Infectious Bursal Disease Virus in Algeria: Persistent Circulation of Very Virulent Strains in Spite of Control Efforts

Abstract

Infectious bursal disease (IBD) is among the most impactful immunosuppressive diseases of poultry. Its agent, infectious bursal disease virus (IBDV), is prone to both mutation and reassortment, resulting in a remarkable variability. Traditionally, IBDV characterization relies on antigenicity and pathogenicity assessment, but multiple phylogenetic classifications have been recently proposed, whose implementation in molecular surveys helps generating informative and standardized epidemiological data. In the present study, the Algerian IBDV scenario was assessed based on the novel classification guidelines by sequencing portions of both genome segments. Seventy pools of bursal samples were collected in 2022-2023 in 11 districts of Northern Algeria, mostly from broiler flocks. Out of 55 (78.6%) positive flocks, 40 (57.1%) were infected by field strains, which were characterized as very virulent strains (genotype A3B2) and phylogenetically related to previously reported Algerian strains. Significant differences in the percentage of field infections were observed between vaccinated (25/52, 46.2%) and unvaccinated (14/17, 82.3%) groups, and also between birds immunized with live intermediate (13/20, 65.0%) and intermediate plus (10/28, 35.7%) vaccines. Nonetheless, the number of field strain detections suggests a high infectious pressure and the inadequacy of current vaccination efforts, demanding a reevaluation of control measures coupled with attentive monitoring activities.

Keywords: Algeria; IBDV; broiler; molecular epidemiology; vaccination.

Figure 1

Phylogenetic trees of VP2 (left) and VP1 (right) sequences, inferred with the Maximum Likelihood Method (1000 bootstraps) adopting the K2+G [37] and GTR+G+I [38] substitution models, respectively. Sequences are color-coded according to their genogroup classification, which refers to the criteria proposed by Islam et al. [18]. Newly obtained sequences are marked with black circles (⬤), whereas other Algerian strains retrieved from GenBank are highlighted with a white square (▢). Node support values are shown only when equal to or higher than 0.7.

Figure 2

Distribution of field A3B2 strain detections at commune level. The area shown in the main map is highlighted in light green in the inset map in the top left corner, which shows the chicken population distribution according to the Gridded Livestock of the World 4 (GLW4) dataset [39].

Figure 3

Mosaic plot showing the relationship between the administered vaccination protocol and the detection of field strains. The size of each cell is proportional to the respective count, while the colors represent standardized residuals.