*Article* **First Detection of the West Nile Virus Koutango Lineage in Sandflies in Niger**

**Gamou Fall 1,\*, Diawo Diallo 2, Hadiza Soumaila 3,4, El Hadji Ndiaye 2, Adamou Lagare 5, Bacary Djilocalisse Sadio 1, Marie Henriette Dior Ndione 1, Michael Wiley 6,7, Moussa Dia 1, Mamadou Diop 8, Arame Ba 1, Fati Sidikou 5, Bienvenu Baruani Ngoy 9, Oumar Faye 1, Jean Testa 5, Cheikh Loucoubar 8, Amadou Alpha Sall 1, Mawlouth Diallo 2 and Ousmane Faye 1**


**Abstract:** West Nile virus (WNV), belonging to the *Flaviviridae* family, causes a mosquito-borne disease and shows grea<sup>t</sup> genetic diversity, with at least eight different lineages. The Koutango lineage of WNV (WN-KOUTV), mostly associated with ticks and rodents in the wild, is exclusively present in Africa and shows evidence of infection in humans and high virulence in mice. In 2016, in a context of Rift Valley fever (RVF) outbreak in Niger, mosquitoes, biting midges and sandflies were collected for arbovirus isolation using cell culture, immunofluorescence and RT-PCR assays. Whole genome sequencing and in vivo replication studies using mice were later conducted on positive samples. The WN-KOUTV strain was detected in a sandfly pool. The sequence analyses and replication studies confirmed that this strain belonged to the WN-KOUTV lineage and caused 100% mortality of mice. Further studies should be done to assess what genetic traits of WN-KOUTV influence this very high virulence in mice. In addition, given the risk of WN-KOUTV to infect humans, the possibility of multiple vectors as well as birds as reservoirs of WNV, to spread the virus beyond Africa, and the increasing threats of flavivirus infections in the world, it is important to understand the potential of WN-KOUTV to emerge.

**Keywords:** West Nile virus; Koutango lineage; high virulence; sandflies; Niger
