Empirical analysis suggests continuous and homogeneous circulation of Newcastle disease virus in a wide range of wild bird species in Africa

Abstract

Newcastle disease (ND) is one of the most important poultry diseases worldwide and can lead to annual losses of up to 80% of backyard chickens in Africa. All bird species are considered susceptible to ND virus (NDV) infection but little is known about the role that wild birds play in the epidemiology of the virus. We present a long-term monitoring of 9000 wild birds in four African countries. Overall, 3·06% of the birds were PCR-positive for NDV infection, with prevalence ranging from 0% to 10% depending on the season, the site and the species considered. Our study shows that ND is circulating continuously and homogeneously in a large range of wild bird species. Several genotypes of NDV circulate concurrently in different species and are phylogenetically closely related to strains circulating in local domestic poultry, suggesting that wild birds may play several roles in the epidemiology of different NDV strains in Africa. We recommend that any strategic plan aiming at controlling ND in Africa should take into account the potential role of the local wild bird community in the transmission of the disease.

Keywords: Zimbabwe.

Fig. 1.

Map of sample sites showing the seven sites where the 34 sampling occasions of the study took place. It also shows that the samples were analysed in two different laboratories following three different procedures. * The size of the circles on the map is proportional to sampling size of the study site. † Samples collected at lakes Chivero and Manyame were analysed by two different techniques at Onderstepoort Veterinary Institute Laboratory: PCR Simplex for samples collected from May 2007 to March 2009; PCR Triplex for samples collected from November 2009 to November 2010.

Fig. 2.

Homogeneous circulation of Newcastle disease virus in wild birds. The figure shows, for a given sampling occasion, similar prevalence rates with 95% confidence intervals across all groups of wild bird species based on cloacal and faecal samples in Mali and Mauritania.

Fig. 3.

Comparison between cloacal-faecal and tracheal PCR-positive rates. Boxplot of the PCR-positive rates of samples collected per country on birds that were tested for both cloacal (C) and tracheal (T) samples. A few birds were positive for both cloacal and tracheal samples (C&T).

Fig. 4.

Seasonal circulation of Newcastle disease virus (NDV) in wild birds. The prevalence rates with 95% confidence intervals are shown for the wild bird community for each sampling occasion per country based on cloacal and faecal samples. The two periods when no PCR-positive samples were detected are displayed as an open red circle. For Zimbabwe, two different graphs are displayed to reflect the two different PCR methods that were used to analyse the samples.

Fig. 5.

Phylogenetic analysis on 167 partial sequence of the F gene nucleic acid sequences of NDV (176 bases). Wild bird sequences from the study are displayed as a black circle and the name of the strain (including the country name) appears in red, blue and purple for strains detected in Mali, Mauritania and Madagascar, respectively. Domestic poultry sequences from the study are displayed as a black square. Trees were constructed using Bayesian inference with 40 000 000 iterations and 1/1000 trees sampled in the Monte Carlo Markov Chain. A burn-in phase for the first 25% of tree samples was used to generate the consensus tree. Convergence of the Markov chains was finally checked by an effective sample size (ESS) for all parameters >6825 and a potential scale reduction factor (PSRF) within 0·99993 and 1·00007. A class I virus sequence was introduced as an outgroup to root the tree. The final tree was drawn using Figtree 1.4.0 (http://tree.bio.ed.ac.uk/software/figtree/) and the tree posterior probabilities are reported on the branches.