They were brooded under deep litter system for 3 weeks after which they were divided into 2 groups, A and B of 45 birds per group. Members of group A were subsequently vaccinated against Newcastle disease using NDV-I2 intraocular. Members of group B received no vaccination. Five ml of blood was collected from 10 birds per group for serology. At 6weeks of age, members of group A were further divided into 3 equal groups A1, A2, and A3 while members of group B were divided into B1, B2 and B3 of 15 birds per group. All groups were housed differently but were fed and managed in similar approach. Five ml of blood were collected from 5 birds per subgroup for serology. Concurrently, 4 birds experimentally infected with kudu-113 strain of Newcastle disease virus were each introduced into pens housing groups A2, A3, B2 and B3 as source of infection, to simulate a natural infection. Groups A1 and B1 controlled their respective groups. Upon manifestation of clinical infections by the 4 exposed groups, all the group members had 5ml of blood collected for serology. Concurrently, members of groups A2 and B2 were vaccinated upon manifestation of clinical infections while A3 and B3 were not. All the groups were monitored till week 10 post hatch when observations were terminated with special attention paid to variables like incubation period, clinical signs, number and pattern of morbidity and mortality including the results of antibody titres of the groups on days 21, 42, 56, 63 and 70 post hatch. The results showed that clinical signs of patent infection were 6 days post infection in the previously unvaccinated groups B2 and B3. However, the period was extended to day 7 PI among the previously vaccinated groups A2 and A3. On day 11 PI, morbidity was 100% among members of groups B2 and 92.86% among groups B3. However, it was 42.86% and 64.29% respectively for members of groups A2 and A3. There were no morbidity nor mortality among members of groups A1 and B1. Mortality started on day 8 PI in the infected B groups and on day 14 PI, it was 100% among group B2 and 71.43% in group B3. However, mortality was first recorded among the infected group A members on day 11 PI. Total mortalities were 14.29% and 50% respectively for groups A2 and A3. The antibody titre among groups A and B members at week 3 of age was zero. On week 6 of age corresponding to 3 weeks post vaccination of group A members, the GMT among groups A1 to A3 was 4096 while among the B groups the value remained zero. On day 56, the mean titre among A1 members dropped to 3026, A3 128 while A2 remained 4096. Among B groups, B2 rose to 115.2, B3 1024 while B1 remained zero as they never received any vaccination/virus. At day 63 post hatch, the GMT among A1 had declined to 106, A3 115.2 while A2 remained 4096. B3 had GMT of 921.6, B2 had recorded 100% mortality while B1 remained zero. At day 70 post hatch, the GMT of A1 was zero, A2 4096 and A3 1945.6. B3 was 768 and B1 zero. This study showed that maternal immunity had waned completely in unvaccinated birds at 3 weeks of age. Also, vaccination of previously unvaccinated birds upon clinical infections showed exacerbated morbidity and mortality relative to those earlier vaccinated. Moreover, vaccination of previously vaccinated birds in the face of clinical Newcastle disease infection proved more beneficial than allowing the infection to run its course.