Infectious bursal disease is among the most economically viral disease affecting poultry industry in the world. IBDV has a different range of virulence, and in particular vvIBDV strains being responsible for the high failures from mortalities and decreased performances. Recently in Morocco, molecular characterisation of full genome IBDV revealed the presence and spread in poultry flocks, the vvIBDV genotype strains including all genetic markers of virulence associated to some amino acids specific to Moroccan IBDV isolates . However, the study of viral virulence evolution of Moroccan vvIBDV isolate would being of great interest to correlate the genotype to the pathotype of the virus strains. In this study, we tried to determine and to evaluate the pathogenicity of one candidate strain of a recent Moroccan vvIBDV designated 1/chicken/Morocco/IB19/2017 (accession no. MK580160 and MK580164) by assessing the clinical signs, mortality rate, the macroscopic and microscopic lesions of different lymphoid organs in commercial broilers (G1) under experimental conditions. The objective of the second experiment was to reproduce the Infectious Bursal Disease in light breed SPF chickens in which the mortality rate could reached 100% . Using SPF chickens is considered as the standard model to assess the pathogenicity of IBDV.
The typical clinical signs of acute IBD including depression, prostration, ruffled feathers and watery or white diarrhoea were observed in both experiments and were similar to those previously described by (27) for acute infectious bursal disease. The mortality occurred at 4dpc for broilers (G1) reached 10% which is very low in comparison to SPF chickens where the mortality appeared at 3 to 5 dpc and reached 84% at 5 days. Therefore, the mortality rate in SPF chickens (G3) is eight time higher than the mortality in broilers (G1), this can be explained by the genetic sensibility of light breed chickens to infection with very virulent IBD viruses than heavy breed as it has been reported in several studies . The mortality rate (10%) in broilers(G1) found in this study was similar to the rate reported by [22, 27] for vvIBDV field outbreaks in broilers. These authors reported respectively mortality rates comprised between 5-15% and 10-25%.
The Gross lesions observed on SPF chickens (G3) found in our work were more severe than the macroscopic lesions observed in the commercial broilers (G1), specially the BF of SPF (G3) was very haemorrhagic giving the appearance of black cherry observed in most of dead birds (G3). Nevertheless, the macroscopic lesions in commercial broilers (G1) dominated by small yellowish Bursae, this is similar to previous descriptions of the disease by [2, 12]. Furthermore, the volume of the BF decreased in challenged broilers (G1), while it moderately increased in control chickens (G2) corresponding to the normal growth of the BF as demonstrated by . We observed significant regression in Bursa Body Ratio in challenged broilers (G1), similarly to the observations of [14, 17] in their studies of the impact of vvIBDV strains in Bursa Body Ratio parameter.
According to , a mean Bursa/Body Index inferior to 0.70 in broilers indicates bursal atrophy. The mean Bursa/Body Index in our experiment on broilers (G1)was 0.54, indicating a severe bursal atrophy.
The Spleen Body Ratio increased in challenged broilers (G1), which mean the presence of splenomegaly at 5, 7 and 9 dpc, this has also been described by  for the vvIBDV 92/04 strain. The Thymus Body Ratio decreased severely in challenged broilers (G1), which could be attributed to thymic atrophy. This observation had also been made by  in their study using Japanese vvIBDV strains. However, thymic microscopic lesions of broilers (G1) were very transient, and only visible in chickens that had died, in contrast to the severe thymic lesions observed in SPF chickens described by [1, 30].
The histological lesions of the BF of challenged broilers (G1) did not show lymphoid regeneration at 9 dpc, which has been described by  as typical of infections by vvIBDV strains higher virulence and the mean lesion score was 3.75. This score is similar to scores obtained by  (3.3) for 90-11 Japanese vvIBDV strain, as well as those obtained by  (3.8) for 849 VB European vvIBDV strain. In addition, the mean lesion scores for the spleen, cecae tonsils and thymus were also similar to the scores obtained in previous studies of very virulent strains from Japan and the Netherlands [14, 32, 33].
Regarding the liver, periportal lymphoid infiltrations have been previously described for infectious bursal disease by . In the kidneys, degeneration of tubular epithelial cells was observed in our study, these lesions were also found in sacrificed chickens after 7dpc, this could be explained by a vvIBDV pathotype of Moroccan strain used in this study.
Finally, the viral load in the BF and viral excretion during an IBDV infection have not been extensively studied before. Li and Co-authors  assessed the viral load in the BF and viral excretion via RT-PCR relative quantification, following an experimental infection with the vvIBDV European strain DK01. At 3dpc, the relative viral load in our study was over 3 times higher than the value obtained by the same authors using DK01 as vvIBDV challenged strain. This suggests a higher replication of Moroccan virus in BF, which clearly indicate a high virulence of Moroccan vvIBD virus used in this study.
The results of the current study showed the high pathogenic potential of Moroccan IBD virus recently isolated and genetically characterized as very virulent virus based on both genome segments and confirm the observations previously reported that SPF light breed chickens are more susceptible to vvIBDV infection than broilers. However, the differences in susceptibility to infection with IBDV based on genetic variations of SPF chickens lines . Nielsen and Collaborators  reported high susceptibility to the infection with IBDV was found in layers chickens vs. the broilers. Further studies of vaccines protection are needed to evaluate the level of protection conferred by available vaccines used in the country against the recent Moroccan vvIBDV isolates.
In conclusion, the vvIBDV Moroccan strain used in this study exhibited a very virulent pathotype according to very virulent genotype finding based on characterization of both genome segments. The pathogenicity of Moroccan vvIBDV is increased when both genome segments A and B are found as very virulent . This synergy between the vv segments A and B generate high pathogenicity of the virus .