SOME EPIDEMIOLOGICAL FACTORS ASSOCIATED WITH PERSISTENT INFECTIOUS BURSAL DISEASE (IBD) VIRUS INFECTION OF CHICKEN IN SOUTH WESTERN NIGERIA

Abstract

Fifty one cases of infectious bursal disease (IBD) were investigated to determine some host and viral factors responsible for recurrent outbreaks in poultry flocks. The disease was confirmed by clinical signs, necropsy findings and detection of specific IBD virus antibody in the blood samples and virus antigen in the bursal of fabricius of infected chickens. The agar gel precipitation test (AGPT) technique was used for the initial antibody detection. Serological screening was carried out on susceptible age groups of chickens. The humoral response of chickens to IBD vaccines in some flocks were then evaluated and the immunogenicity of four different commercial IBD vaccines determined experimentally. The proportion of chicks that received maternal antibodies from their parents was determined by longitudinal follow-up of parent stocks and their progenies for 5 weeks. Commercially prepared enzyme linked immunosorbent assay (ELISA) kits were used to determine the decay of passive IBD virus antibody in baby chicks. An antibody detection ELISA. was developed for detection and quantification of IBD virus antibody using Positive/Negative ratio method of analysis from single strum dilution. Out of the 51 outbreaks, 31 (60.78%) were among pullet flocks, 17 (33.33%) among broiler flocks, while 2 (3.92%) were cockerels. Seven (13.73%) of the outbreaks were caused by very virulent strains of IBD virus. Thirteen different imported and one locally produced IBD vaccines were identified as being used by farmers during the period of investigation. Specific IBD virus antibodies were detected in 26, 25, 15, 12, 33.33, 71, 43, 63. 3 and 72.12 percent of birds on farms at 8, 15, 22, 29, 36, 43 , and 50 days of age

respectively. Following IBD vaccination, 0, 22. 50, 26.67 and 77.78 per cent of flock at 12, 19, 26 and 33 days post vaccination were positive for IBD virus antibody, respectively. Out of the four commercial vaccines tested, one did not induce effective immunisation until 21 days post-vaccination. Antibodies to IBD virus were present in 29.73% of vaccinated broiler breeders and 44.21% of their progenies. Using the commercial ELISA kits, maternal antibody level decayed to non-interference level of 396 on the 10th, 15th and 13th days of age in three different flocks, respectively. The mean Optical density (0:D:) for negative sera for the spot ELISA developed was 0.085 with upper limit of 0.17 at A₄₉₂. A standard prediction equation was derived by regression analysis of ELISA titres at 300⁻¹ dilution of sera samples as: ET=P/N-1.96/804X10⁶ - for measurement of IBD antibody titre. Surveillance through the pre- and post- vaccination monitoring to determine effectiveness immunisation, optimum age to vaccinate and type of vaccine to use are recommended as short- term measures to control IBD. A national Policy on chicken breeder and progeny vaccination programme against IBD and determination of appropriate antigenic vaccine stains to use in Nigeria is recommended as a long term measure to control IBD. This work showed that combinations of factors are responsible for the persistence of the infectious bursal disease among poultry flocks and could be controlled by a combination of serological monitoring and the use of the appropriate vaccine.