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.
