Genetic susceptibility to West Nile virus infection in Camargue horses

Highlights

• Horses from the Camargue region exposed to West Nile virus infection were studied.

• Immunity gene markers associate with susceptibility to horse West Nile encephalitis

• Immunity gene markers associated with antibody response to horse West Nile virus

Abstract

West Nile virus (WNV) is a mosquito-borne zoonotic neurotropic virus capable to cause lethal meningoencephalitis (WNE) in infected hosts such as birds, horses, and humans. Due to their sensitivity, horses serve as sentinel species in areas at risk. We studied a population of Camargue horses living in Southern France in two zones with endemic WNV circulation where WNV outbreaks were recorded in 2000 and 2003–4. Two sets of microsatellite markers located in MHC and Ly49 genomic regions were genotyped as well as multiple SNPs in ten immunity-related candidate gene regions. Associations between genetic polymorphisms and resistance/susceptibility to WNE were tested. While single marker associations were weak, compound two-gene genotypes of SNPs located within the MAVS, NCR2 and IL-10 genes and microsatellites HMS082 and CZM013 were associated with susceptibility to WNE. Combinations of microsatellite markers CZM009, ABGe17402 and ABGe9019 were associated with simple seroconversion without clinical signs of WNE (resistance). In addition, a distribution of polymorphic markers between WNV-IgG seropositive horses and a control group of WNV-IgG seronegative horses was tested. One SNP in the OAS1 gene (NC_009151.3:g.21961328A>G) was significantly associated with the seropositive phenotype (p_corr = 0.023; OR = 40.5 CI (4.28; 383.26); RR = 8.18 CI (1.27; 52.89) in the Camargue breed. In compound genotypes, SNP markers for SLC11A1, MAVS, OAS1, TLR4, ADAM17 and NCR2 genes and ten microsatellites showed non-random distribution between seropositive and seronegative groups of horses. Further analysis of associated markers could contribute to our understanding of anti-WNV defense mechanisms in horses.

Introduction

Flavivirus is a genus of the large family Flaviviridae. This genus comprises more than fifty arthropod-borne viruses including dengue virus, yellow fever virus or tick-borne encephalitis virus, which may cause significant morbidity and mortality in humans. Another member of the genus, the West Nile virus (WNV), has recently emerged as a significant cause of viral meningoencephalitis in humans and animals. WNV is a mosquito-borne zoonotic neurotropic positive-sense single-stranded (ss+) RNA virus, which can infect different vertebrate hosts, including birds, horses, and humans. In natural environments, it is maintained in an enzootic cycle between mosquitoes and birds (Chancey et al., 2015), while horses and humans are considered to be dead-end hosts due to a short and low viraemia (Sejvar, 2014). West Nile virus was first isolated in Uganda in 1937 and later it spread to Eurasia, Australia and the United States (Petersen and Roehrig, 2001). The disease caused by WNV had been only sporadically appearing in Europe since 1950´s until the first large outbreak was recognized in Romania in 1996, followed by outbreaks in South France, Tuscany, Italy and various other European countries (Panthier, 1968; Tsai et al., 1998; Murgue et al., 2001; Zeller and Schuffenecker, 2004).

Horses are considered to be particularly susceptible to WNV. Similarly to humans, the course of infection is subclinical in about 80–90% of infected horses. A severe neuroinvasive disease (West Nile encephalitis, WNE) with a mortality rate up to 40% may develop in old or immunocompromised horses (Angenvoort et al., 2013; Sejvar, 2014). Clinical signs of the disease involve fever, muscle tremor and/or ataxia and other neurological symptoms. The pathogenesis of WNV-induced disease in horses remains unclear. Probably, both innate and adaptive immune mechanisms are required for rapid clearance of the virus from the organism and to set up long-lasting protective immunity (Quicke and Suthar, 2013; Bielefeldt-Ohmann et al., 2017). Possible immunopathological mechanisms involved in the pathogenesis of encephalitis were also suggested (Bielefeldt-Ohmann et al., 2017; Rossini et al., 2013). Due to their susceptibility to infection, horses serve as sentinel species in areas at risk. In such regions, cases of suspected viral encephalitis in horses alert a local surveillance system (Leblond et al., 2007; Rockx et al., 2006).

Association analyses represent a suitable tool for identifying genes involved in mechanisms of disease. Genes associated with the susceptibility to WNV infection were reported in humans, mice and several other mammalian species. In 2002, a nonsense mutation in the gene encoding 2′-5′-oligoadenylate synthetase 1 isoform (OAS1) was identified in specific laboratory strains of mice (Mashimo et al., 2002). Since then, OAS1 and other immune response genes polymorphisms were intensively studied for their associations with WNV infection outcome and disease progression in humans (Bigham et al., 2011; Cahill et al., 2018). In horses, six SNPs in the OAS1 promoter region associated with the susceptibility to WNV-induced clinical disease were identified (Rios et al., 2010). Several studies also suggested possible role of complex genomic regions such as MHC and/or NK cell receptor genes in the host genetic susceptibility to WNV (Spiroski et al., 2013; Sarri et al., 2016; Strauss-Albee et al., 2015).

Genome-wide associations studies (GWAS) are a prime tool for detecting candidate genomic regions. However, they require large numbers of well-defined cases and are rather expensive. Targeted candidate-gene approaches aim to identify polymorphisms and genes associated with disease based on their hypothesized role in the disease mechanisms. The statistical nature of association analyses requires limiting as many variables as possible. For this purpose, model populations are often used.

Horses, despite several limitations, such as generation interval and breeding management, are a suitable model species due to their susceptibility to infection, its clinical manifestation and their role in disease surveillance systems (Chevalier et al., 2011; Angelini et al., 2010). They are also a more appropriate model for human studies compared to rodents (Bielefeldt-Ohmann et al., 2017). The Camargue region, situated in the delta of the Rhone River, harboring large mosquito populations and numerous colonies of both migratory and settled birds, is inhabited with a specific local breed, the Camargue horse, along with other horse breeds used mostly for touristic purposes. In the South of France, repeated outbreaks of the WNV infection were reported from horses from two zones with endemic WNV circulation - Camargue and Var (Durand et al., 2005). The first WNV infection outbreak in the Camargue region was observed in 1962 and lasted three years (Panthier, 1968). Later, outbreaks were recorded in 2000 and 2003–4. In contrast to the large epidemics in the US, outbreaks in Camargue could be more properly described as sporadic outbreaks separated by long periods of silence (Beck et al., 2013). The Camargue horses are likely descendants of old Berber horses brought to this specific area 2000 ago (http://www.haras-nationaux.fr/information/accueil-equipaedia/races-dequides/chevaux-de-sang/camargue.html). Taking into consideration their long-term interactions with the enzootic WNV, their adaptation to the pathogen at the genomic level may be expected.

Based on these assumptions, we analyzed associations between polymorphisms in selected immune-related candidate genes and microsatellite markers and the resistance/susceptibility to WNV-induced clinical disease. In addition, differences in the distribution of the same polymorphic markers between WNV-IgG seropositive and seronegative horses were tested.