Long-Term Humoral Immune Response After West Nile Virus Convalescence in Horses in a Geographic Area of Multiple Orthoflavivirus Co-Circulation.

BACKGROUND

In the last three decades, West Nile virus (WNV, Flaviviridae, Orthoflavivirus genus) has become one of the most important encephalitic agents worldwide, causing substantial numbers of cases in humans and horses every year by re-emerging in endemic areas and emerging in new territories. It is considered that after natural WNV infection, humans and birds develop long-term immunoprotection, but data on immunoprotection in horses is scarce.

HYPOTHESIS

West Nile virus infection provides long-term humoral immunity in subclinically infected horses.

ANIMALS

Client-owned, naturally WNV subclinically infected non-WNV-vaccinated, healthy horses.

METHODS

In this prospective cohort study, anti-WNV neutralizing antibody (nAb) titers of 25 horses were monitored for 5 consecutive years in Hungary. Serum samples were collected annually. First, a WNV immunoglobulin G (IgG) ELISA was performed, followed by virus neutralization tests (VNT) for endemic orthoflaviviruses. A VNT titer > 8 was considered positive.

RESULTS

The mean WNV titer of horses was 260.64 ± 336.74 in 2019, 114.32 ± 107.36 in 2020, 95.38 ± 115.56 in 2021, 22.53 ± 25.71 in 2022 and 6.31 ± 5.15 in 2023. A significant decrease (p < 0.001) in the nAb titers occurred over time. In 2023, 88% of the horses had WNV VNT titers below the cut-off value.

CONCLUSION AND CLINICAL RELEVANCE

Our results showed a significant decrease in WNV titers over time. Because nAbs correlate best with orthoflavivirus protection, our findings suggest that horses might not be protected against re-infection. We recommend regular nAb titer testing or vaccination in endemic areas.