Insect-borne viruses circulating among horses and birds in northeastern Spain

Results point to widespread circulation of flaviviruses in Catalonia, in particular West Nile, but also Usutu virus.
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Testing indicates that West Nile virus and Usutu virus are circulating widely among birds and horses in Catalonia, in northeastern Spain.

Tick-borne encephalitis virus was also sporadically detected in horses, Sebastian Napp and his fellow researchers reported in the journal Viruses.

Their study looked at the incidence of flaviviruses in horses and birds in the region between 2010 and 2019.

Flaviviruses include important pathogens that are capable of infecting humans, such as dengue virus, yellow fever virus, West Nile virus, tick-borne encephalitis virus, and Japanese encephalitis virus.

Like West Nile virus, others in the genus, such as Usutu virus and Bagaza virus, can cause severe disease in animals. They are transmitted to susceptible animals through the likes of blood-feeding mosquitoes or ticks.

In recent years, the areas in Europe reporting flavivirus infections, particularly those caused by West Nile virus, Usutu virus or tick-borne encephalitis virus, have increased significantly, the authors noted.

“Many of these viruses are currently endemic in several areas of the continent.”

Of the 3791 serum samples collected from birds in Catalonia between 2010 and 2019, 380, or 10 percent, were positive for a flavivirus.

Of these, there was enough serum in 205 cases for more precise analysis. In all, 118 showed specific antibodies for West Nile virus, 19 for Usutu virus, and 68 were classified as an undetermined flavivirus. None was classified as Bagaza virus.

Of the 1856 serum samples collected from horses in the same time period, 182, or 9.8 percent, tested positive. Of them, 164 had enough serum for further testing. Ninety-two were found to be positive for West Nile virus, 11 for Usutu, four for tick-borne encephalitis virus, and 57 were classified as infected by an undetermined flavivirus.

The authors said there may have been some bias in the selection of animals for the study. In the case of birds, some species were over-represented either because they were useful for West Nile surveillance (such as chickens or magpies), were found in wildlife recovery centers (such as raptors), or because samples were commonly collected within other surveillance programs.

In horses, the distribution of samples may have been influenced by the proximity to the Equine Veterinary Hospital. Finally, some regions, mainly the areas where West Nile circulation was detected, were also over-represented as a result of the specific surveys implemented there.

The researchers noted that a third of the samples tested were inconclusive by microneutralization test in terms of identifying the specific virus, highlighting the limitations of current diagnostic techniques.

“Our results evidenced the widespread circulation of flaviviruses, in particular West Nile virus, but also Usutu virus, and suggest that chicken and horses could serve as sentinels for both viruses,” they said.

They noted that, so far, no human infections by West Nile, Usutu or tick-borne encephalitis viruses have been reported in Catalonia.

“However, considering that these viruses have been circulating for some time in different areas of Catalonia means that they need to be kept under surveillance, and the most efficient way to do that is within a One Health framework.

“That includes raising awareness among medical practitioners (clinicians), to consider these infections in the differential diagnosis of patients with compatible clinical symptoms.”

Given the widespread distribution of West Nile virus and Usutu virus in Catalonia, microneutralization tests for both viruses should be included as a routine technique for the correct diagnosis of flavivirus infections. “Failure to do so results in both under-detection and misclassification,” they said.

“Considering the sporadic detection of tick-borne encephalitis virus, further studies of the risk of introduction and establishment would be needed.”

Napp, S.; Llorente, F.; Beck, C.; Jose-Cunilleras, E.; Soler, M.; Pailler-García, L.; Amaral, R.; Aguilera-Sepúlveda, P.; Pifarré, M.; Molina-López, R.; Obón, E.; Nicolás, O.; Lecollinet, S.; Jiménez-Clavero, M.Á.; Busquets, N. Widespread Circulation of Flaviviruses in Horses and Birds in Northeastern Spain (Catalonia) between 2010 and 2019. Viruses 2021, 13, 2404.

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

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