Défense de thèse de doctorat en sciences vétérinaires "West Nile virus "

Pathotyping of European West Nile virus (WNV) strains and analysis of the impact of viral molecular diversity on host-pathogen interaction

Jury

Ute ZIEGLER (Friedrich Loeffler Institut, Allemagne), Sylvie LECOLLINET (ANSES, France), Alain VANDERPLASSCHEN (ULg), Bénédicte LAMBRECHT, co-promoteur (CODA-CERVA), Jean-Jacques LETESSON, président (UNamur), Benoît MUYLKENS, promoteur (UNamur)

Résumé

A wide range of West Nile virus (WNV) phylogenetic variants has been circulating in Europe and the Mediterranean Basin countries since the mid 1990’s when this re-emerging arbovirus caused outbreaks in humans and horses. WNV genetic diversity is further amplified by its existence in nature as a quasispecies. Given the rare WNV-associated bird mortalities so far recorded in Europe, and despite their role as reservoirs and amplifying hosts for WNV, European birds were only scarcely targeted for pathogenicity and transmission studies. Moreover, if the NS3_Thr249Pro substitution was correlated to an increased virulence of WNV in American crows (Corvus brachyrhynchos), the epidemiological data recorded in Eurasia does not always support this mutation as a molecular marker of virulence.

In this thesis, we have first shown that lineage 1 WNV strains characterized by differential epidemiological profiles can be discriminated in day-old SPF chickens on the basis of mortality rates. In order to confirm the predictivity of the SPF chicken model and investigate the susceptibility of a common European corvid to WNV infection, we subsequently developed a wild-caught Carrion crow (Corvus corone) model for WNV infection that proved to be sensitive to WNV and also allowed the pathotyping of different WNVs based on multiple parameters. In an effort to explore WNV molecular determinants of virulence, we characterized highly related Eastern-European lineage 2 WNV strains as well as lineage 1-derived cloned viruses that differ as of the residue at position 249 (proline vs. threonine or histidine) of the viral helicase NS3 in both previously-developed avian models of WNV infection. In this context, we have shown that, outside the frame of some restricted viral genetic backbones, the NS3_249Pro  genotype is neither sufficient nor necessary to render any given WNV strain pathogenic for birds. In a final study, we sought to investigate whether WNV genetic diversification would differ between experimentally-infected wild-caught Carrion crows and SPF chickens using the deep-sequencing technology. We indeed showed that WNV quasispecies population dynamics differ between SPF chicks and Carrion crows, given that the latter allow more abundant and diverse single nucleotide variants (SNVs) to appear, what may correlate with the higher susceptibility of crows to WNV infection compared to chickens. 

The research carried out in the frame of this thesis is coherent with the Belgian preparedness efforts for dealing with a possible incursion of WNV. We indeed developed an SPF chicken infection model that would allow the pathotyping of unknown WNV isolates, and we proved that the Carrion crow, which is the main species targeted by the Belgian WNV surveillance program, is susceptible to WNV infection. These tools are of paramount importance given that our results suggest that WNV pathogenicity cannot be easily predicted yet as the same single-point mutations are not always correlated with comparable impacts on virulence. Moreover, the situation is further complicated given the differing sets of quasispecies that can be selected depending on the avian hosts WNV passes through.

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