Catalan researchers have detected mosquitoes carrying the West Nile virus at a horse riding school in Riudoms (Tarragona). Last year there was an outbreak of the virus among horses.
Although the Generalitat has been detecting cases of West Nile virus in horses and birds since 2017, it is the first time specimens of the transmitting mosquito have been found.
West Nile virus generally develops very mildly in humans (four out of five cases are asymptomatic). Less than 1% of infected people become seriously ill from the virus. 80% notice nothing and 20% have flu-like symptoms. However, the disease caused the death of an elderly man (77) and woman (85) in Andalucia in 2021. This region, together with Extremadura, is the region where the most cases of West Nile virus are detected; there were 100 in 2020 alone.
Alarm protocol level 2
The virus is mainly transmitted between birds. The detection of the mosquitoes does not mean that the disease is officially diagnosed; although the government has activated the level 2 alert protocol.
“The fact the mosquito has been detected means the virus is circulating.” , explains Sebastian Napp, expert in veterinary epidemiology at the Institut de Recerca i Tecnologia Agroalimentàries (IRTA). He has confirmed the presence of 30 Culex pipiens after placing traps in the equestrian centre.
So far, since the first detection in a bird in 2017, the cases found have come from line 2, the same ones circulating in much of central and eastern Europe, where West Nile virus is already widespread. Napp explains that pending the analyses, it is most likely the mosquitoes found with the virus also belong to strain 2, which, unlike type 1, is more pathogenic for humans and less for horses.
“The virus tends to disappear in the winter and return in the summer. It is not possible to predict exactly how it will develop in the coming years,” adds Napp. He sees climate change as an important factor in explaining its expansion in colder countries like Germany and the Netherlands.
More control needed
Napp adds the disease doesn’t usually affect farm birds, although antibodies have been detected in some birds. Also, the symptoms in horses range from lack of coordination to paralysis; and that the best way to stop a spread is to extend the controls. Although, he admits, the mechanism to trap the mosquitoes is expensive.
The Catalan government is now carrying out a sampling of birds and horses within a radius of 10 kilometres.
How does the virus spread?
The West Nile virus does not spread through direct contact between humans. It is spread by mosquitoes between birds in the warmer seasons. Birds can have a large amount of virus in their blood. Mosquitoes that bite such a bird become infected and then bite a human or other animal (such as a horse). Humans have so little West Nile virus in their blood that a mosquito cannot spread it from person to person.