The wetlands of the Murcia region are a risk area for Nile virus infection.

Andalusia is facing its biggest outbreak of West Nile virus since the first human cases of the mosquito-borne disease were discovered in Spain in 2010. Since July, nine people in the neighboring village have died from meningoencephalitis, the most serious complication associated with the virus. There is also another outbreak in Extremadura, with 25 people currently infected but no deaths.

Surveillance systems have not detected any traces of West Nile virus in the Murcia region to date, but experts remain vigilant. The Department of Agriculture lists the region’s wetlands, such as San Pedro Salinas, among the areas at risk for surveillance due to the presence of migratory birds. This is stated in several reports, the latest of which is dated 2024. Birds are the main hosts of the virus, which is transmitted by Culex mosquitoes.

Christian de la Fe, professor of animal health at UMU, and Enrique Bernal, head of infectious diseases at Reina Sofía, agree that it is “a matter of time” before a case of Nile fever appears in the region. “We have the conditions for this: vectors (mosquitoes), hosts (migratory birds) and high temperatures throughout the year due to climate change. We must be clear that this will remain forever,” explains De la Fe.

The fact that the virus has not yet been detected does not mean that it has not been circulating. “The presence of infected mosquitoes cannot be ruled out,” admits Francisco Collantes, professor in the Department of Zoology at UMU and coordinator of the Mosquito Pathogen Tracking Program at the Ministry of Health. Since April last year, Collantes and his team have been collecting Culex specimens in traps located in Salinas de San Pedro and other areas of the region (Salinas de Marjamalo, Mazarrón, Alhama, Calasparra, Cartagena). These are samples that remain frozen pending PCR tests to detect the possible presence of both Nile fever and other mosquito-borne viruses.

The goal is to create a system that will allow these PCR tests to be performed quickly to enable “real-time” monitoring. But to do this, you will have to wait until the personnel who will be responsible for this equipment undergo the necessary training.

Last year, mosquito samples were sent to the Biomedical Research Center of La Rioja (Siberia). As LA VERDAD reported, tests revealed the first case of Usutu virus infection in mosquitoes in the region. The virus currently poses less of a public health threat than Nile fever.

80% asymptomatic

Another factor complicating surveillance of West Nile virus is the high percentage of infected people who do not develop symptoms. Up to 80% of people infected by the mosquitoes that carry the virus have no symptoms. Serious conditions account for less than 1%. Patients who develop meningoencephalitis or other complications often have risk factors such as immunosuppression. This means that if around 80 cases and 9 deaths were detected in Andalusia this summer, it is because there are “significantly more” infected people, summarizes Enrique Bernal. That is, the circulation of the virus in the affected provinces, mainly in Seville, is very high.

In 2010, the first cases were identified in Spain, but starting in 2020, significant outbreaks began to appear. To date, human cases have been described in Andalusia, Extremadura, Catalonia, Castile-La Mancha and the Valencian Community.

At the end of July, the head of the infectious diseases department at the Reina Sofia Hospital, Enrique Bernal, was treating a patient from Cuba who developed symptoms that made him suspicious. Tests confirmed he had Oropouche virus, a disease transmitted by mosquito bites and endemic to the Caribbean island. The Ministry of Health subsequently confirmed a second imported case of Oropos virus in the Murcia region. Cases have also been identified in other localities such as Galicia, Madrid, the Canary Islands, the Basque Country or Andalusia. The disease causes fever, severe headache, muscle and joint pain, among other symptoms.

To date, no cases of Oropouche have been reported in Spain. Yes, there is dengue fever, another infectious disease transmitted by mosquitoes, in this case Aedes mosquitoes or tiger mosquitoes. This summer, Tarragona had the largest outbreak recorded in Spain, with eight cases, surpassing the figure that left the Murcia region with five infections in 2018.