Two mosquito-borne viruses that can infect horses are now endemic in eastern Austria, results show

West Nile virus and tick-borne encephalitis virus are now endemic in horses and donkeys in eastern Austria, researchers report.

Phebe de Heus and his fellow researchers, writing in the journal Virus, described their efforts to find evidence of both viruses, as well as the Usutu virus, in blood samples from 348 horses and donkeys taken the following year in eastern Austria.

All three mosquito-borne viruses have been associated with neurological signs and are known to circulate in Austria. Cases fluctuate considerably from year to year, depending on factors that influence vectors, hosts, route of transmission and viral replication.

The transmission cycles of West Nile virus and Usutu virus are similar, with birds acting as reservoir and enhancer host, and mosquitoes being the primary vector.

West Nile virus has been reported in Austria since 2008 in several species, including birds of prey, humans and mosquitoes.

The first equine case of West Nile neuroinvasive disease in Austria was diagnosed in 2016 at the University of Veterinary Medicine in Vienna.

The researchers noted that, of the 11 confirmed cases of West Nile neuroinvasive disease treated at the university’s equine hospital through March 2021, two presented in 2016, three in 2017, two in 2018, three in 2019 and one in 2020.

All of the cases presented with gait abnormalities and most presented with muscle contractions and a change in mental activity. Six were euthanized because of the seriousness of their problems.

In 2001, the Usutu virus was detected for the first time on the European continent, more precisely in Austria, and has been associated with bird mortality. Human cases are mostly asymptomatic, but in rare cases they can cause neurological problems. Evidence of past infection was found in healthy horses in Croatia, Poland and Italy, but its occurrence in Austria was unknown.

Small mammals are the main reservoir hosts for tick-borne encephalitis virus. Horses and humans can be infected, but are hopeless hosts. Historically, a high number of human cases in Austria has triggered decades of mass surveillance and vaccination programs. No confirmed clinical case of tick-borne encephalitis in equines has been diagnosed at the University of Veterinary Medicine in Vienna during the past 20 years.

Samples from the 334 horses and 14 donkeys to be analyzed were all taken in 2017 at the height of mosquito season.

The researchers found neutralizing antibodies against West Nile virus – excluding vaccinated equines – in 5.3% of the animals; that’s 18 in all. The prevalence of antibodies against tick-borne encephalitis virus was 15.5%, while no antibodies against Usutu virus were detected.

The authors said only four of the 18 horses could be truly identified as having evidence of old West Nile infections that may have occurred locally. These four people were documented as being born in Austria and had no documentation available indicating a trip abroad.

Of the remaining 14 HIV-positive equines, one was vaccinated against the virus, 13 had a questionable place of birth, a foreign passport, a questionable travel history, or were documented as having been imported or traveled abroad.

No West Nile virus RNA was detected, the presence of which would have indicated a current infection.

“This study confirms the endemicity of West Nile virus and tick-borne encephalitis virus in horses in eastern Austria, but could not unequivocally demonstrate Usutu virus infections in horses. equines “, concluded the study team.

“Since humans and equines as dead-end hosts share epidemiological aspects, equine surveillance studies like this one are relevant to public health. “

They said continuous surveillance in horses could be used as a predictor of virus circulation, allowing medical and veterinary authorities to take action before a large-scale human infection occurs.

The study team consisted of Phebe de Heus, Jolanta Kolodziejek, Katharina Dimmel, Norbert Nowotny and Jessika-MV Cavalleri, all from the University of Veterinary Medicine in Vienna; Zdenĕk Hubálek, with the Czech Academy of Sciences; and Victoria Racher, with the University of Salzburg.

from Heus, P .; Kolodziejek, J .; Hubálek, Z .; Dimmel, K .; Racher, V.; Nowotny, N .; Cavalleri, J.-MV West Nile virus and tick-borne encephalitis virus are endemic in equines in eastern Austria. Virus 2021, 13, 1873. https://doi.org/10.3390/v13091873

The study, published under a Creative Commons License, can be read here.