Evaluation of Cross-Protection of a Lineage 1 West Nile Virus Inactivated Vaccine against Natural Infections from a Virulent Lineage 2 Strain in Horses, under Field Conditions

Abstract

Although experimental data regarding cross-protection of horse West Nile virus (WNV) vaccines against lineage 2 infections exist, the cross-protective efficacy of these vaccines under field conditions has not been demonstrated. This study was conducted to evaluate the capability of an inactivated lineage 1 vaccine (Equip WNV) to protect against natural infections from the Nea Santa-Greece-2010 lineage 2 strain. In total, 185 WNV-seronegative horses in Thessaloniki, Greece, were selected during 2 consecutive years (2011 and 2012); 140 were immunized, and 45 were used as controls. Horses were examined for signs compatible with WNV infection. Neutralizing antibody titers against the Greek strain and the PaAn001/France lineage 1 strain were determined in immunized horses. WNV circulation was detected during both years in the study area. It was estimated that 37% and 27% of the horses were infected during 2011 and 2012, respectively. Three control animals developed clinical signs, and the WNV diagnosis was confirmed. Signs related to WNV infection were not observed in the vaccinated animals. The nonvaccinated animals had a 7.58% ؎ 1.82% higher chance of exhibiting signs than immunized animals (P < 0.05). Neutralizing antibodies raised against both strains in all immunized horses were detectable 1 month after the initial vaccination course. The cross-protective capacity of the lowest titer (1:40) was evident in 19 animals which were subsequently infected and did not exhibit signs. Neutralizing antibodies were detectable until the annual booster, when strong anamnestic responses were observed (geometrical mean titer ratio [GMTR] for lineage 1 of 30.2; GMTR for lineage 2 of 27.5). The results indicate that Equip WNV is capable of inducing cross-protection against natural infections from a virulent lineage 2 WNV strain in horses. West Nile virus (WNV) is a single-stranded RNA virus within the Japanese encephalitis virus serocomplex, which belongs to the genus Flavivirus (family Flaviviridae) (1). WNV is maintained in nature by enzootic transmission cycles between certain bird species and ornithophilic mosquitoes (2). Mosquitoes mainly belonging to the genus Culex can also act as bridge vectors, transmitting the virus to other animal species, including incidental hosts (3-6). Humans and horses are regarded as incidental (deadend) hosts, as the virus titer developed in their blood is generally too low to infect mosquitoes (7). Nevertheless, WNV infection in susceptible hosts may eventually cause neurological disease (8). Regarding horses, the reported clinical signs may vary, and these include fever, paraparesis or tetraparesis, and ataxia, recumbency, and behavioral changes, while in many clinically affected horses muscle fasciculation and tremors are also present. It is expected that deaths will occur in a small percentage of the affected animals (9-13).Phylogenetic analyses of WNV strains isolated worldwide have resulted in the identification of 8 genetic lineages of the virus so far (14). Until 2004, only viral strains...

Integrated analysis of human-animal-vector surveillance: West Nile virus infections in Austria, 2015–2016

“…Vaccination is the primary method for reducing the risk of infection with WNV. While vaccines against WNV for horses are commercially available 43 , for humans they are still in development 44 . Therefore, other preventive public health measures, such as individual prevention to mosquito bites and possible reduction of breeding habitats, are advised to the public during peak WNV activity.…”

Pathogenicity and virulence of West Nile virus revisited eight decades after its first isolation

“…WNV veterinary vaccines have proved to be protective, and their use greatly contributed to reduce the incidence of WN disease in horses in the US [ 255 , 256 ]. Although most vaccines have been based on American strains from Lin 1, the high degree of cross-reactivity between viruses from Lin 1 and 2 supports the efficacy of veterinary vaccines to prevent outbreaks associated with both lineages [ 184 , 257–260 ]. Notwithstanding, despite the proven efficacy of veterinary vaccines, they still exhibit some limitations, like the requirement for repeated administrations for initial immunization, and the relatively short duration of the immunity that makes annual boosters necessary.…”

Comparative Pathology of West Nile Virus in Humans and Non-Human Animals

“…As the species most frequently and significantly affected by WNV infections aside from birds and humans, horses have been used in pathogenesis work and in efficacy and safety studies required for approval of WNV equine vaccines [191][192][193][194][195][196]. There are currently four United States Department of Agriculture-licensed equine WNV vaccines available in the United States [197].…”