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

The transmission cycles of West Nile Virus and Usutu virus are similar, with birds acting as a reservoir and amplifying host, and mosquitoes being the principal vector.
Photo by Annika Treial

West Nile Virus and Tick-Borne Encephalitis Virus are now endemic among horses and donkeys in eastern Austria, researchers report.

Phebe de Heus and her fellow researchers, writing in the journal Viruses, have described their efforts to find evidence of the two viruses, as well as 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 neurologic signs and have been known to circulate in Austria. Cases fluctuate widely on a yearly basis, depending on factors that influence vectors, hosts, transmission route, and viral replication.

The transmission cycles of West Nile Virus and Usutu virus are similar, with birds acting as a reservoir and amplifying host, and mosquitoes being the principal 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 Vienna’s University of Veterinary Medicine.

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

All cases showed gait abnormalities and most displayed muscle twitches and a change in mental activity. Six were euthanized because of the severity of their problems.

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

Small mammals are the main reservoir hosts of the Tick-Borne Encephalitis Virus. Horses and humans can be infected, but are dead-end hosts. Historically, high case numbers among humans in Austria triggered monitoring and mass vaccination programs over decades. No confirmed equine clinical cases of Tick-Borne Encephalitis have been diagnosed at the University of Veterinary Medicine Vienna during the last 20 years.

The samples from the 334 horses and 14 donkeys for analysis were all taken in 2017 at the height of the mosquito season.

The researchers found neutralizing antibodies for West Nile Virus — excluding vaccinated equids — 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 former West Nile infections that are likely to have arisen locally. These four were documented to be born in Austria and had no available documentation indicating travel abroad.

Of the remaining 14 seropositive equids, one was vaccinated against the virus, 13 had a questionable birthplace, a foreign passport, a questionable travel history, or were documented to have been imported or traveled abroad.

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

“This study substantiates West Nile Virus and Tick-Borne Encephalitis Virus endemicity in horses in eastern Austria, but could not unequivocally demonstrate Usutu Virus infections in equids,” the study team concluded.

“Since humans and equids as dead-end hosts share epidemiologic aspects, surveillance studies in equids like this are relevant for public health.”

They said continuous surveillance of horses may be used as a predictor for virus circulation, which would allow medical and veterinary authorities to take action before large-scale infection of humans occurs.

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

de Heus, P.; Kolodziejek, J.; Hubálek, Z.; Dimmel, K.; Racher, V.; Nowotny, N.; Cavalleri, J.-M.V. West Nile Virus and Tick-Borne Encephalitis Virus Are Endemic in Equids in Eastern Austria. Viruses 2021, 13, 1873.

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

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