June 26, 2025 Author : Saraiva da Cunha (infectious diseases) The West Nile virus was first isolated and identified in 1937 in the West Nile region of Uganda, in a sick woman during an investigation into yellow fever. It is an RNA virus belonging to the Flaviviridae family. Its study over the following three decades led to a thorough understanding in several aspects, including: Its geographical distribution, at the time limited mainly to a few countries in Africa and the Middle East; The role of wild birds as the virus’s reservoir and various mosquito species as transmission vectors; The potential for human infection, which occurred mainly in rural populations, almost always resulting in asymptomatic infections or only mild, self-limiting symptoms. However, starting in the 1990s, the importance of West Nile Fever as a public health issue increased significantly . Three large outbreaks of human infection occurred in urban populations - in Romania, Russia, and the United States (New York) - with cases of severe and often fatal neurological illness. Since then, West Nile virus has become endemic in North American countries. In several European countries outbreaks of infection have occurred regularly, especially in the warmer months, when mosquito activity is more intense. Currently, West Nile virus is the arbovirus with the widest geographical distribution in the world, present on all five continents. Virus mutations, migratory bird movements, and climate changes which promotes mosquito expansion into temperate regions are among the factors believed to have contributed to the broad geographical spread of the virus and cases of West Nile Fever, particularly in humans and horses. Biological cycle and infection in mammals West Nile virus is present in nature in various species of wild birds, which are its natural reservoirs. Several species of mosquitoe act as its transmission vectors . They become infected when feeding on infected birds, and later transmit the virus to other birds. Domestic birds are not affected by the virus. In mammals, especially humans and equines (horses, donkeys, among others), the virus can cause illness, but it does not multiply significantly. Therefore, these species are considered dead-end hosts and do not spread the virus further. Mosquitoes of the Culex genus are the most frequently involved in transmission , both in Europe and North America. Other, rarer, forms of transmission in humans include blood transfusions, organ and tissue transplants, and vertical transmission (from mother to child during pregnancy). Equines are particularly susceptible to West Nile virus infection . Around 10% of infected animals develop neurological symptoms of varying severity. In 2000, a major outbreak of West Nile Fever occurred in horses in France, after more than three decades without cases. Since then, the disease has been identified in several European countries. Symptoms, diagnosis, treatment, and prevention In humans, the incubation period for West Nile Fever ranges from 2 to 21 days after infection. The majority - around 80% of people infected with West Nile virus - show no symptoms. The most common presentation is West Nile fever, with symptoms that may be mild but vary in severity . They appear suddenly and may include: Fever and general discomfort; Headaches and muscle pain; Nausea and vomiting; Skin rashes; Eye pain. In less than 1% of cases, especially in high-risk groups, such as the elderly, those with chronic illnesses, or immunocompromised individuals, severe neurological disease may develop. This is called West Nile neuroinvasive disease and involves the central nervous system. In very rare cases, the infection may lead to Guillain-Barré syndrome. Diagnosis is made based on medical history and symptoms, the epidemiological context, and, if needed, laboratory tests to identify the virus. There is no targeted antiviral treatment for West Nile virus. Treatment focuses on managing symptoms and providing any necessary supportive care . Recovery can be quick, but may also take months, and severe cases can be fatal. The mortality rate in neuroinvasive forms can reach around 17%. Currently, there is no human vaccine to prevent West Nile Fever, although there are vaccines available for equines. Human infection prevention relies mainly on avoiding mosquito bites controlling mosquito populations . Epidemiological surveillance Due to the potential for West Nile fever to become a serious public health concern, and because of the animal and economic losses associated with the disease in horses, Portugal has implemented an active surveillance program. Its goal is early detection and monitoring of the virus circulation, along with the implementation of specific measures when needed, to protect both human and animal health. In several countries, horses are used as sentinel species for West Nile Fever surveillance. The Portuguese surveillance plan includes : Monitoring of wild birds (eg, migratory birds from North Africa, crows, among others) through testing of dead birds and collection of samples from live ones; Mosquito capture and virus testing in horse farms where West Nile Fever is suspected; Monitoring of equines , including disease awareness campaigns for producers and veterinarians, examinations of animals with suspicious symptoms, and sample collection from animals in risk zones, among other measures. Since 2015, outbreaks of West Nile fever have been regularly identified in horses, mostly in southern Portugal. In 2022, three outbreaks were confirmed in Alcácer do Sal, Portalegre, and Castro Verde. West Nile fever in the European Union and in Portugal West Nile fever is a notifiable disease in Portugal, both in humans and animals . Within the European Union, disease surveillance is mandatory in most member countries. Throughout the 21st century, cases of West Nile fever have increased in several European countries. In 2018, the number of reported cases exceeded the total from the previous eight years combined. In the last five years: After a peak in 2022 with 1,140 suspected and confirmed cases across European countries, 751 cases were recorded in 2023 - the second highest figure to date; Of these, 713 cases were locally acquired, reported by the countries themselves, while the remaining were imported from other EU nations; The countries reporting the most cases were Italy (341), Greece (99), Romania (83), France (33), Hungary (22), and Spain (22); According to ECDC epidemiological data, in 2024, nineteen European countries recorded 1,436 human cases; In 2025, up to the beginning of June, no human cases had been reported in Europe, reinforcing the seasonal nature of the disease, which typically occurs in the summer months. Despite the climate in Portugal being favorable to mosquito activity, particularly in the south, and despite documented cases in horses, the country has shown a different pattern from other Mediterranean nations, with only four confirmed human cases of West Nile fever in the past 40 years. It is suspected that more cases may exist, but go undetected due to lack of laboratory confirmation and thus are not officially reported. Sources Direção Geral de Alimentação e Veterinária. Plano de vigilância da febre do Nilo Ocidental . European Centre for Disease Prevention and Control. West Nile virus infection . European Food Safety Authority, European Centre for Disease Prevention and Control. The European Union One Health 2023 Zoonoses Report. Lourenço J, Barros SC, Zé-Zé L, et al. West Nile virus transmission potential in Portugal. Commun Biol 2022;5:6. Meet the infectious diseases clinical team from Hospital da Luz network. The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
