The Regional Ministry of Health has stepped up surveillance of West Nile virus after detecting a positive case in a horse in Menorca, the first confirmed case of West Nile fever in the Balearics. In a press release, the Department of Health reported that, following the detection and notification of this first case by the Department of Agriculture, Fisheries and the Environment in an equine with clinical symptoms compatible with West Nile fever in the municipality of Maó, and its official confirmation by the Central Veterinary Laboratory, it will launch a reinforced surveillance programme to determine the extent of West Nile virus circulation in the islands and assess the possible risks. In turn, the Regional Ministry of Health has announced that it has alerted the Balearic Health System to reinforce surveillance and early detection of possible human cases.
West Nile fever is currently present in Spain in other regions where cases have been detected in horses, humans, birds and mosquitoes. Andalusia is the most affected autonomous community, followed by Extremadura, Catalonia and Valencia. It should be noted that, in the Balearics, no positive cases of local origin have been detected so far in the analyses carried out by the Blood and Tissue Bank Foundation since the summer of 2024. These analyses are carried out on all blood donations and other blood components in order to determine the possible presence of the West Nile virus and other arboviruses such as dengue, Zika, etc.
The Public Health vector surveillance programme will be reinforced with a series of measures aimed at determining the presence of the virus directly in the vectors and thus being able to assess the degree of circulation of the virus in this autonomous community and the possibilities of transmission. In addition, the entire healthcare system, both public and private, has been alerted to this disease in order to increase surveillance and diagnostic suspicion in cases with symptoms compatible with West Nile fever for early detection.
In order to reduce the risk of exposure to the West Nile virus, the Regional Ministry of Health recommends taking a series of protective measures to avoid mosquito bites and thus avoid exposure to the virus, especially in risk groups. The main measures outdoors are to avoid exposure to mosquitoes by using authorised repellents or wearing long-sleeved clothing. In addition, it is recommended to eliminate small accumulations of water, as they can be a breeding ground. At home, mosquitoes should be prevented from entering by using mosquito nets and repellents.
West Nile Valley Fever is a zoonosis caused by certain strains of the West Nile virus, which is mainly transmitted by mosquito bites. This virus is maintained through a mosquito-bird-mosquito transmission cycle, in which birds act as a reservoir for the disease and mosquitoes as vectors that can transmit the disease to other hosts such as horses or humans. Mosquitoes are not infected when they bite horses, nor is the disease transmitted between horses and humans.
The disease cycle is seasonal, occurring mainly during the months when mosquitoes are active, between April and late October and early November. It is particularly associated with wetlands, where waterfowl stop during their migration and introduce viruses from other latitudes. Transmission of the virus between birds and mosquitoes amplifies the virus, and both horses and humans act as accidental hosts. Although they may manifest the disease, they do not contribute to its maintenance.
As for the clinical course of the infection in humans, it is estimated that around 80 per cent of those infected are asymptomatic and the rest present mild symptoms with fever and myalgia. Approximately less than one per cent may present with severe neurological involvement (encephalitis). Very rarely, other symptoms such as hepatitis, pancreatitis and myocarditis may appear.