The Department of Agriculture of the Generalitat has confirmed sixteen positive cases of West Nile fever in equids in Catalonia. Of this total, eleven are located in the municipality of Almacelles, in the Segrià district (Lleida), and five in Pau, in the Alt Empordà district. The results, verified on 11 October, mark the detection of the infection in horses in the province of Lleida after more than five years without active records.

Distribution of cases and surveillance

Official data indicate a concentration of infections in the central-western part of the demarcation. The municipality of Almacelles accounts for the majority of the detected positive cases. The remaining five cases have been identified on the eastern coast.

This distribution corresponds to the presence of habitats conducive to the reproduction of the Culex genus, the main vector for the transmission of the disease. These mosquitoes proliferate in stagnant waters and wetland areas, characteristic of the geography of the inner delta and the irrigated zones of Lleida.

Since October 2020, no confirmed cases in horses had been recorded in the province. The current detection has led the Department of Agriculture to establish a surveillance perimeter in the area affected by the cases in Almacelles and to communicate the alert to the competent health authorities.

Transmission and human risk

West Nile fever is not transmitted from animal to animal, nor directly between people. The main transmission cycle involves wild birds, which act as reservoirs of the virus, and mosquitoes of the Culex genus, which act as vectors. An infected mosquito that bites a horse or a person may transmit the virus.

Horses and humans are regarded as “dead-end” hosts. This means that while they may become infected, they do not develop sufficiently high levels of viraemia (the concentration of the virus in the blood) to re-infect other mosquitoes. Consequently, the presence of cases in equids does not indicate an outbreak of direct human-to-human transmission, but it does serve as an epidemiological indicator of active viral circulation in the environment.

Human infection is possible through the bite of an infected mosquito. Symptoms can range from mild presentations of fever, headache and myalgia to severe forms affecting the central nervous system (meningoencephalitis). No specific antiviral treatment exists; management is therefore supportive.

Epidemiological context in the Segrià

The detection of cases in Almacelles is part of an established surveillance framework. During the current year, health and veterinary monitoring systems have recorded the presence of the virus in birds in the Segrià district, including sentinel chickens and other avian species.

The Institute for Research in Animal Health Sciences (IRTA-CRESA) has been involved in confirming the cases through the analysis of biological samples. Coordination between this body and the Department of Agriculture enables a coordinated response to the detection of new foci.

The combination of climatic factors, the abundance of irrigation zones and wetlands, and the passage of migratory birds through the Lleida area creates conditions that facilitate the survival of the virus in the environment.

Control and prevention measures

Following the confirmation of positive cases, authorities have activated a protocol of preventive measures. These actions focus on two main pillars: vector control and health surveillance.

At the local level, municipal councils in the area, including Almacelles, are required to manage mosquito breeding sites. Measures include the removal of stagnant water from containers, cisterns, gardens, and other areas where mosquitoes may breed.

In the health sector, clinical surveillance has been strengthened among professionals at local health centres. The clinical presentation of West Nile fever is included in the differential diagnosis for patients exhibiting compatible symptoms, particularly fever and neurological disturbances.

Blood banks are maintaining their screening protocols to detect potential infections in donations. This is a standard precautionary measure to prevent transfusion-transmitted infection, a rare but acknowledged risk within public health strategy.

Recommendations for the public and livestock farmers

For the livestock sector, authorities recommend close monitoring of animals. Clinical signs of West Nile fever in horses include fever, muscular weakness, ataxia (difficulty maintaining balance), and paralysis. In the event of a suspected case, it is mandatory to notify the registered veterinary surgeon and health authorities for sample collection.

Vaccination is a preventive tool available for equines. Although it is not mandatory across the entire region, its consideration is advised in high-risk areas such as the Segrià, particularly during periods of peak mosquito activity. The decision should be assessed by the veterinarian based on the farm’s history and location.

For the general public, bite prevention is the most effective measure. Recommendations include:

  • Use of repellents effective against mosquitoes.
  • Wearing clothing that covers the arms and legs.
  • Installing screens on doors and windows.
  • Avoiding outdoor activities at dawn and dusk, times when the vector is most active.

Surveillance

The epidemiological situation is under active surveillance. The Department of Agriculture and IRTA-CRESA will continue monitoring equids and birds in the Lleida province. The detection of new positive cases could lead to an expansion of the surveillance perimeter and an intensification of mosquito control efforts.

Health authorities have not established any animal movement restrictions or border closures. The current recommendation is for breeders to exercise caution and adhere to biosecurity measures, and for the public to follow preventive measures, in order to minimise the risk of exposure to the virus.