The current outbreak of Oropouche virus (OROV) infection in the Amazonas and Acre states of Brazil presents considerable public health concerns, including the risk to travellers visiting these regions. Given the endemic nature of OROV infection in these areas, the US Centers for Disease Control and Prevention has recently elevated Brazil to a level 1 Travel Health Alert.

Oropouche fever, resulting from OROV infection, is an emerging zoonotic disease transmitted primarily via the bites of infected Culicoides paraensis, commonly known as biting midges, and sometimes via mosquitoes. C paraensis is commonly found in water bodies such as ponds, lakes, and rivers and humid tropical regions, particularly in some areas of different South American countries, and it plays an important role in the transmission dynamics of OROV.

The clinical manifestations of OROV infection include high fever, headache, myalgia, arthralgia, nausea, vomiting, chills, and photophobia, with the symptoms bearing a striking resemblance to those caused by other Latin American endemic arboviruses such as dengue, chikungunya, and Zika viruses, all of which cocirculate.

Although most OROV infection cases are self-limiting and typically resolve within 1 week, there is potential for severe complications such as meningitis and encephalitis, which can lead to fatal outcomes, and the complications are more likely to manifest in cases having CNS involvement, which require immediate medical intervention and thorough care.

OROV was first identified among forest workers in Trinidad in 1955, and in 1960, the virus was identified in a sloth in Brazil. An outbreak in 1961 in Brazil resulted in approximately 11 000 cases. Over 500 000 cases have been reported in the Americas, although the actual number and extent of disease transmission could be underestimated.

The recent increase in the number of OROV infection cases in Brazil, notably Amazonas, emphasises the importance of increased awareness, preventive measures, and enhanced surveillance for the population living in endemic areas and for travellers. In 2023, 832 cases were reported. In 2024 alone, as of May 9 (epidemiological weeks 1–18), 5913 cases have been recorded, and 2910 cases (approx. 93%) have been reported in Amazonas, with most cases from the Manaus area.

Furthermore, cases have been reported in Rondônia (n=1113), Acre (n=163), Para (n=52), Roraima (n=7), Amapa (n=1), Rio de Janeiro (n=10), Santa Catarina (n=7), and Paraná (n=1), and cases in Piaui (n=10), Bahia (n=273), and Espirito-Santo (n=33) are under investigation to establish the site of infection.

During the rainy season, Oropouche, dengue, chikungunya, Zika, Mayaro, and Venezuelan equine encephalitis viruses emerge as causes of fever, which requires differential diagnosis and poses a substantial health concern in the Americas. Additionally, urban outbreaks of OROV infection and Mayaro fever emerged in Acre, Brazil, in 2023, with over 60 cases clustered in areas such as Acrelândia, Feijó, and Rio Branco.

Cases of Mayaro fever have been documented in Cruzeiro do Sul and other western municipalities. Moreover, among neighbouring countries, Peru reported 94 cases of OROV fever between 2016 and 2022 and Colombia reported 87 cases between 2019 and 2021. Ecuador, French Guiana, Panama, and Trinidad and Tobago have also reported cases. Thus far in 2024 (as of May 9), Brazil (over 5000 cases), Bolivia (1856 suspected cases, with 313 cases confirmed using RT-PCR), Peru (259 cases, the highest number of cases reported to date in Peru), Cuba (n=74), and Colombia (38 cases) have reported a total of over 6000 OROV infection cases.

Factors contributing to such disease outbreaks include urbanisation, deforestation, and climate change, which are associated with increasing global temperatures, extreme heat temperatures, droughts, rain, and floods, all of which improve the climatic suitability for midges and mosquitoes.

Given the overlapping symptoms with other arboviruses and the absence of targeted treatments or vaccines for OROV, precise diagnosis and early medical intervention are crucial.

Considering these developments, it is important for travellers visiting endemic regions in Brazil, especially Amazonas and Acre, to adopt preventive measures such as using the Environmental Protection Agency-approved insect repellents, wearing protective clothing, opting for lodgings with mosquito protection, treating clothing and equipment with permethrin, and seeking medical attention promptly if symptoms arise to minimise the risk of OROV infection (appendix 1).8 Brazilian health authorities consider OROV infection a threat and have conducted further investigations to understand the disease status in different areas.

As researchers and health-care professionals continue to study and manage OROV outbreaks, it is crucial to disseminate reliable information and promote preventive strategies to safeguard public health. By raising awareness and implementing effective strategies, the effect of OROV can be mitigated, and individuals travelling to endemic regions can be protected.

*This article originally appeared in The Lancet under the title Oropouche fever outbreak in Brazil: an emerging concern in Latin America.