The West Nile Virus-Like Flavivirus Koutango Is Highly Virulent in Mice due to Delayed Viral Clearance and the Induction of a Poor Neutralizing Antibody Response

Abstract

The mosquito-borne West Nile virus (WNV) is responsible for outbreaks of viral encephalitis in humans, horses, and birds, with particularly virulent strains causing recent outbreaks of disease in eastern Europe, the Middle East, North America, and Australia. Previous studies have phylogenetically separated WNV strains into two main genetic lineages (I and II) containing virulent strains associated with neurological disease. Several WNV-like strains clustering outside these lineages have been identified and form an additional five proposed lineages. However, little is known about whether these strains have the potential to induce disease. In a comparative analysis with the highly virulent lineage I American strain (WNV NY99 ), the low-pathogenicity lineage II strain (B956), a benign Australian strain, Kunjin (WNV KUN ), the African WNV-like Koutango virus (WNV KOU ), and a WNV-like isolate from Sarawak, Malaysia (WNV Sarawak ), were assessed for neuroinvasive properties in a murine model and for their replication kinetics in vitro . While WNV NY99 replicated to the highest levels in vitro , in vivo mouse challenge revealed that WNV KOU was more virulent, with a shorter time to onset of neurological disease and higher morbidity. Histological analysis of WNV KOU - and WNV NY99 -infected brain and spinal cords demonstrated more prominent meningoencephalitis and the presence of viral antigen in WNV KOU -infected mice. Enhanced virulence of WNV KOU also was associated with poor viral clearance in the periphery (sera and spleen), a skewed innate immune response, and poor neutralizing antibody development. These data demonstrate, for the first time, potent neuroinvasive and neurovirulent properties of a WNV-like virus outside lineages I and II. IMPORTANCE In this study, we characterized the in vitro and in vivo properties of previously uncharacterized West Nile virus strains and West Nile-like viruses. We identified a West Nile-like virus, Koutango virus (WNV KOU ), that was more virulent than a known virulent lineage I virus, WNV NY99 . The enhanced virulence of WNV KOU was associated with poor viral clearance and the induction of a poor neutralizing antibody response. These findings provide new insights into the pathogenesis of West Nile virus.