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

Abstract

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.

Keywords: antibody; cross‐reaction; equine; serology.

FIGURE 1

Location of the stables in Hungary. Stable 1—Győr‐Moson‐Sopron County, Stable 2—Pest County, Stable 3—Baranya County, Stable 4—Pest County, Stable 5—Hajdu‐Bihar County, Stable 6—Békés County.

FIGURE 2

The WNV neutralizing antibody titers (y‐axis) of the 25 included horses during the study period (x‐axis). The mean WNV titer of horses was 260.64 (SD = 336.739) in 2019, 114.32 (SD = 107.36) in 2020, 95.38 (SD = 115.56) in 2021, 22.53 (SD = 25.71) in 2022 and 6.31 (SD = 5.15) in 2023. There was a significant decrease (p < 0.001) in the serum neutralizing antibody levels over time.