SEROPREVALENCE, MOLECULAR AND CLINICAL EPIDEMIOLOGY OF AVIAN METAPNEUMOVIRUS INFECTION IN THREE SELECTED CLIMATIC ZONES IN NIGERIA

Abstract

Avian metapneumovirus (aMPV) causes an immunosuppressive disease of the upper respiratory tract of chickens and turkeys, leading to substantial economic loss in poultry production. Despite the significant global burden of this disease, little is known about its endemicity, distinguishing clinical features, circulating virus subtypes and the role of climate in its occurrence in Nigeria. This study was designed to investigate aMPV seroprevalence, circulating subtypes, clinical presentation and predisposing factors in Nigeria. Using a cross-sectional study design and simple random sampling technique, blood was collected from 480 apparently healthy commercial chickens from states within three climatic zones of Nigeria: near-temperate (Plateau, n=160), rainforest (Oyo, n=160) and semi-arid (Sokoto, n=160) during the dry and wet seasons between December 2018 and September 2019. Harvested sera were tested for aMPV antibodies using indirect ELISA. A total of 168 tissue samples including conjunctivae, turbinates, tracheae and lungs (n=42 each) were collected from carcasses from chicken flocks with signs of respiratory distress presented at Veterinary diagnostic facilities in the study areas between December 2019 and April 2020 for virus detection using RT-PCR to amplify the N- and G-genes of the virus. Amplicons were sequenced using Sanger’s method and phylogenetic analysis was performed with appropriate software. Pretested questionnaires were administered to 42 owners of the sampled flocks to access information on clinical presentations and antibiotic usage during respiratory disease outbreaks. Thereafter, RT-PCR-positive samples were processed for virus isolation in Specific-Antibody-Negative embryonated chicken eggs. Bacteria associated with aMPV-positive tissues were isolated and identified using standard methods. Data were analysed using descriptive statistics and ANOVA at α0.05. The aMPV seroprevalence rates were 100.0, 48.8 and 56.2% for Plateau, Oyo and Sokoto states, respectively, during the dry season and 52.5, 36.2 and 65.0%, in the wet season. Mean antibody titers were significantly higher in the dry season (4757.9±223.5, 1414.0±158.0 and 2800.9±313.1) than wet season (670.7±74.9, 499.4±55.8 and 548.8±61.4) for Plateau, Oyo and Sokoto states, respectively. Turbinate and conjunctiva samples from five flocks (11.9 %) of layer chickens of all age groups were positive for aMPV in Plateau State with significant association between near temperate zone and the occurrence of the disease. Phylogenetic analysis revealed that the Nigerian aMPV strain clustered with European and Asian subtype B strains with unique mutations (T12I, G223E and A238V) in the G-gene. Clinical signs presented by aMPV-positive flocks included rales, coughing, sneezing and dyspnoea while the commonly used antibiotics by farmers were tylosin (71.4%), doxycycline (66.7%) and enrofloxacin (59.5%), without prescription. Virus isolation from aMPV-positive tissues was unsuccessful while secondary bacteria isolated included Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae. Avian metapneumovirus infection associated with a more virulent Subtype B strain was widespread in commercial layers in the study areas, with the turbinate and conjunctivae being the predilection sites. Associations with Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae were established, while transmission was aided by low environmental temperature and humidity. Routine vaccination of commercial chickens using homologous virus strains is recommended.