A meta-analysis of observational epidemiological studies of Newcastle disease in African agro-systems, 1980-2009

Abstract

Newcastle disease (ND) is one of the most important and widespread avian pests. In Africa, backyard poultry production systems are an important source of protein and cash for poor rural livelihoods. ND mortality in these production systems is important and seriously disrupts benefits derived from it. This study undertook an African continental approach of ND epidemiology in backyard poultry. After a systematic literature review of studies published from 1980 to 2009, a meta-analysis of spatio-temporal patterns of serological prevalence and outbreak occurrence was performed. Average ND serological prevalence was estimated at 0·67 [95% confidence interval (CI) 0·58-0·75] in regions characterized by humid ecosystems, high human and poultry densities and low altitudes; 0·36 (95% CI 0·30-0·41) in dry ecosystems at intermediate altitude where human and poultry densities are low and 0·27 (95% CI 0·19-0·38) in mountain ecosystems where human and poultry densities are intermediate. In terms of seasonality, ND outbreaks occur mostly during the dry seasons in Africa, when environmental conditions are likely to be harshest for backyard poultry. In addition, a phylogeographical analysis revealed the regionalization of ND virus strains, their potential to evolve towards a higher pathogenicity from the local viral pool and suggests a risk for vaccine strains to provide new wild strains. These results present for the first time a continent-wide approach to ND epidemiology in Africa. More emphasis is needed for ND management and control in rural African poultry production systems.

Fig. 1.

Newcastle disease and virus eco-epidemiological characteristics: survival of the virus for different substrates, temperatures and pHs; incubation periods and transmission modes. (a) [17], (b) [95], (c) [8], (d) [96], (e) [97], (f) [1], (g) [4].

Fig. 2.

Locations of Newcastle serological prevalence studies selected for the meta-analyses on a biome map (adapted from [39]).

Fig. 3.

A median-joining network depicting the relationships between African Newcastle strains. Node size is proportional to haplotype frequencies in the dataset. Colours refer to countries. Red spots indicate a hypothetical missing haplotype and length of lines between nodes are proportional to the genetic distance between nodes.

Fig. 4.

Spatial distribution of the Newcastle strains in Africa.

Fig. 5.

Individual study serological prevalences (open symbols) and estimates of mean serological prevalences (filled symbols) with associated 95% confidence interval for Newcastle disease in three biogeographical categories defined according to biome (BI), altitude (AL), poultry density (PD), human density (HD) and year (Y).