High seroprevalence of West Nile Virus in western Germany’s horse population – study

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A micrograph of the West Nile Virus, appearing in yellow.
A micrograph of the West Nile Virus, appearing in yellow. Image: Cynthia Goldsmith, P.E. Rollin, USCDCP, public domain

West Nile Virus circulation in horses in eastern Germany is now established, a fresh study has shown.

However, there was no clear evidence the mosquito-borne virus is yet among horses in the eastern part of Germany.

Felicitas Bergmann and her fellow researchers noted that, since the confirmed introduction of West Nile to eastern Germany in 2018, increasing infections have been diagnosed in birds, equines, and humans over time, while the spread into western Germany remained unclear.

In their study, reported in the journal Viruses, the researchers screened serum samples collected over three years, from 2018 to 2020, from 437 equines for West Nile-specific antibodies and evidence of the viral genome. The authors excluded horses vaccinated against the virus.

The convenience sample of horses lived in eastern and western parts of Germany. The eastern samples were collected from horses in the Berlin/Brandenburg region, the western samples from the North Rhine-Westphalia region.

Clinical presentations, final diagnoses, and demographic data were also recorded.

In the eastern part, a total of eight horses were found to be seropositive for West Nile in 2019, representing a seroprevalence of 8.16%. In 2020, the number of seropositive horses rose to 27, for a seroprevalence of 13.77%.

Eight horses in eastern Germany carried antibodies against the virus, but only four showed typical clinical signs.

The study team said there were also two clinically unsuspected horses with West Nile-specific antibodies in the western part from 2020. However, their travel history could be excluded as a possible source of their exposure.

One of the horses originally came from Austria and was introduced to western Germany five years earlier. The other equine, a racehorse, had been in western Germany for only 18 months, having moved from France. In both cases, the researchers could not rule out short-term stays for sporting events in endemic regions.

None of the horse samples in the study contained West Nile genomes.

In about half of cases, recent exposure to West Nile – as indicated by a positive antibody test – corresponded with clinical signs. However, frequently reported clinical findings in this study, including muscle twitching or altered skin sensibility, are less frequently reported in association with a West Nile infection.

In this study, these signs occurred alone or in combination with the more commonly reported poor co-ordination, gait deficit, or reduced proprioception in affected horses.

In accordance with previous findings, fever was reported in only one of the cases.

In one case, neurological signs rapidly worsened, and the horse became recumbent and had to be euthanized.

The authors said the results underline the difficulty of detecting a West Nile infection in a horse based solely on clinical signs.

They characterised the rate of West Nile seroprevalence among the tested horses in eastern Germany as high.

“Future serosurveys of horse populations in areas previously assumed to be unaffected will help to reveal the spread of the virus at an early stage,” they said.

“As long as the vaccination rates for West Nile Virus in these areas remain on a low level, horses can be important sentinels.”

When neurological signs occur in horses, or there are nonspecific signs of unclear cause, such as the presence of colic or hard faeces in areas with high mosquito abundance, West Nile infection should be included as an important differential diagnosis, they said.

The study team comprised Bergmann, Martin Groschup and Ute Ziegler, with the Friedrich-Loeffler Institute; Dagmar Trachsel, Sabita Stoeckle and Heidrun Gehlen, with the Free University of Berlin; and Joke Bernis Sierra and Stephan Lübke, with the Equine Veterinary Practice in Neunkirchen-Seelscheid, Germany.

Bergmann, F.; Trachsel, D.S.; Stoeckle, S.D.; Bernis Sierra, J.; Lübke, S.; Groschup, M.H.; Gehlen, H.; Ziegler, U. Seroepidemiological Survey of West Nile Virus Infections in Horses from Berlin/Brandenburg and North Rhine-Westphalia, Germany. Viruses 2022, 14, 243. https://doi.org/10.3390/v14020243

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

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