ABSTRACT

Mareks disease (MD) a lymphoproliferative, neoplastic disease of birds with worldwide distribution and significant economic consequences has not been studied in Abia State or its environs. In order to understudy its status, occurrence, vaccination dynamics and pathophysiological changes that can be associated with its viral etiology, structured, open and close ended questionnaires were employed in cross sectional study to obtain information on clinicopathological, post mortem, host and type factors influencing MD occurrence in the three senatorial zones. A second questionnaire was employed with oral interviews to obtain data on vaccine use and application on the same farms in addition to the hatcheries they patronize. An indirect ELISA was used to audit difference vaccination protocol adopted in the different hatcheries. These protocols were experimentally replicated in unvaccinated d.o.c. and the sera harvested for eight weeks and subjected to ELISA. Tissue samples including feather tips suspected of MD were collected within the study region. The tissues were processed for histopathological examination. Total DNA was extracted from the feather tips through PCR technique. Meq gene analysis of the DNA showed five MD positive samples. Phylogenetic analysis of three of the samples was conducted using the deduced amino acid sequences of the Meq gene. Results showed a retrieval rate of 52.1% and a positive occurrence of MD in 76% of analyzed cases. Significant occurrence of MD was recorded in all the zones with Abia Central having the highest. The classical form of MD was dominant with severe weight loss being the most observed clinical sign. Though 77.5% of farmers were aware of MD vaccination need, only 8.5% revaccinated their flock. Use of antibiotics for brooding significantly affected MD occurrence. 70% of hatcheries patronized by farmers were localized in the South West of Nigeria. Whereas 80% of the hatcheries were using Rispens alone or in combination with HVT, 20% used HVT alone and they contributed to the highest number of outbreak in farms sourcing d.o.c from them. ELISA evaluation of vaccine protocols showed sustained seroconversion in vaccinated birds but with Rispens group having better results especially form the second week. Reduction in vaccine dose significantly reduced antibody titers across the groups. Boaster administration of HVT after a week showed slight advantage over the rest of the protocols. Histopathological examination of most of the suspected cases revealed consistent pleomorphism which is diagnostic of MD. Phylogenetic analysis of the Meq gene in MD positive samples showed 5 non-synonymous and one synonymous mutations. Sequences clustered in two clads. Two sequences showed close to 91% identity to the very virulent Egyptian vv+ strain while the other that segregated with Netherland strain was of the vv strain. Diagnostic challenges and vaccination aberration appears to be the major limiting factors in MD control in the study region. Novel mutations recorded in this study can represent a drive towards higher virulence in the study region.