West Nile Virus circulation in Portugal concentrated in south, study finds

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Researchers say more active surveillance will be required to keep watch on the virus in Portugal, which can infect horses and humans.
A working horse in Portugal. Image by Frank Nürnberger

West Nile Virus circulation in Portugal appears to be mostly restricted to the south of the country, based on molecular and serological evidence, researchers report.

West Nile virus is a mosquito-borne virus first identified in 1937 in the West Nile district of Uganda. The virus has since spread widely around the world, including into North America and Europe.

It has a transmission cycle maintained between mosquitoes and wild birds. The virus can infect a range of species, including horses and humans. About 80% of infections in humans are symptom-free, while the rest may develop mild or severe disease of a neurological nature.

In horses, West Nile infection can range from mild to potentially fatal inflammation of the brain. Although most horses recover, some experience long-term effects.

José Lourenço and his fellow researchers, in a study published in the journal Communications Biology, took a comprehensive, data-driven look at the virus in Portugal.

The available data, they said, pointed to a north-south discrepancy in West Nile circulation in Portugal, with the virus seemingly more prevalent in the south, based on existing molecular and serological information.

“It is still unclear whether West Nile Virus circulates endemically in any region of the country,” they said, “but sufficient evidence exists to support ongoing seasonal circulation.”

The south of Portugal may be more suitable for West Nile Virus transmission, they said, because of the effects of climate and land type, but the possibility of the north supporting transmission in shorter periods of each year cannot be rejected.

“Climate change is gradually increasing suitability for West Nile Virus in Portugal. A shift from passive to active One Health surveillance will be necessary to manage future epidemics, which may result in public health emergencies, as reported recently in other European countries.”

Discussing their findings, the authors noted that several European countries in the Mediterranean basin have progressed from sporadically reporting West Nile infections to reporting yearly epidemic activity affecting both humans and equines. They include Spain, France and Greece.

However, despite Portugal sharing similar climate types and bird migration routes with nearby countries that have experienced local West Nile dissemination, no events of sustained epidemic activity have been reported. “Without such reports, the perception of the public health importance of West Nile Virus in Portugal remains minimal.”

They said the large European West Nile epidemic in 2018 provided key opportunities to assess existing surveillance and public health mechanisms surrounding the virus.

Countries implementing early-warning systems based on a One Health approach incorporated surveillance of mosquitoes, birds, equines and humans.

The study noted that, in Portugal, mosquito surveillance remains an exception in what they described as an otherwise fragmented and insufficient West Nile surveillance infrastructure.

“Key improvements would include sustainability of laboratory capacities and their timely responses, active One Health surveillance beyond the mosquito level, and investment in awareness and training across public health domains and the general public, in particular in the south of Portugal.”

They said a shift from passive to active One Health surveillance will be necessary to manage future epidemics, which may result in public health emergencies, as reported recently in other European countries.

The study team comprised Lourenço, Sílvia Barros, Líbia Zé-Zé, Daniel Damineli, Marta Giovanetti, Hugo Osório, Fátima Amaro, Ana Henriques, Fernanda Ramos, Tiago Luís, Margarida Duarte, Teresa Fagulha, Maria Alves and Uri Obolski, variously affiliated with a range of institutions in Portugal, England, Brazil and Israel.

Lourenço, J., Barros, S.C., Zé-Zé, L. et al. West Nile virus transmission potential in Portugal. Commun Biol 5, 6 (2022). https://doi.org/10.1038/s42003-021-02969-3

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

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