Climate change has affected the spread of West Nile fever. This observation was confirmed in an Italian Ministry of Health note reporting 94 confirmed cases of infection. Of those cases, 55 were neuroinvasive, 19 were from blood donors, 19 were associated with fever, and in one case, the patient was symptomatic. Seven deaths have occurred since the start of the summer season, particularly in northern Italy.
Entomologists and veterinarians have confirmed the presence of West Nile virus (WNV) in a pool of 100 mosquitoes, 15 birds from targeted species, and 10 wild birds from passive surveillance. Four cases have been reported in horses in which clinical symptoms were attributable to a WNV infection. No cases of infection with Usutu virus (USUV) have been registered in humans. USUV is a virus in the same family as WNV. It was first identified in South Africa in the 1950s and is capable of causing encephalitis. The viral genome has been detected in a pool of 33 mosquitoes and four birds.
Currently, the regions where the circulation of WNV has been confirmed are Emilia-Romagna, Veneto, Piedmont, Lombardy, Sardinia, and Friuli Venezia Giulia. To date, USUV has been detected in Le Marche, Lombardy, Umbria, Emilia Romagna, Friuli Venezia Giulia, Lazio, and Veneto.
Current climate conditions favor the reproduction of the vector (mosquitoes of the Culex genus) and the subsequent viral circulation among wildlife, the natural reservoir of the virus, and mammals (including humans). The 2022 epidemic season is peculiar in comparison with seasons from the past 3 years. Viral circulation has started early, and a greater number of cases have been observed in the avifauna and in the mosquito pool, and there has been an increase in the number of cases in humans.
For these reasons, and considering the significance of the infection for public health, it is necessary to put all useful measures in place to limit the risk of further transmission among humans and animals.
As specified on the Italian National Institute for Health website, West Nile fever is caused by the homonymous virus of the Flaviviridae family, which was isolated for the first time in Uganda in 1937. The virus has spread to almost all continents.
The virus reservoirs are wild birds and mosquitoes (more frequently of the Culex genus). Other means of transmission, although very rare, are organ transplants, blood transfusions, and transmission from mother to fetus. West Nile fever cannot be transmitted from person to person. The virus infects other mammals, especially horses, and in some cases, dogs and rabbits.
Incubation and symptoms
The incubation period from the time of being bitten by an infected mosquito ranges from 2 to 14 days but can be up to 21 days in immunocompromised patients.
Most infected people do not show any symptoms. In around 20% of symptomatic cases, patients present with mild symptoms: fever, headache, nausea, vomiting, enlarged lymph nodes, and skin rashes. These symptoms may only last a few hours, but in rare cases, they may last a few weeks. Symptoms vary significantly, depending on the patient’s age. In children, a mild fever is most common, whereas in young people, symptoms are characterized by a fairly high fever, redness of the eyes, headache, and muscle pains. In the elderly and in debilitated patients, symptoms can be more severe.
The most serious symptoms are seen in fewer than 1% of infected patients (1 in 150 people) and include a high fever, a severe headache, muscle weakness, disorientation, tremors, visual disturbances, listlessness, and seizures, leading to paralysis and coma. Some neurologic effects may be permanent. In the most severe cases (around 1 in 1,000), the virus can cause terminal encephalitis.