Brazil has registered new cases of West Nile Fever, with confirmations in São Paulo and Santa Catarina. In São Paulo, two human cases were confirmed, while Santa Catarina recorded two, including one death. With these diagnoses, the country totals four confirmed cases in 2026, in addition to one probable case identified in Piauí, according to data from the Ministry of Health.
A worrying aspect of the disease is that approximately 80% of infected people do not show any symptoms, which complicates the identification of areas of viral circulation and reinforces the need for rigorous monitoring of mosquitoes and animals.
How the virus is transmitted
The transmission of the West Nile virus occurs mainly through mosquitoes belonging to the genus Culex, popularly known as biting flies or gnats. The virus's life cycle is closely linked to birds. Mosquitoes acquire the pathogen by biting these birds and can subsequently transmit it to other birds.
Humans and horses are infected accidentally. In the human case, the viral concentration in the blood does not reach sufficient levels to contaminate new mosquitoes. The cycle is established when infected birds contaminate Culex mosquitoes, which in turn infect more birds; humans and equines are affected incidentally.
In São Paulo, a 34-year-old resident of Ribeirão Preto recovered after reporting a recent trip to the Amazon. The second patient, 18 years old and a resident of São José do Rio Preto, remains hospitalized. The site of infection for these two patients has not yet been determined, although the first patient's trip is being investigated, without proving that the transmission occurred during the travel. For the second case, there is no record of similar exposure outside the state.
Carla Kobayashi, an infectious disease specialist at Sírio-Libanês Hospital in São Paulo, warned that it is essential to obtain evidence of circulation between animals and mosquitoes before stating that the virus is active in São Paulo or Santa Catarina.
Situation in Santa Catarina
In Santa Catarina, a 77-year-old woman from Imbituba died on August 8th. A 56-year-old man from Caçador was diagnosed and discharged. The probable sites of contagion in these occurrences are still under investigation.
When symptoms appear, they can include fever, headache, general malaise, body aches, nausea, vomiting, and skin rashes. It is important to note that this clinical picture can be confused with diseases such as dengue, zika, and chikungunya.
In more severe situations, the infection can affect the central nervous system, resulting in encephalitis, meningitis, confusion, seizures, muscle weakness, and flaccid paralysis.
Luana Araújo, an infectious disease specialist, epidemiologist, and president of the One Health Scientific Committee of the Brazilian Society of Infectology (SBI), emphasized that the low rate of symptoms (80% asymptomatic) decreases surveillance sensitivity, making reliance on animal and entomological surveillance crucial for understanding viral circulation.
The virus belongs to the same group as dengue, zika, and yellow fever, which can lead to cross-reactions in certain antibody tests, complicating the analysis of results. Therefore, depending on the phase of the disease and initial exams, other tests may be necessary to confirm the infection. The complete investigation must cover mosquitoes, birds, and horses to help locate viral circulation and human case hotspots.