The fifth death from West Nile fever since the start of the year has put authorities on alert, but the virus had already been frequently infecting humans since 2020. This year, however, seems to mark a turning point in the dangerousness of this emerging disease.
The number of confirmed cases is 65 (53 in Andalusia and 12 in Extremadura), triple that of last year, and there are already five deaths. Only 2020 exceeds the number of infections, but it is expected to lag behind: In other years, cases start appearing at this point in the summer, leaving plenty of room for growth..
The first two cases detected in Spain were reported in 2010 (another in 2004 was identified retrospectively). In the last decade, new cases were not recorded until 2016, but as of 2020, things have changed.
That year, 77 cases were reported (with eight deaths) spread across three provinces: Seville, Cádiz and Badajoz. It is believed that the disruptions caused by the pandemic have affected the fight against mosquitoes, insects that transmit the virus to humans from the reservoir of migratory birds.
“Where there are more migratory birds carrying the virus, that’s where there may be more cases in humans,” he explains. Diego Garciaspokesperson for the Spanish Society of Infectious Diseases and Clinical Microbiology (Seimc).
“And in Doñana (which extends to Huelva, Seville and Cádiz) there are many birds but, being a marshy area, there are also many mosquitoes,” because they usually breed in rivers and stagnant waters. The perfect storm for the West Nile virus.
Since 2020, not only have cases been reported every year, but they have also been reported in new provinces. In 2022, it was Tarragona and Córdoba. In 2023, Barcelona, Cáceres, Huelva, Valencia and Toledo.
The 2023 epidemiological report from the Carlos III Health Institute already indicated that it was likely that “since what happened in 2020, the sensitivity of the diagnosis has increased due to a higher degree of suspicion”, but other factors such as the impact of climate change.
“If we have less harsh winters, fewer mosquitoes die, they continue to infect birds and move to other areas, so the virus can reach more environments,” says García.
Earlier detection
So far this year, 53 cases have already been reported (the disease has been notifiable since 2015) in Andalusia and five in Extremadura. It would not be strange if the total number of cases exceeded that of 2020: that year, that is when they began to be reported.
“We have detected mosquitoes carrying the virus since the beginning of June, whereas it usually started in mid-July,” explains the clinical microbiologist.
However, those reported so far are symptomatic cases, “a number much lower than the total that may exist, because 80% of infections are usually asymptomatic“.
It is estimated that one in 100 to 150 cases are serious and one in 500 to 1,000 develop neurological symptoms such as meningitis and encephalitis, which can lead to death.
“If there are more serious cases, that means there are more asymptomatic cases,” García explains. “And there are more serious cases.” The math is simple.
The Director General of Public Health of the Government of Andalusia, Jorge del DiegoIt should be recalled that cases of encephalitis (inflammation of the brain) of viral origin that were previously not identified as infections have been associated with this virus, since the detection capacity has improved.
But he warns: “In 2020, the year there was a peak, there were only 44 confirmed cases. This year we have already had 53. If we know the disease and we know that about 1% of cases are those who show their faces, We are talking about 5,000 or 6,000 real infections.by applying pure statistics.”
The elderly, usually men, and immunocompromised people are most exposed to viral infections. In addition, there are no vaccines for human use or specific treatments, only symptomatic vaccines.
Diego García points out that it is now very complicated to stop the current expansion of the virus. “It was time to act before this happened, focusing on the reproduction process of mosquitoes and larvae.”
The administrations carry out active and passive surveillance, in the natural reservoir (birds, mainly migratory but also free-ranging birds) as well as in other infected animals such as horses. Entomological surveillance is also carried out, detecting mosquitoes carrying the virus.
These are usually mosquitoes of the genus Culex those that transmit it, although it is not exclusive to this genus. The problem is that these mosquitoes are very widespread: among the species of this genus is the common mosquito.
West Nile virus is the most widespread arbovirus in the world, occurring on all continents except Antarctica. In recent years, it has occurred in epidemics with a high proportion of severe cases in temperate regions of Europe and North America.
