TDK conference 2020

The West Nile Virus (WNV) is a zoonotic arbovirus, which belongs to the genus Flavivirus in the family of Flaviviridae. In addition to WNV, other Flaviviruses have been shown to be present in the geographical region of Hungary, out of them Usutu Virus together with WNV is a member of the Japanese Encephalitis Virus Complex (JEV). WNV causes nervous system disorders, especially in birds, humans and horses, with different frequencies by species. In 2016, an epidemic with a significant number of diseases developed in Hungary, and in 2018, record number of human and equine cases occurred in our country, and WNV became endemic in previously uninfected areas of Europe as well.

In addition to the increased number of infections seen in recent years, protection has become a key point, but in the course of vaccination, we should also pay close attention to immunity from previous latent and symptomatic infections, and to the possibility of cross-reactions within Flaviviruses, especially between JEV complex members. In our study, we would like to answer the question, according to literature notes, whether long-term immunity of Flaviviruses is characteristic in horses after natural infection with WNV. In addition, we tested the presence of antibodies against Usutu Virus (USUV) and Thick-borne Encephalitis Virus (TBEV) in horses infected with WNV.

A total of 21 horses with acute neurological symptoms were included in our study. These horses were tested positive for WNV during the officially performed IgM ELISA test between 2008 and 2019, thus the disease was diagnosed. In our research the samples were first subjected to WNV IgG ELISA test, and then in virus neutralization (VN) assays we measured accurate titers to WNV, and against USUV and TBEV, which viruses also belong to Flaviviruses.

Only one horse was tested negative on IgG ELISA, and since this assay shows total antibody levels, the negative horse was not tested further. All of the 20 horses, which were positive for the preliminary IgG ELISA test also showed positive titers during virus neutralization. WNV VN titer values ranged from 1:45 to 1:1024. In the majority of cases, titers of 1:128 and 1:724 were measured, these titers occurred in 20-20% of the cases.

We have measured a positive 1:23 titer against Tick-borne Encephalitis Virus in one horse, while in the VN studies for Usutu Virus we obtained low, but positive 1:2 and 1:4 titer results in several samples. In these cases, we assume the formation of a cross-reaction characteristic of Flaviviruses following WNV-induced disease, since significantly higher WNV titer values were measured in all cases.

From our results, we can conclude that horses with proven WNV induced neurological disease have long-term immunity, but further studies are needed to determine accurately the length and characteristics of WNV induced immunity.